The WP Travel Engine WordPress plugin before 6.8.2 does not verify that an incoming PayPal payment notification was sent to the sites configured merchant account, nor that the paid amount matches the order total, before marking a booking as paid, allowing unauthenticated attackers to mark bookings as fully paid using a token payment made to an attacker-controlled account.
The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.