In FusionPBX up to 4.5.7, the file app\fifo_list\fifo_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-2/
https://github.com/fusionpbx/fusionpbx/commit/d3679bbeface57a21f6623cbc193b04a7fc0a885