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.