The Easy Appointments WordPress plugin before 3.12.28 does not verify that the appointment targeted by its customer-data update action belongs to the current user; the action only checks a shared nonce that any authenticated user can obtain from their own appointment's edit form. A subscriber-level user with an appointment of their own can therefore reuse that nonce to overwrite the customer metadata (email, name, phone, description) of another user's appointment. Because the Easy Appointments WordPress plugin before 3.12.28 then treats that metadata as the appointment's contact data, a subsequent administrator status change with customer notifications enabled delivers the victim's appointment notification to the attacker-controlled email address.