The Simply Schedule Appointments WordPress plugin before 1.5.7.7 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/12062d78-7a0d-4dc1-9bd6-6c54aa6bc761