source: https://www.securityfocus.com/bid/14702/info

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

An unauthorized user can retrieve arbitrary files by supplying directory traversal strings '../' to the vulnerable parameter.

Exploitation of this vulnerability could lead to a loss of confidentiality. Information obtained may aid in further attacks against the underlying system; other attacks are also possible. 

http://www.example.com/index.php?mod=read&id=../forum/users/[user].php%00