Easy!Appointments is a self hosted appointment scheduler. Versions prior to 1.6.0 allow administrators to define a custom "booking disabled"

Easy!Appointments is a self hosted appointment scheduler. Versions prior to 1.6.0 allow administrators to define a custom "booking disabled" message through the booking settings page. That value is stored in the disable_booking_message setting via a rich-text editor and later passed directly to the public booking_message view without escaping or sanitization. An authenticated administrator can store HTML or JavaScript in this field, enable disabled-booking mode, and trigger stored XSS in every unauthenticated visitor who opens the public booking page. Version 1.6.0 fixes the issue.