Subscriptions for WooCommerce < 2.0.1 - Payment Bypass via Attacker-Supplied PayPal Capture Token

The Subscriptions for WooCommerce WordPress plugin before 2.0.1 does not validate the payment amount or bind the PayPal order token to the order being completed on the WooCommerce order-received flow: it captures a client-supplied token and marks the order paid whenever the capture status is COMPLETED, without comparing the captured amount to the order total. This allows an attacker (unauthenticated where guest checkout is enabled) to substitute an approved, uncaptured PayPal order token and have an expensive order marked paid without paying its price.