Easy Appointments 4.0 - 4.0.2.1 - Unauthenticated Arbitrary Appointment Modification and Deletion via IDOR

The Easy Appointments WordPress plugin before 4.0.2.2 does not perform an ownership or authorization check on its unauthenticated appointment-reservation endpoint before updating an existing appointment identified by a request-supplied id, allowing unauthenticated attackers to overwrite, and through a follow-on cleanup delete, arbitrary appointments.