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[apache] / docs / manual / misc / security_tips.xml

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<manualpage metafile="security_tips.xml.meta">
  <relativepath href=".." />
  <parentdocument href="./">Miscellaneous Documentation</parentdocument>

  <title>Security Tips</title>
  
  <summary>
    <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>
  </summary>
  
  <section id="uptodate"><title>Keep up to Date</title>

    <p>The Apache HTTP Server has a good record for security and a
    developer community highly concerned about security issues.  But
    it is inevitable that some problems -- small or large -- will be
    discovered in software after it is released.  For this reason, it
    is crucial to keep aware of updates to the software.  If you have
    obtained your version of the HTTP Server directly from Apache, we
    highly recommend you subscribe to the <a
    href="http://httpd.apache.org/lists.html#http-announce">Apache
    HTTP Server Announcements List</a> where you can keep informed of
    new releases and security updates.  Similar services are available
    from most third-party distributors of Apache software.</p>

    <p>Of course, most times that a web server is compromised, it is
    not because of problems in the HTTP Server code.  Rather, it comes
    from problems in add-on code, CGI scripts, or the underlying
    Operating System.  You must therefore stay aware of problems and
    updates with all the software on your system.</p>

  </section>

  <section id="serverroot">
  
    <title>Permissions on ServerRoot Directories</title>
    
    <p>In typical operation, Apache is started by the root user, and it 
    switches to the user defined by the <directive 
    module="mpm_common">User</directive> 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  /usr/local/apache then it is suggested that you create 
    that directory as root, with commands like these:</p>
    
    <example>
      mkdir /usr/local/apache <br />
      cd /usr/local/apache <br />
      mkdir bin conf logs <br />
      chown 0 . bin conf logs <br />
      chgrp 0 . bin conf logs <br />
      chmod 755 . bin conf logs
    </example>
    
    <p>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:</p>
    
    <example>
      cp httpd /usr/local/apache/bin <br />
      chown 0 /usr/local/apache/bin/httpd <br />
      chgrp 0 /usr/local/apache/bin/httpd <br />
      chmod 511 /usr/local/apache/bin/httpd
    </example>
    
    <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>
    
  </section>
  
  <section id="ssi">
  
    <title>Server Side Includes</title>
    
    <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.</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 <directive
    module="core">Options</directive> directive.  Note that users may 
    still use &lt;--#include virtual="..." --&gt; to execute CGI scripts if 
    these scripts are in directories desginated by a <directive
    module="mod_alias">ScriptAlias</directive> directive.</p>
    
  </section>
  
  <section id="cgi">
  
    <title>CGI in General</title>
    
    <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 e.g. 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://cgiwrap.unixtools.org/">CGIWrap</a>.</p>
    
  </section>

  <section id="nsaliasedcgi">
  
    <title>Non Script Aliased CGI</title>
    
    <p>Allowing users to execute CGI scripts in any directory should only be 
    considered if:</p>
    
    <ul>
      <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>
    </ul>
    
  </section>
  
  <section id="saliasedcgi">
  
    <title>Script Aliased CGI</title>
    
    <p>Limiting CGI to special directories gives the admin control over what 
    goes into those directories. This is inevitably more secure than non 
    script aliased CGI, but only if users with write access to the 
    directories are trusted 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>
    
  </section>

   <section id="dynamic">

  <title>Other sources of dynamic content</title>

  <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 <directive 
  module="mpm_common">User</directive> 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>

  </section>
  
  <section id="systemsettings">
  
    <title>Protecting System Settings</title>
    
    <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>
    
    <example>
      &lt;Directory /&gt; <br />
        AllowOverride None <br />
      &lt;/Directory&gt;
    </example>
    
    <p>This prevents the use of <code>.htaccess</code> files in all 
    directories apart from those specifically enabled.</p>
    
  </section>
  
  <section id="protectserverfiles">
  
    <title>Protect Server Files by Default</title>
    
    <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>
    
    <example>
      # cd /; ln -s / public_html <br />
      Accessing <code>http://localhost/~root/</code>
    </example>
    
    <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>
    
    <example>
      &lt;Directory /&gt; <br />
      Order Deny,Allow <br />
      Deny from all <br />
      &lt;/Directory&gt;
    </example>
    
    <p>This will forbid default access to filesystem locations. Add 
    appropriate <directive module="core">Directory</directive> blocks to 
    allow access only in those areas you wish. For example,</p>
    
    <example>
      &lt;Directory /usr/users/*/public_html&gt; <br />
        Order Deny,Allow <br />
        Allow from all <br />
      &lt;/Directory&gt; <br />
      &lt;Directory /usr/local/httpd&gt; <br />
        Order Deny,Allow <br />
        Allow from all <br />
      &lt;/Directory&gt;
    </example>
    
    <p>Pay particular attention to the interactions of <directive
    module="core">Location</directive> and <directive 
    module="core">Directory</directive> directives; for instance, even 
    if <code>&lt;Directory /&gt;</code> denies access, a <code>
    &lt;Location /&gt;</code> directive might overturn it</p>
    
    <p>Also be wary of playing games with the <directive
    module="mod_userdir">UserDir</directive> directive; setting it to 
    something like "./" 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>
    
    <example>
      UserDir disabled root
    </example>
    
  </section>
  
  <section id="watchyourlogs">
  
    <title>Watching Your Logs</title>
    
    <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>
    
    <example>
      grep -c "/jsp/source.jsp?/jsp/ /jsp/source.jsp??" access_log <br />
      grep "client denied" error_log | tail -n 10
    </example>
    
    <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>
    
    <example>
      [Thu Jul 11 17:18:39 2002] [error] [client foo.bar.com] client denied 
      by server configuration: /usr/local/apache/htdocs/.htpasswd
    </example>
    
    <p>As you can see, the log files only report what already has happend, so 
    if the client had been able to access the <code>.htpasswd</code> file you 
    would have seen something similar to:</p>
    
    <example>
      foo.bar.com - - [12/Jul/2002:01:59:13 +0200] "GET /.htpasswd HTTP/1.1"
    </example>
    
    <p>in your <a href="../logs.html#accesslog">Access Log</a>. This means 
    you probably commented out the following in your server configuration 
    file:</p>
    
    <example>
      &lt;Files ~ "^\.ht"&gt; <br />
        Order allow,deny <br />
        Deny from all <br />
      &lt;Files&gt;
    </example>
    
  </section>
  
</manualpage>