Cross-site scripting (XSS) vulnerability in the CSS user subpage preview feature in MediaWiki before 1.23.15, 1.26.x before 1.26.4, and 1.27.x before 1.27.1 allows remote attackers to inject arbitrary web script or HTML via the edit box in Special:MyPage/common.css.
https://phabricator.wikimedia.org/T133147
https://lists.wikimedia.org/pipermail/mediawiki-announce/2016-August/000195.html