Online Scheduling and Appointment Booking System <= 28.4 - Insecure Direct Object Reference to Authenticated (Subscriber+) Privilege Escalation via 'id' and 'wp_user_id' Parameters via Query String / JSON Body

The Online Scheduling and Appointment Booking System – Bookly plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 28.4 via the 'id, wp_user_id' parameter due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with subscriber-level access and above, to overwrite the WordPress account binding of any Bookly staff record, including those owned by administrators, effectively hijacking a higher-privileged staff member's account association and escalating privileges. Exploitation requires the attacking user to be linked to at least one Bookly staff record, which occurs by default whenever an admin enables the bookly_gen_allow_staff_edit_profile option (active on fresh installs) and links a WordPress user to a staff entry.