MediaWiki 1.21.x, 1.22.x before 1.22.14, and 1.23.x before 1.23.7, when $wgContentHandlerUseDB is enabled, allows remote attackers to conduct cross-site scripting (XSS) attacks by setting the content model for a revision to JS.
https://phabricator.wikimedia.org/T72901
https://lists.wikimedia.org/pipermail/mediawiki-announce/2014-November/000170.html