The Post Index WordPress plugin is vulnerable to Cross-Site Request Forgery via the OptionsPage function found in the ~/php/settings.php file which allows attackers to inject arbitrary web scripts, in versions up to and including 0.7.5.
https://www.wordfence.com/vulnerability-advisories/#CVE-2021-34637
https://plugins.trac.wordpress.org/browser/post-index/trunk/php/settings.php#L86