ProfilePress < 4.16.12 - Subscriber+ Membership Payment Bypass

Description

The plugin is vulnerable to unauthorized membership payment bypass 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.