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233 lines
7.4 KiB
HTML
233 lines
7.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<TITLE>Apache HTTP Server: Security Tips</TITLE>
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</HEAD>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<BODY
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#000080"
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ALINK="#FF0000"
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>
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<!--#include virtual="header.html" -->
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<H1 ALIGN="CENTER">Security Tips for Server Configuration</H1>
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<HR>
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<P>Some hints and tips on security issues in setting up a web server. Some of
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the suggestions will be general, others specific to Apache.
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<HR>
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<H2><A NAME="serverroot">Permissions on ServerRoot Directories</A></H2>
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<P>In typical operation, Apache is started by the root
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user, and it switches to the user defined by the <A
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HREF="../mod/core.html#user"><STRONG>User</STRONG></A> directive to serve hits.
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As is the case with any command that root executes, you must take care
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that it is protected from modification by non-root users. Not only
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must the files themselves be writeable only by root, but so must the
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directories, and parents of all directories. For example, if you
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choose to place ServerRoot in <CODE>/usr/local/apache</CODE> then it is
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suggested that you create that directory as root, with commands
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like these:
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<BLOCKQUOTE><PRE>
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mkdir /usr/local/apache
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cd /usr/local/apache
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mkdir bin conf logs
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chown 0 . bin conf logs
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chgrp 0 . bin conf logs
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chmod 755 . bin conf logs
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</PRE></BLOCKQUOTE>
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It is assumed that /, /usr, and /usr/local are only modifiable by root.
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When you install the httpd executable, you should ensure that it is
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similarly protected:
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<BLOCKQUOTE><PRE>
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cp httpd /usr/local/apache/bin
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chown 0 /usr/local/apache/bin/httpd
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chgrp 0 /usr/local/apache/bin/httpd
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chmod 511 /usr/local/apache/bin/httpd
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</PRE></BLOCKQUOTE>
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<P>You can create an htdocs subdirectory which is modifiable by other
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users -- since root never executes any files out of there, and shouldn't
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be creating files in there.
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<P>If you allow non-root users to modify any files that root either
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executes or writes on then you open your system to root compromises.
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For example, someone could replace the httpd binary so that the next
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time you start it, it will execute some arbitrary code. If the logs
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directory is writeable (by a non-root user), someone
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could replace a log file with a symlink to some other system file,
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and then root might overwrite that file with arbitrary data. If the
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log files themselves are writeable (by a non-root user), then someone
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may be able to overwrite the log itself with bogus data.
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<P>
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<HR>
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<H2>Server Side Includes</H2>
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<P>Server side includes (SSI) can be configured so that users can execute
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arbitrary programs on the server. That thought alone should send a shiver
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down the spine of any sys-admin.<P>
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One solution is to disable that part of SSI. To do that you use the
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IncludesNOEXEC option to the <A HREF="../mod/core.html#options">Options</A>
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directive.<P>
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<HR>
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<H2>Non Script Aliased CGI</H2>
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<P>Allowing users to execute <STRONG>CGI</STRONG> scripts in any directory
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should only
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be considered if;
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<OL>
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<LI>You trust your users not to write scripts which will deliberately or
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accidentally expose your system to an attack.
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<LI>You consider security at your site to be so feeble in other areas, as to
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make one more potential hole irrelevant.
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<LI>You have no users, and nobody ever visits your server.
