Cross-site scripting (XSS) vulnerability in the Parser::replaceInternalLinks2 method 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 vectors involving replacement of percent encoding in unclosed internal links.
https://phabricator.wikimedia.org/T137264
https://lists.wikimedia.org/pipermail/mediawiki-announce/2016-August/000195.html