Mediawiki before 1.28.1 / 1.27.2 / 1.23.16 contains a XSS vulnerability in SearchHighlighter::highlightText() with non-default configurations.
https://security-tracker.debian.org/tracker/CVE-2017-0365
https://phabricator.wikimedia.org/T144845
https://lists.wikimedia.org/pipermail/mediawiki-announce/2017-April/000207.html