Download Monitor <= 5.1.7 - Insecure Direct Object Reference to Unauthenticated Arbitrary Order Completion via 'token' and 'order_id'

The Download Monitor plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 5.1.7 via the executePayment() function due to missing validation on a user controlled key. This makes it possible for unauthenticated attackers to complete arbitrary pending orders by exploiting a mismatch between the PayPal transaction token and the local order, allowing theft of paid digital goods by paying a minimal amount for a low-cost item and using that payment token to finalize a high-value order.