ProfilePress <= 4.16.12 - Missing Authorization to Authenticated (Subscriber+) Inactive Membership Plan Subscription

The Paid Membership Plugin, Ecommerce, User Registration Form, Login Form, User Profile & Restrict Content – ProfilePress plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 4.16.12. This is due to the 'process_checkout' function not properly enforcing the plan active status check when a 'change_plan_sub_id' parameter is provided. This makes it possible for authenticated attackers, with Subscriber-level access and above, to subscribe to inactive membership plans by supplying an arbitrary 'change_plan_sub_id' value in the checkout request.