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</OL><P>
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<HR>
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<H2>Script Alias'ed CGI</H2>
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<P>Limiting <STRONG>CGI</STRONG> to special directories gives the admin
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control over
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what goes into those directories. This is inevitably more secure than
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non script aliased CGI, but <STRONG>only if users with write access to the
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directories are trusted</STRONG> or the admin is willing to test each new CGI
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script/program for potential security holes.<P>
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Most sites choose this option over the non script aliased CGI approach.<P>
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<HR>
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<H2>CGI in general</H2>
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<P>Always remember that you must trust the writers of the CGI script/programs
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or your ability to spot potential security holes in CGI, whether they were
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deliberate or accidental.<P>
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All the CGI scripts will run as the same user, so they have potential to
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conflict (accidentally or deliberately) with other scripts <EM>e.g.</EM>
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User A hates User B, so he writes a script to trash User B's CGI
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database. One program which can be used to allow scripts to run
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as different users is <A HREF="../suexec.html">suEXEC</A> which is
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included with Apache as of 1.2 and is called from special hooks in
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the Apache server code. Another popular way of doing this is with
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<A HREF="http://wwwcgi.umr.edu/~cgiwrap/">CGIWrap</A>. <P>
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<HR>
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<H2>Stopping users overriding system wide settings...</H2>
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<P>To run a really tight ship, you'll want to stop users from setting
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up <CODE>.htaccess</CODE> files which can override security features
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you've configured. Here's one way to do it...<P>
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In the server configuration file, put
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<BLOCKQUOTE><CODE>
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<Directory /> <BR>
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AllowOverride None <BR>
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Options None <BR>
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allow from all <BR>
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</Directory> <BR>
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</CODE></BLOCKQUOTE>
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Then setup for specific directories<P>
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This stops all overrides, Includes and accesses in all directories apart
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from those named.<P>
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<HR>
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<H2>
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Protect server files by default
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</H2>
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<P>
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One aspect of Apache which is occasionally misunderstood is the feature
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of default access. That is, unless you take steps to change it, if the
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server can find its way to a file through normal URL mapping rules, it
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can serve it to clients.
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</P>
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<P>
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For instance, consider the following example:
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</P>
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<OL>
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<LI><SAMP># cd /; ln -s / public_html</SAMP>
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</LI>
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<LI>Accessing <SAMP>http://localhost/~root/</SAMP>
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</LI>
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</OL>
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<P>
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This would allow clients to walk through the entire filesystem. To work
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around this, add the following block to your server's configuration:
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</P>
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<PRE>
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<Directory />
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Order deny,allow
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Deny from all
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</Directory>
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</PRE>
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<P>
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This will forbid default access to filesystem locations. Add
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appropriate
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<A
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HREF="../mod/core.html#directory"
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><SAMP><Directory></SAMP></A>
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blocks to allow access only
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in those areas you wish. For example,
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</P>
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<PRE>
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<Directory /usr/users/*/public_html>
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Order deny,allow
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Allow from all
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</Directory>
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<Directory /usr/local/httpd>
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Order deny,allow
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Allow from all
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</Directory>
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</PRE>
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<P>
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Pay particular attention to the interactions of
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<A
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HREF="../mod/core.html#location"
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><SAMP><Location></SAMP></A>
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and
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<A
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HREF="../mod/core.html#directory"
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><SAMP><Directory></SAMP></A>
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directives; for instance, even if <SAMP><Directory /></SAMP>
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denies access, a <SAMP><Location /></SAMP> directive might
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overturn it.
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</P>
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<P>
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Also be wary of playing games with the
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<A
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HREF="../mod/mod_userdir.html#userdir"
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>UserDir</A>
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directive; setting it to something like <SAMP>"./"</SAMP>
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would have the same effect, for root, as the first example above.
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If you are using Apache 1.3 or above, we strongly recommend that you
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include the following line in your server configuration files:
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</P>
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<DL>
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<DD><SAMP>UserDir disabled root</SAMP>
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</DD>
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</DL>
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<HR>
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<P>Please send any other useful security tips to The Apache Group
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by filling out a
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<A HREF="http://www.apache.org/bug_report.html">problem report</A>.
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If you are confident you have found a security bug in the Apache
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source code itself, <A
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HREF="http://www.apache.org/security_report.html">please let us
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know</A>.
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<P>
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<HR>
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<!--#include virtual="footer.html" -->
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</BODY>
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</HTML>
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