The WP Booking Calendar WordPress plugin before 10.6.5 does not sanitise and escape some of its settings, which could allow high privilege users such as admin to perform Stored Cross-Site Scripting attacks even when the unfiltered_html capability is disallowed (for example in multisite setup).
https://wpscan.com/vulnerability/a230a552-3fda-4145-810f-58af540107db/