source: http://www.securityfocus.com/bid/25116/info

phpCoupon is prone to a remote payment-bypass vulnerability because the application fails to properly secure PayPal payment transactions.

Successfully exploiting this issue allows remote attackers to perform payment transactions in the application without actually paying money. This allows them to obtain services for free. 

The following URI demonstrates this issue:

http://www.example.com/path/user.php?REQ=auth&billing=141&status=success&custom=upgrade5

The '141' and the 'upgrade5' values may vary from installation to installation. 