Phorum 3.3.2a allows remote attackers to execute arbitrary commands via an HTTP request to (1) plugin.php, (2) admin.php, or (3) del.php that modifies the PHORUM[settings_dir] variable to point to a directory that contains a PHP file with the commands.
http://www.iss.net/security_center/static/9107.php
http://archives.neohapsis.com/archives/bugtraq/2002-05/0153.html
http://archives.neohapsis.com/archives/bugtraq/2002-05/0147.html