Session fixation vulnerability in the CentralAuth extension for MediaWiki before 1.18.6, 1.19.x before 1.19.3, and 1.20.x before 1.20.1 allows remote attackers to hijack web sessions via the centralauth_Session cookie.
https://bugzilla.wikimedia.org/show_bug.cgi?id=40962
http://lists.wikimedia.org/pipermail/mediawiki-announce/2012-November/000122.html