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apache/docs/manual/misc/security_tips.html.en
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<?xml version="1.0" encoding="ISO-8859-1"?>
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<title>Security Tips - Apache HTTP Server</title>
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.3</p>
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<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
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<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.3</a> &gt; <a href="./">Miscellaneous Documentation</a></div><div id="page-content"><div id="preamble"><h1>Security Tips</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/misc/security_tips.html" title="English">&nbsp;en&nbsp;</a> |
<a href="../ko/misc/security_tips.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
</div>
<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>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#uptodate">Keep up to Date</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#dos">Denial of Service (DoS) attacks</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#serverroot">Permissions on ServerRoot Directories</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#ssi">Server Side Includes</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cgi">CGI in General</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#nsaliasedcgi">Non Script Aliased CGI</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#saliasedcgi">Script Aliased CGI</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#dynamic">Other sources of dynamic content</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#systemsettings">Protecting System Settings</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#protectserverfiles">Protect Server Files by Default</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#watchyourlogs">Watching Your Logs</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="uptodate" id="uptodate">Keep up to Date</a></h2>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="dos" id="dos">Denial of Service (DoS) attacks</a></h2>
<p>All network servers can be subject to denial of service attacks
that attempt to prevent responses to clients by tying up the
resources of the server. It is not possible to prevent such
attacks entirely, but you can do certain things to mitigate the
problems that they create.</p>
<p>Often the most effective anti-DoS tool will be a firewall or
other operating-system configurations. For example, most
firewalls can be configured to restrict the number of simultaneous
connections from any individual IP address or network, thus
preventing a range of simple attacks. Of course this is no help
against Distributed Denial of Service attacks (DDoS).</p>
<p>There are also certain Apache HTTP Server configuration
settings that can help mitigate problems:</p>
<ul>
<li>The <code class="directive"><a href="../mod/core.html#timeout">TimeOut</a></code> directive
should be lowered on sites that are subject to DoS attacks.
Setting this to as low as a few seconds may be appropriate.
As <code class="directive"><a href="../mod/core.html#timeout">TimeOut</a></code> is currently
used for several different operations, setting it to a low value
introduces problems with long running CGI scripts.</li>
<li>The <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>
directive may be also lowered on sites that are subject to DoS
attacks. Some sites even turn off the keepalives completely via
<code class="directive"><a href="../mod/core.html#keepalive">KeepAlive</a></code>, which has of course
other drawbacks on performance.</li>
<li>The values of various timeout-related directives provided by
other modules should be checked.</li>
<li>The directives
<code class="directive"><a href="../mod/core.html#limitrequestbody">LimitRequestBody</a></code>,
<code class="directive"><a href="../mod/core.html#limitrequestfields">LimitRequestFields</a></code>,
<code class="directive"><a href="../mod/core.html#limitrequestfieldsize">LimitRequestFieldSize</a></code>,
<code class="directive"><a href="../mod/core.html#limitrequestline">LimitRequestLine</a></code>, and
<code class="directive"><a href="../mod/core.html#limitxmlrequestbody">LimitXMLRequestBody</a></code>
should be carefully configured to limit resource consumption
triggered by client input.</li>
<li>On operating systems that support it, make sure that you use
the <code class="directive"><a href="../mod/core.html#acceptfilter">AcceptFilter</a></code> directive
to offload part of the request processing to the operating
system. This is active by default in Apache httpd, but may
require reconfiguration of your kernel.</li>
<li>Tune the <code class="directive"><a href="../mod/mpm_common.html#maxclients">MaxClients</a></code> directive to allow
the server to handle the maximum number of simultaneous
connections without running out of resources. See also the <a href="perf-tuning.html">performance tuning
documentation</a>.</li>
<li>The use of a threaded <a href="../mpm.html">mpm</a> may
allow you to handle more simultaneous connections, thereby
mitigating DoS attacks. Further, the experimental
<code class="module"><a href="../mod/event.html">event</a></code> mpm
uses asynchronous processing to avoid devoting a thread to each
connection. At the current point of time this
