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

The MailPoet plugin before 3.23.2 for WordPress allows remote attackers to inject arbitrary web script or HTML using extra parameters in the URL (Reflective Server-Side XSS).