The Multiplayer Games WordPress plugin is vulnerable to Reflected Cross-Site Scripting due to the use of $_SERVER['PHP_SELF'] in the ~/multiplayergames.php file which allows attackers to inject arbitrary web scripts, in versions up to and including 3.7.
https://www.wordfence.com/vulnerability-advisories/#CVE-2021-34644
https://plugins.trac.wordpress.org/browser/multiplayer-plugin/trunk/multiplayergames.php#L662