Easy Appointments <= 3.12.27 - Missing Authorization to Authenticated (Author+) Bulk Appointment Manipulation

The Easy Appointments plugin for WordPress is vulnerable to authorization bypass in all versions up to, and including, 3.12.27. This is due to the plugin not properly verifying that a user is authorized to perform an action. This makes it possible for authenticated attackers, with author-level access and above, to cancel all upcoming appointments site-wide by marking every future appointment stored by the plugin as abandoned. The nonce required to authenticate the cancellation request is printed on the Appointments admin page, which is itself gated only by the edit_posts capability that Authors possess, making the nonce readily accessible to low-privileged users.