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3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen    <h1 align="CENTER">Security Tips for Server Configuration</h1>
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen    <ul>
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen      <li><a href="#serverroot">Permissions on ServerRoot
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen      Directories</a></li>
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen      <li><a href="#ssi">Server Side Includes</a></li>
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen      <li><a href="#cgi">CGI in General</a></li>
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen      <li><a href="#nsaliasedcgi">Non Script Aliased CGI</a></li>
3be1d7e0c5bf60658d34eb6311d4e77c6803578cTom Gundersen
      <li><a href="#saliasedcgi">Script Aliased CGI</a></li>

     <li><a href="#systemsettings">Protecting System
      Settings</a></li>

      <li><a href="#protectserverfiles">Protect Server Files by
      Default</a></li>

      <li><a href="#watchyourlogs">Watching Your Logs</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/mpm_common.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 so must 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="/docs/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 &lt;--#include virtual="..." --&gt; to execute
    CGI scripts if these scripts are in directories desginated by a <a
    href="/mod/mod_alias.html#ScriptAlias">ScriptAlias</a>
    directive.</p>

    <hr />

    <h2><a id="cgi" name="cgi">CGI in General</a></h2>

    <p>First of all, you always have to remember that you must trust
    the writers of the CGI scripts/programs or your ability to spot
    potential security holes in CGI, whether they were deliberate or
    accidental.  CGI scripts can run essentially arbitrary commands
    on your system with the permissions of the web server user and can
    therefore be extremely dangerous if they are not carefully
    checked.</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>

    <p></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>

    <p></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>&lt;Directory /&gt;<br />
       AllowOverride None<br />
       &lt;/Directory&gt;<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>
 &lt;Directory /&gt;
     Order Deny,Allow
     Deny from all
 &lt;/Directory&gt;
</pre>

    <p>This will forbid default access to filesystem locations. Add
    appropriate <a href="/mod/core.html#directory"><samp>
    &lt;Directory&gt;</samp></a> blocks to allow access only in
    those areas you wish. For example,</p>
<pre>
 &lt;Directory /usr/users/*/public_html&gt;
     Order Deny,Allow
     Allow from all
 &lt;/Directory&gt;
 &lt;Directory /usr/local/httpd&gt;
     Order Deny,Allow
     Allow from all
 &lt;/Directory&gt;
</pre>

    <p>Pay particular attention to the interactions of <a
    href="/mod/core.html#location"><samp>
    &lt;Location&gt;</samp></a> and <a
    href="/mod/core.html#directory"><samp>
    &lt;Directory&gt;</samp></a> directives; for instance, even if
    <samp>&lt;Directory /&gt;</samp> denies access, a <samp>
    &lt;Location /&gt;</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&nbsp;disabled&nbsp;root</samp></dd>
    </dl>

    <p></p>
    <hr />

    <h2><a id="watchyourlogs" name="watchyourlogs">
    Watching Your Logs</a></h2>

    <p>To keep up-to-date with what is actually going on against your
    server you have to check the <a href="/logs.html">Log Files</a>.
    Even though the log files only reports what has already happend,
    they will give you some understanding of what attacks is thrown
    against the server and allows you to check if the necessary level
    of security is present.</p>

    <p>A couple of examples:</p>
   <ol>
   <li><samp>grep -c "/jsp/source.jsp?/jsp/ /jsp/source.jsp??"
   access_log</samp></li> <li><samp>grep "client denied" error_log |
   tail -n 10 </samp></li>
    </ol>

   <p>The first example will list the number of attacks trying to
   exploit the <a
   href="http://online.securityfocus.com/bid/4876/info/">Apache Tomcat
   Source.JSP Malformed Request Information Disclosure
   Vulnerability</a>, the second example will list the ten last denied
   clients, for example:</p>

  <dl>
  <dd><samp>[Thu Jul 11 17:18:39 2002] [error] [client foo.bar.com]
  client denied by server configuration:
  /usr/local/apache/htdocs/.htpasswd</samp></dd>
  </dl>

    <p>As you can see, the log files only report what already has
    happend, so if the client had been able to access the
    <samp>.htpasswd</samp> file you would have seen something similar
    to <samp>foo.bar.com - - [12/Jul/2002:01:59:13 +0200] "GET
    /.htpasswd HTTP/1.1"</samp> in your <a
    href="/logs.html#accesslog">Access Log</a>. This means you
    probably commented out the following in your server configuration
    file:</p>

   <pre>
   &lt;Files ~ "^\.ht"&gt;
    Order allow,deny
    Deny from all
   &lt;/Files&gt;
   </pre>

    <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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