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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
<title>Apache HTTP Server: Security Tips</title>
</head>
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<h1 align="CENTER">Security Tips for Server Configuration</h1>
<ul>
<li><a href="#serverroot">Permissions on ServerRoot
Directories</a></li>
<li><a href="#ssi">Server Side Includes</a></li>
<li><a href="#nsaliasedcgi">Non Script Aliased CGI</a></li>
<li><a href="#saliasedcgi">Script Aliased CGI</a></li>
<li><a href="#cgi">CGI in General</a></li>
<li><a href="#dynamic">Other sources of dynamic content</a></li>
<li><a href="#systemsettings">Protecting System
Settings</a></li>
<li><a href="#protectserverfiles">Protect Server Files by
Default</a></li>
</ul>
<hr />
<p>Some hints and tips on security issues in setting up a web
server. Some of the suggestions will be general, others
specific to Apache.</p>
<hr />
<h2><a id="serverroot" name="serverroot">Permissions on
ServerRoot Directories</a></h2>
<p>In typical operation, Apache is started by the root user,
and it switches to the user defined by the <a
href="../mod/core.html#user"><strong>User</strong></a>
directive to serve hits. As is the case with any command that
root executes, you must take care that it is protected from
modification by non-root users. Not only must the files
themselves be writeable only by root, but also the
directories and parents of all directories. For example, if
you choose to place ServerRoot in
<code>/usr/local/apache</code> then it is suggested that you
create that directory as root, with commands like these:</p>
<blockquote>
<pre>
mkdir /usr/local/apache
cd /usr/local/apache
mkdir bin conf logs
chown 0 . bin conf logs
chgrp 0 . bin conf logs
chmod 755 . bin conf logs
</pre>
</blockquote>
It is assumed that /, /usr, and /usr/local are only modifiable
by root. When you install the httpd executable, you should
ensure that it is similarly protected:
<blockquote>
<pre>
cp httpd /usr/local/apache/bin
chown 0 /usr/local/apache/bin/httpd
chgrp 0 /usr/local/apache/bin/httpd
chmod 511 /usr/local/apache/bin/httpd
</pre>
</blockquote>
<p>You can create an htdocs subdirectory which is modifiable by
other users -- since root never executes any files out of
there, and shouldn't be creating files in there.</p>
<p>If you allow non-root users to modify any files that root
either executes or writes on then you open your system to root
compromises. For example, someone could replace the httpd
binary so that the next time you start it, it will execute some
arbitrary code. If the logs directory is writeable (by a
non-root user), someone could replace a log file with a symlink
to some other system file, and then root might overwrite that
file with arbitrary data. If the log files themselves are
writeable (by a non-root user), then someone may be able to
overwrite the log itself with bogus data.</p>
<hr />
<h2><a id="ssi" name="ssi">Server Side Includes</a></h2>
<p>Server Side Includes (SSI) present a server administrator
with several potential security risks.</p>
<p>The first risk is the increased load on the server. All
SSI-enabled files have to be parsed by Apache, whether or not
there are any SSI directives included within the files. While
this load increase is minor, in a shared server environment it
can become significant.</p>
<p>SSI files also pose the same risks that are associated with
CGI scripts in general. Using the "exec cmd" element,
SSI-enabled files can execute any CGI script or program under
the permissions of the user and group Apache runs as, as
configured in httpd.conf. That should definitely give server
administrators pause.</p>
<p>There are ways to enhance the security of SSI files while
still taking advantage of the benefits they provide.</p>
<p>To isolate the damage a wayward SSI file can cause, a server
administrator can enable <a
href="../suexec.html">suexec</a> as described in the <a
href="#cgi">CGI in General</a> section.</p>
<p>Enabling SSI for files with .html or .htm extensions can be
dangerous. This is especially true in a shared, or high
traffic, server environment. SSI-enabled files should have a
separate extension, such as the conventional .shtml. This helps
keep server load at a minimum and allows for easier management
of risk.</p>
<p>Another solution is to disable the ability to run scripts
and programs from SSI pages. To do this, replace
<code>Includes</code> with <code>IncludesNOEXEC</code> in the
<a href="../mod/core.html#options">Options</a> directive. Note
that users may still use <--#include virtual="..." --> to
execute CGI scripts if these scripts are in directories
designated by a <a
href="../mod/mod_alias.html#scriptalias">ScriptAlias</a>
directive.</p>
<hr />
<h2><a id="nsaliasedcgi" name="nsaliasedcgi">Non Script Aliased
CGI</a></h2>
<p>Allowing users to execute <strong>CGI</strong> scripts in
