The ProfilePress plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 4.16.11. This is due to missing ownership validation on the change_plan_sub_id parameter in the process_checkout() function. The ppress_process_checkout AJAX handler accepts a user-controlled subscription ID intended for plan upgrades, loads the subscription record, and cancels/expires it without verifying the subscription belongs to the requesting user. This makes it possible for authenticated attackers, with Subscriber-level access and above, to cancel and expire any other user's active subscription via the change_plan_sub_id parameter during checkout, causing immediate loss of paid access for victims.