Appointment Booking Calendar <= 1.3.96 - Missing Authorization to Arbitrary Booking Confirmation via 'cpabc_ipncheck' Parameter

The Appointment Booking Calendar plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 1.3.96. This is due to the plugin exposing an unauthenticated booking processing endpoint (cpabc_appointments_check_IPN_verification) that trusts attacker-supplied payment notifications without verifying their origin, authenticity, or requiring proper authorization checks. This makes it possible for unauthenticated attackers to arbitrarily confirm bookings and insert them into the live calendar via the 'cpabc_ipncheck' parameter, triggering administrative and customer notification emails and disrupting operations.