The Appointment Booking Plugin WordPress plugin before 5.7.6 does not escape several booking-form step content settings before rendering them on the public booking form, allowing high-privilege users such as administrators to inject JavaScript that executes in the browser of any visitor who loads the form; on multisite, where administrators do not hold the unfiltered_html capability, this crosses a privilege boundary to unauthenticated visitors.
https://wpscan.com/vulnerability/134c2bda-aa25-4c79-b2b0-171b17e18591/