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Some hints and tips on security issues in setting up a web server. Some of the suggestions will be general, others specific to Apache.
In typical operation, Apache is started by the root user,
and it switches to the user defined by the User
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 also 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:
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
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:
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
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.
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.
Server Side Includes (SSI) present a server administrator with several potential security risks.
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.
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. That should definitely give server administrators pause.
There are ways to enhance the security of SSI files while still taking advantage of the benefits they provide.
To isolate the damage a wayward SSI file can cause, a server administrator can enable suexec as described in the CGI in General section.
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.
Another solution is to disable the ability to run scripts
and programs from SSI pages. To do this, replace
Includes with IncludesNOEXEC in the
Options directive. Note
that users may still use <--#include virtual="..." --> to
execute CGI scripts if these scripts are in directories
designated by a ScriptAlias
directive.
Allowing users to execute CGI scripts in any directory should only be considered if;
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.
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.
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 User 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.
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
</Directory>
This prevents the use of .htaccess files in all
directories apart from those specifically enabled.
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:
This would allow clients to walk through the entire filesystem. To work around this, add the following block to your server's configuration:
<Directory />
Order Deny,Allow
Deny from all
</Directory>
This will forbid default access to filesystem locations. Add appropriate <Directory> blocks to allow access only in those areas you wish. For example,
<Directory /usr/users/*/public_html>
Order Deny,Allow
Allow from all
</Directory>
<Directory /usr/local/httpd>
Order Deny,Allow
Allow from all
</Directory>
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. If you are using Apache 1.3 or above, we strongly recommend that you include the following line in your server configuration files:
Please send any other useful security tips to The Apache Group by filling out a problem report. If you are confident you have found a security bug in the Apache source code itself, please let us know.
