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12.2. Server Security
There are many changing aspects to securing a server, but the following points should get you started. All of these need to be checked regularly and by someone other than the normal sys admin. Two sets of eyes find more problems, and an independent and knowledgeable review ensures trust.
12.2.1. Root Password
The root password on your server is the linchpin of your security. Do not let people write it on the wall over their monitors or otherwise expose it.
12.2.2. File Positions and Ownerships
File security is a fundamental aspect of web server security. These are rules to follow for file positions and ownership:
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Files should not be owned by the user(s) that services (http, ftpd, sendmail...) run as — each service should have its own user. Ideally, ownership of files and services should be as finely divided as possible — for instance, the user that the Apache daemon runs as should probably be different from the user that owns its configuration files — this prevents the server from changing its own configuration even if someone does manage to subvert it. Each service should also have its own user, to increase the difficulty of attacks that use multiple servers. (With different users, it is likely that files dropped off using one server can't be accessed from another, for example). Qmail, a secure mail server, for instance, uses no less than six different users for different parts of its service, and its configuration files are owned by yet another user, usually root.
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Services shouldn't share file trees.
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Don't put executable files in the web tree — that is, on or below Apache's DocumentRoot.
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Don't put service control files in the web tree or ftp tree or anywhere else that can be accessed remotely.
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Ideally, run each service on a different machine.
These are rules to follow for file permissions:
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If files are owned by someone else, you have to grant read permissions to the group that includes the relevant service. Similarly, you have to grant execute permissions to compiled binaries. Compiled binaries don't need read permissions, but shell scripts do. Always try to grant the most restrictive permissions possible — so don't grant write permission to the server for configuration files, for instance.
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In the upgrade procedure (see later) make handoff scripts set permissions and ownerships to avoid mistakes.
12.2.3. The Apache Web Site
The Apache web site offers some hints and tips on security issues in setting up a web server. Some of the suggestions will be general; others specific to Apache.
12.2.3.1. 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 nonroot users. Not only must the files themselves be writable 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 that 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 nonroot 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 writable (by a nonroot 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 writable (by a nonroot user), then someone may be able to overwrite the log itself with bogus data.
12.2.3.2. 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.
12.2.3.3. Nonscript-aliased CGI
Allowing users to execute CGI scripts in any directory should only be considered if:
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You trust your users not to write scripts that will deliberately or accidentally expose your system to an attack.
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You consider security at your site to be so feeble in other areas as to make one more potential hole irrelevant.
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You have no users, and nobody ever visits your server.
12.2.3.4. Script-aliased CGI
Limiting CGI to special directories gives the sys admin control over what goes into those directories. This is inevitably more secure than nonscript-aliased CGI, but only if users with write access to the directories are trusted or the sys admin is willing to test each new CGI script/program for potential security holes.
Most sites choose this option over the nonscript-aliased CGI approach.
12.2.3.5. 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 the potential to conflict (accidentally or deliberately) with other scripts. For example, User A hates User B, so she writes a script to trash User B's CGI database. One program that 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.
12.2.3.6. Stopping users overriding system-wide settings...
To run a really tight ship, you'll want to stop users from setting up .htaccess files that can override security features you've configured. Here's one way to do it: in the server configuration file, add the following:
<Directory /> AllowOverride None Options None Allow from all </Directory>
then set up for specific directories. This stops all overrides, includes, and accesses in all directories apart from those named.
12.2.3.7. 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:
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# cd /; ln -s / public_html
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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:
<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 earlier. 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
TIP: 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.
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