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).
In the process of testing the MailPoet WordPress plugin before 5.3.2, a vulnerability was found that allows you to implement Stored XSS on behalf of the editor by embedding malicious script, which entails account takeover backdoor