Paid Membership Plugin, Ecommerce, User Registration Form, Login Form, User Profile & Restrict Content – ProfilePress <= 4.16.11 - Missing Authorization to Authenticated (Subscriber+) Membership Payment Bypass

The Paid Membership Plugin, Ecommerce, User Registration Form, Login Form, User Profile & Restrict Content – ProfilePress plugin for WordPress is vulnerable to unauthorized membership payment bypass in all versions up to, and including, 4.16.11. This is due to a missing ownership verification on the `change_plan_sub_id` parameter in the `process_checkout()` function. This makes it possible for authenticated attackers, with subscriber level access and above, to reference another user's active subscription during checkout to manipulate proration calculations, allowing them to obtain paid lifetime membership plans without payment via the `ppress_process_checkout` AJAX action.