The MPL-Publisher WordPress plugin is vulnerable to Stored Cross-Site Scripting due to insufficient input validation and sanitization via several parameters found in the ~/libs/PublisherController.php file which allowed attackers with administrative user access to inject arbitrary web scripts, in versions up to and including 1.30.2. This affects multi-site installations where unfiltered_html is disabled for administrators, and sites where unfiltered_html is disabled.
https://www.wordfence.com/vulnerability-advisories/#CVE-2021-39343
https://plugins.trac.wordpress.org/browser/mpl-publisher/trunk/libs/PublisherController.php#L35
https://github.com/BigTiger2020/word-press/blob/main/MPL-Publisher.md