any directory should only be considered if;</p>
<ol>
<li>You trust your users not to write scripts which will
deliberately or accidentally expose your system to an
attack.</li>
<li>You consider security at your site to be so feeble in
other areas, as to make one more potential hole
irrelevant.</li>
<li>You have no users, and nobody ever visits your
server.</li>
</ol>
<hr />
<h2><a id="saliasedcgi" name="saliasedcgi">Script Aliased
CGI</a></h2>
<p>Limiting <strong>CGI</strong> to special directories gives
the admin control over what goes into those directories. This
is inevitably more secure than non script aliased CGI, but
<strong>only if users with write access to the directories are
trusted</strong> or the admin is willing to test each new CGI
script/program for potential security holes.</p>
<p>Most sites choose this option over the non script aliased
CGI approach.</p>
<hr />
<h2><a id="cgi" name="cgi">CGI in General</a></h2>
<p>Always remember that you must trust the writers of the CGI
script/programs or your ability to spot potential security
holes in CGI, whether they were deliberate or accidental.</p>
<p>All the CGI scripts will run as the same user, so they have
potential to conflict (accidentally or deliberately) with other
scripts <em>e.g.</em> User A hates User B, so he writes a
script to trash User B's CGI database. One program which can be
used to allow scripts to run as different users is <a
href="../suexec.html">suEXEC</a> which is included with Apache
as of 1.2 and is called from special hooks in the Apache server
code. Another popular way of doing this is with <a
href="http://wwwcgi.umr.edu/~cgiwrap/">CGIWrap</a>.</p>
<hr />
<h2><a id="dynamic" name="dynamic">Other sources of dynamic
content</a></h2>
<p>Embedded scripting options which run as part of the server itself, such
as mod_php, mod_perl, mod_tcl, and mod_python, run under the identity of
the server itself (see the <a href="../mod/core.html#user">User</a>
directive), and therefore scripts executed by these engines
potentially can access anything the server user can. Some scripting
engines may provide restrictions, but it is better to be safe and assume
not.</p>
<hr />
<h2><a id="systemsettings" name="systemsettings">Protecting
System Settings</a></h2>
<p>To run a really tight ship, you'll want to stop users from
setting up <code>.htaccess</code> files which can override
security features you've configured. Here's one way to do
it.</p>
<p>In the server configuration file, put</p>
<blockquote>
<code><Directory /><br />
AllowOverride None<br />
</Directory><br />
</code>
</blockquote>
<p>This prevents the use of <code>.htaccess</code> files in all
directories apart from those specifically enabled.</p>
<hr />
<h2><a id="protectserverfiles"
name="protectserverfiles">Protect Server Files by
Default</a></h2>
<p>One aspect of Apache which is occasionally misunderstood is
the feature of default access. That is, unless you take steps
to change it, if the server can find its way to a file through
normal URL mapping rules, it can serve it to clients.</p>
<p>For instance, consider the following example:</p>
<ol>
<li><samp># cd /; ln -s / public_html</samp></li>
<li>Accessing <samp>http://localhost/~root/</samp></li>
</ol>
<p>This would allow clients to walk through the entire
filesystem. To work around this, add the following block to
your server's configuration:</p>
<pre>
<Directory />
Order Deny,Allow
Deny from all
</Directory>
</pre>
<p>This will forbid default access to filesystem locations. Add
appropriate <a
href="../mod/core.html#directory"><samp><Directory></samp></a>
blocks to allow access only in those areas you wish. For
example,</p>
<pre>
<Directory /usr/users/*/public_html>
Order Deny,Allow
Allow from all
</Directory>
<Directory /usr/local/httpd>
Order Deny,Allow
Allow from all
</Directory>
</pre>
<p>Pay particular attention to the interactions of <a
href="../mod/core.html#location"><samp><Location></samp></a>
and <a
href="../mod/core.html#directory"><samp><Directory></samp></a>
directives; for instance, even if <samp><Directory
/></samp> denies access, a <samp><Location /></samp>
directive might overturn it.</p>
<p>Also be wary of playing games with the <a
href="../mod/mod_userdir.html#userdir">UserDir</a> directive;
setting it to something like <samp>"./"</samp> would have the
same effect, for root, as the first example above. If you are
using Apache 1.3 or above, we strongly recommend that you
include the following line in your server configuration
files:</p>
<dl>
<dd><samp>UserDir disabled root</samp></dd>
</dl>
<hr />
<p>Please send any other useful security tips to The Apache
Group by filling out a <a
href="http://bugs.apache.org/">problem report</a>. If you are
confident you have found a security bug in the Apache source
code itself, <a
href="http://httpd.apache.org/bug_report.html">please let us
know</a>.</p>
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