MediaWiki before 1.18.5, and 1.19.x before 1.19.2 does not send a restrictive X-Frame-Options HTTP header, which allows remote attackers to conduct clickjacking attacks via an embedded API response in an IFRAME element.
https://phabricator.wikimedia.org/T41180
https://lists.wikimedia.org/pipermail/mediawiki-announce/2012-August/000119.html
https://bugzilla.redhat.com/show_bug.cgi?id=853426
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=686330