The Hydra Booking — Appointment Scheduling & Booking Calendar plugin for WordPress is vulnerable to missing payment verification to unauthenticated payment bypass in all versions up to, and including, 1.1.27. This is due to the plugin accepting client-controlled payment confirmation data in the tfhb_meeting_paypal_payment_confirmation_callback function without server-side verification with PayPal's API. This makes it possible for unauthenticated attackers to bypass payment requirements and confirm bookings as paid without any actual payment transaction occurring.