Apache Server Security - Tips
This article will provide some basic tips from the Apache site since so many people fail to read the Apache manual. Some of the suggestions will be general, others specific to Apache.
Permissions on ServerRoot Directories
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 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:
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
Server Side Includes (SSI) presents 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 allow 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 to execute CGI scripts if these scripts are in directories desginated by a ScriptAlias directive.
Non Script Aliased CGI
Allowing users to execute CGI scripts in any directory should only be considered if you trust your users not to write scripts which will deliberately or accidentally expose your system to an attack. You consider security at your site to be so feeble in other areas, as to make one more potential hole irrelevant. You have no users, and nobody ever visits your server.
Script Aliased 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.
Protecting System 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:
AllowOverride None
This prevents the use of .htaccess files in all directories apart from those specifically enabled.
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:
# cd /; ln -s / public_html
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:
Order Deny,Allow
Deny from all
This will forbid default access to filesystem locations. Add appropriate blocks to allow access only in those areas you wish.
For example,
Order Deny,Allow
Allow from all
Order Deny,Allow
Allow from all
Pay particular attention to the interactions of and directives; for instance, even if denies access, a 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:
UserDir disabled root
Running CGI Programs as Other Users
CGI programs always run as the same user that owns the Apache server process. This is set with the User directive in the config file, and is typically a normal user such as 'www', or the 'nobody' user. In most cases, this is fine, since CGI scripts should run with few privileges to limit any potential malicious damage to the system. However, in some cases it would be nice to be able to run CGI programs as other users. For example, on a virtual host system, with multiple customers, CGIs could run as the customer's user, to let them read and write to the customer's files. On other systems with multiple users, CGIs in home directories could run as that user.
The ability to run CGI programs as other users is referred to as 'running setuid', after the UNIX filesystem ability to run a program as another user. The biggest problem with having a setuid CGI facility on a web server is security. It has to be very careful to ensure that the program running setuid cannot be invoked to do malicious damage to the system. Having setuid programs on a system can be dangerous, particularly if you do not trust all the other users on the system (which would be the case with both the example above). The risk is that other users could run the setuid program manually (from the command line) and give it an environment or command arguments that make it perform undesired activities. The risks of setuid programs are well-known to UNIX system administrators, but a lot of web administrators do not have so much experience of UNIX or setuid security programming.
The suEXEC program included with Apache provides one method of running CGI programs as other users.
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