The Leaky Paywall WordPress plugin is vulnerable to Stored Cross-Site Scripting due to insufficient input validation and sanitization via the ~/class.php file which allowed attackers with administrative user access to inject arbitrary web scripts, in versions up to and including 4.16.5. This affects multi-site installations where unfiltered_html is disabled for administrators, and sites where unfiltered_html is disabled.
https://wordfence.com/vulnerability-advisories/#CVE-2021-39357
https://plugins.trac.wordpress.org/changeset/2615195/leaky-paywall/trunk/class.php