In FusionPBX up to v4.5.7, the file app\conferences_active\conference_interactive.php uses an unsanitized "c" variable coming from the URL, which is reflected in HTML, leading to XSS.
https://resp3ctblog.wordpress.com/2019/10/19/fusionpbx-xss-19/
https://github.com/fusionpbx/fusionpbx/commit/83123e314a2e4c2dd0815446f89bcad97278d98d