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<h1 align="CENTER">Security Tips for Server Configuration</h1>
<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
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 so must the
directories, and parents of all directories. For example, if
you choose to place ServerRoot in
create that directory as root, with commands like these:</p>
<blockquote>
<pre>
mkdir bin conf logs
chown 0 . bin conf logs
chgrp 0 . bin conf logs
chmod 755 . bin conf logs
</pre>
</blockquote>
by root. When you install the httpd executable, you should
ensure that it is similarly protected:
<blockquote>
<pre>
</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>Server Side Includes</h2>
<p>Server side includes (SSI) can be configured so that users
can execute arbitrary programs on the server. That thought
alone should send a shiver down the spine of any sys-admin.</p>
<p>One solution is to disable that part of SSI. To do that you
use the IncludesNOEXEC option to the <a
<hr />
<h2>Non Script Aliased CGI</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>Script Alias'ed CGI</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
<p>Most sites choose this option over the non script aliased
CGI approach.</p>
<hr />
<h2>CGI in general</h2>
<p>Always remember that you must trust the writers of the CGI
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
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
<hr />
<h2>Stopping users overriding system wide settings...</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 />
Options None<br />
Allow from all<br />
</Directory><br />
</code>
</blockquote>
Then setup for specific directories
<p>This stops all overrides, Includes and accesses in all
directories apart from those named.</p>
<hr />
<h2>Protect server files by default</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>
</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
blocks to allow access only in those areas you wish. For
example,</p>
<pre>
Order Deny,Allow
Allow from all
</Directory>
Order Deny,Allow
Allow from all
</Directory>
</pre>
<p>Pay particular attention to the interactions of <a
and <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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