is work in progress and not fully implemented. Especially the
<code class="module"><a href="../mod/event.html">event</a></code> mpm is currently incompatible with
<code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> and other input filters.</li>
<li>There are a number of third-party modules available through
<a href="http://modules.apache.org/">http://modules.apache.org/</a>
that can restrict certain client behaviors and thereby mitigate
DoS problems.</li>
</ul>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="serverroot" id="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 <code class="directive"><a href="../mod/mpm_common.html#user">User</a></code> 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>
<div class="example"><p><code>
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
</code></p></div>
<p>It is assumed that /, /usr, and /usr/local are only modifiable by
root. When you install the <code class="program"><a href="../programs/httpd.html">httpd</a></code> executable, you
should ensure that it is similarly protected:</p>
<div class="example"><p><code>
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
</code></p></div>
<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 <code class="program"><a href="../programs/httpd.html">httpd</a></code> 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="ssi" id="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.</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 <code class="directive"><a href="../mod/core.html#options">Options</a></code> directive. Note that users may
still use &lt;--#include virtual="..." --&gt; to execute CGI scripts if
these scripts are in directories desginated by a <code class="directive"><a href="../mod/mod_alias.html#scriptalias">ScriptAlias</a></code> directive.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="cgi" id="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 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="nsaliasedcgi" id="nsaliasedcgi">Non Script Aliased CGI</a></h2>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="saliasedcgi" id="saliasedcgi">Script Aliased CGI</a></h2>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="dynamic" id="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 <code class="directive"><a href="../mod/mpm_common.html#user">User</a></code> 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="systemsettings" id="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>
<div class="example"><p><code>
&lt;Directory /&gt; <br />
AllowOverride None <br />
&lt;/Directory&gt;
</code></p></div>
<p>This prevents the use of <code>.htaccess</code> files in all
directories apart from those specifically enabled.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="protectserverfiles" id="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>
<div class="example"><p><code>
# cd /; ln -s / public_html <br />
Accessing <code>http://localhost/~root/</code>
</code></p></div>
<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>
<div class="example"><p><code>
&lt;Directory /&gt; <br />
Order Deny,Allow <br />
Deny from all <br />
&lt;/Directory&gt;
</code></p></div>
<p>This will forbid default access to filesystem locations. Add
appropriate <code class="directive"><a href="../mod/core.html#directory">Directory</a></code> blocks to
allow access only in those areas you wish. For example,</p>
<div class="example"><p><code>
&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;
</code></p></div>
<p>Pay particular attention to the interactions of <code class="directive"><a href="../mod/core.html#location">Location</a></code> and <code class="directive"><a href="../mod/core.html#directory">Directory</a></code> 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 <code class="directive"><a href="../mod/mod_userdir.html#userdir">UserDir</a></code> 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>
<div class="example"><p><code>
UserDir disabled root
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="watchyourlogs" id="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 happened, 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>
<div class="example"><p><code>
grep -c "/jsp/source.jsp?/jsp/ /jsp/source.jsp??" access_log <br />
grep "client denied" error_log | tail -n 10
</code></p></div>
<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>
<div class="example"><p><code>
[Thu Jul 11 17:18:39 2002] [error] [client foo.bar.com] client denied
by server configuration: /usr/local/apache/htdocs/.htpasswd
</code></p></div>
<p>As you can see, the log files only report what already has happened, so
if the client had been able to access the <code>.htpasswd</code> file you
would have seen something similar to:</p>
<div class="example"><p><code>
foo.bar.com - - [12/Jul/2002:01:59:13 +0200] "GET /.htpasswd HTTP/1.1"
</code></p></div>
<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>
<div class="example"><p><code>
&lt;Files ~ "^\.ht"&gt; <br />
Order allow,deny <br />
Deny from all <br />
&lt;/Files&gt;
</code></p></div>
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/misc/security_tips.html" title="English">&nbsp;en&nbsp;</a> |
<a href="../ko/misc/security_tips.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
</div><div id="footer">
<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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