The Appointment Booking Calendar — Simply Schedule Appointments Booking Plugin plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 1.6.9.29. This is due to the `get_item_permissions_check` method granting access to users with the `ssa_manage_appointments` capability without validating staff ownership of the requested appointment. This makes it possible for authenticated attackers, with custom-level access and above (users granted the ssa_manage_appointments capability, such as Team Members), to view appointment records belonging to other staff members and access sensitive customer personally identifiable information via the appointment ID parameter.