What is mod_security or modsecurity?
ModSecurity is an open source intrusion detection and prevention engine for web applications. It operates embedded into the web server, acting as a powerful umbrella - shielding applications from attacks. ModSecurity supports both branches of the Apache web server.
Rfx Networks Recommended:
"mod_security is great and I encourage it be used by everyone; it does have the potential to break some web applications but so far iv seen very few issues to say the least. Likewise it is easy to fix any applications that may break with the granular filter rules that can be setup to either deny or allow certain content. Overall mod_security is a needed addition to apache, providing a layer of security yet unseen for apache. I highly encourage you read the reference document on the modsecurity.org site (under documentation) to better understand each directive and the role it plays in protecting your server and sites."

Apache Web Server 1.3x or 2.x

Note: We have confirmed this security addon works with Cpanel based servers.

UPDATE: Sept. 15, 2004:
Changed # Prevent path traversal (..) attacks rules to fix a typo in tutorial.
How to install?
Login to your server through SSH and su to the root user.

2. First your going to start out by grabbing the latest version of mod_security
wget http://www.modsecurity.org/download/mod_security-1.7.4.tar.gz

3. Next we untar the archive and cd into the directory:
tar zxvf mod_security-1.7.4.tar.gz
cd mod_security-1.7.4/

4. Now you need to determine which version of apache you use:
APACHE 1.3.x users
cd apache1/
APACHE 2.x users
cd apache2/

5.Lets Compile the module now:
/usr/local/apache/bin/apxs -cia mod_security.c

6. Ok, now its time to edit the httpd conf file. First we will make a backup just incase something goes wrong:
cp /usr/local/apache/conf/httpd.conf /usr/local/apache/conf/httpd.conf.backup

7. Now that we have backed it all up, we can edit the httpd.conf. Replace pico with nano depending on what you have
pico /usr/local/apache/conf/httpd.conf

8. Lets look for something in the config, do this by holding control and pressing W and you are going to search for
(altho any of the IfModules would work fine)
9. Now add this

# Turn the filtering engine On or Off
SecFilterEngine On

# Change Server: string
SecServerSignature " "

# Make sure that URL encoding is valid
SecFilterCheckURLEncoding On

# This setting should be set to On only if the Web site is
# using the Unicode encoding. Otherwise it may interfere with
# the normal Web site operation.
SecFilterCheckUnicodeEncoding Off

# Only allow bytes from this range
SecFilterForceByteRange 1 255

# The audit engine works independently and
# can be turned On of Off on the per-server or
# on the per-directory basis. "On" will log everything,
# "DynamicOrRelevant" will log dynamic requests or violations,
# and "RelevantOnly" will only log policy violations
SecAuditEngine RelevantOnly

# The name of the audit log file
SecAuditLog /var/log/httpd/audit_log

# Should mod_security inspect POST payloads
SecFilterScanPOST On

# Action to take by default
SecFilterDefaultAction "deny,log,status:500"

# Require HTTP_USER_AGENT and HTTP_HOST in all requests
SecFilterSelective "HTTP_USER_AGENT|HTTP_HOST" "^$"

# Prevent path traversal (..) attacks
SecFilter "../"

# Weaker XSS protection but allows common HTML tags
SecFilter "<[[:space:]]*script"

# Prevent XSS atacks (HTML/Javascript injection)
SecFilter "<(.|n)+>"

# Very crude filters to prevent SQL injection attacks
SecFilter "delete[[:space:]]+from"
SecFilter "insert[[:space:]]+into"
SecFilter "select.+from"

# Protecting from XSS attacks through the PHP session cookie
SecFilterSelective ARG_PHPSESSID "!^[0-9a-z]*$"
SecFilterSelective COOKIE_PHPSESSID "!^[0-9a-z]*$"

10. Save the file Ctrl + X then Y

11. Restart Apache

/etc/rc.d/init.d/httpd stop
/etc/rc.d/init.d/httpd start

You've successfully installed mod_security!