source: http://www.securityfocus.com/bid/17533/info

MODxCMS is prone to a directory-traversal vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input. 

An attacker can exploit this vulnerability to retrieve arbitrary files from the vulnerable system in the context of the affected application. Information obtained may aid attackers in further attacks.

http://www.example.com/[modx_dir]/index.php?id=[parameter][reverse_derectory]%00