Subscriptions & Memberships for PayPal <= 1.1.7 - Unauthenticated Fake Payment Creation

The Subscriptions & Memberships for PayPal plugin for WordPress is vulnerable to fake payment creation in all versions up to, and including, 1.1.7. This is due to the plugin not properly verifying the authenticity of an IPN request. This makes it possible for unauthenticated attackers to create fake payment entries that have not actually occurred.