Security Tips for Server Configuration

Some hints and tips on security issues in setting up a web server. Some of the suggestions will be general, others specific to Apache.

Permissions on Log File Directories

When Apache starts, it opens the log files as the user who started the server before switching to the user defined in the User directive. Anyone who has write permission for the directory where any log files are being written to can append pseudo-arbitrary data to any file on the system which is writable by the user who starts Apache. Since the server is normally started by root, you should NOT give anyone write permission to the directory where logs are stored unless you want them to have root access.

Server Side Includes

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.

One solution is to disable that part of SSI. To do that you use the IncludesNOEXEC option to the Options directive.

Non Script Aliased CGI

Allowing users to execute CGI scripts in any directory should only be considered if;

1. You trust your users not to write scripts which will deliberately or accidentally expose your system to an attack.
2. You consider security at your site to be so feeble in other areas, as to make one more potential hole irrelevant.
3. You have no users, and nobody ever visits your server.

Script Alias'ed CGI

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.

Most sites choose this option over the non script aliased CGI approach.

CGI in general

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.

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 suEXEC 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 CGIWrap.

Stopping users overriding system wide settings...

To run a really tight ship, you'll want to stop users from setting up .htaccess files which can override security features you've configured. Here's one way to do it...

In the server configuration file, put

<Directory />
AllowOverride None
Options None
allow from all
</Directory>


This stops all overrides, Includes and accesses in all directories apart from those named.

Protect server files by default

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.

For instance, consider the following example:

1. # cd /; ln -s / public_html
2. Accessing http://localhost/~root/

This would allow clients to walk through the entire filesystem. To work around this, add the following block to your server's configuration:

This will forbid default access to filesystem locations. Add appropriate <Directory> blocks to allow access only in those areas you wish. For example,

Pay particular attention to the interactions of <Location> and <Directory> directives; for instance, even if <Directory /> denies access, a <Location /> directive might overturn it.

Also be wary of playing games with the UserDir directive; setting it to something like "./" would have the same effect, for root, as the first example above.

Please send any other useful security tips to The Apache Group by filling out a problem report, or by sending mail to apache-bugs@mail.apache.org