Paid Member Subscriptions < 3.0.8 - Subscriber+ Cross-User Subscription Hijack via process_checkout

The Paid Membership Subscriptions WordPress plugin before 3.0.8 does not verify that the subscription being modified through its change-subscription checkout belongs to the current user, allowing any authenticated user with Subscriber-level access and above to take over another member's subscription and overwrite its plan, status and expiration.