An issue was discovered in WPO-Foundation WebPageTest 3.0. The vulnerability exists due to insufficient filtration of user-supplied data passed to the "webpagetest-master/www/compare-cf.php" URL. An attacker could execute arbitrary HTML and script code in a browser in the context of the vulnerable website.
https://github.com/WPO-Foundation/webpagetest/issues/820
https://github.com/MarkLee131/awesome-web-pocs/blob/main/CVE-2017-6396.md