Apache mod_security: Rule bypass

Background

mod_security is an Apache module designed for enhancing the security of the Apache web server.

Description

Stefan Esser discovered that mod_security processes NULL characters as terminators in POST requests using the application/x-www-form-urlencoded encoding type, while other parsers used in web applications do not.

Impact

A remote attacker could send a specially crafted POST request, possibly bypassing the module ruleset and leading to the execution of arbitrary code in the scope of the web server with the rights of the user running the web server.

Workaround

There is no known workaround at this time.

Resolution

All mod_security users should upgrade to the latest version:

 # emerge --sync
 # emerge --ask --oneshot --verbose ">=www-apache/mod_security-2.1.1"