The (1) Special:Preferences and (2) Special:UserLogin pages in MediaWiki before 1.19.20, 1.22.x before 1.22.12 and 1.23.x before 1.23.5 allows remote authenticated users to conduct cross-site scripting (XSS) attacks or have unspecified other impact via crafted CSS, as demonstrated by modifying MediaWiki:Common.css.
https://lists.wikimedia.org/pipermail/mediawiki-announce/2014-October/000163.html
https://bugzilla.wikimedia.org/show_bug.cgi?id=70672
http://www.securityfocus.com/bid/70238
http://www.debian.org/security/2014/dsa-3046