Vulnerable Code:
Line 53 of the validate.php file
Lines 198 through 202 and 234 in the includes/functions_fees.php file
Proof of Concept:
validate.php?toocheckout=asdf calls the toocheckout_validate() function
toocheckout_validate() takes unsanitized post input from 2 different parameters (total and cart_order_id)
toocheckout_validate() calls callback_process() if the post parameter credit_card_processed is equal to 'Y'
The unsanitized parameters are using in an UPDATE query:
$query = "UPDATE " . $DBPrefix . "users SET balance = balance + " . $payment_amount . $addquery . " WHERE id = " . $custom_id;
This allows an attacker to retrieve data using a time-based blind injection technique or by updating a pre-existing value to the output of an embedded query.