The Online Scheduling and Appointment Booking System – Bookly plugin for WordPress is vulnerable to price manipulation via the 'tips' parameter in all versions up to, and including, 27.0. This is due to the plugin trusting a user-supplied input without server-side validation against the configured price. This makes it possible for unauthenticated attackers to submit a negative number to the 'tips' parameter, causing the total price to be reduced to zero.

The Bookly WordPress plugin before 28.3 does not validate client-supplied booking quantity values on the server before computing the appointment total, allowing unauthenticated users to reduce the total to zero and book paid services for free while bypassing the payment step.