The Registrations for the Events Calendar WordPress plugin before 2.13.4 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).
The Registrations for the Events Calendar WordPress plugin before 2.12.4 does not sanitise and escape some parameters when accepting event registrations, which could allow unauthenticated users to perform Cross-Site Scripting attacks.