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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HEAD>
<TITLE>Apache HTTP Server: Security Tips</TITLE>
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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.
<HR>
<H2><A 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 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:
<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>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>
One solution is to disable that part of SSI. To do that you use the
IncludesNOEXEC option to the <A HREF="../mod/core.html#options">Options</A>
directive.<P>
<HR>
<H2>Non Script Aliased CGI</H2>
<P>Allowing users to execute <STRONG>CGI</STRONG> scripts in any directory
should only
be considered if;
<OL>
<LI>You trust your users not to write scripts which will deliberately or
accidentally expose your system to an attack.
<LI>You consider security at your site to be so feeble in other areas, as to
make one more potential hole irrelevant.
<LI>You have no users, and nobody ever visits your server.
</OL><P>
<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
script/program for potential security holes.<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 script/programs
or your ability to spot potential security holes in CGI, whether they were
deliberate or accidental.<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>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>
In the server configuration file, put
<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>
<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://www.apache.org/bug_report.html">problem report</A>.
If you are confident you have found a security bug in the Apache
source code itself, <A
HREF="http://www.apache.org/security_report.html">please let us
know</A>.
<P>
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