In Joomla! 1.5.0 through 3.6.5 (fixed in 3.7.0), inadequate filtering of specific HTML attributes leads to XSS vulnerabilities in various components.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2017-16956
https://developer.joomla.org/security-centre/686-20170404-core-xss-vulnerability