The MailPoet WordPress plugin before 5.5.2 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/7945f52d-364d-438c-84f2-cf19b4250056/