The WP Hotel Booking WordPress plugin before 2.3.3 does not ensure that room quantities and the resulting order total are non-negative when placing a booking, and relies on client-controlled cart data, allowing unauthenticated users to create confirmed reservations for free or at an arbitrarily reduced price.
https://wpscan.com/vulnerability/9c1df017-dfa5-43c4-b31e-43c9315edab1/