MediaWiki before 1.23.15, 1.26.x before 1.26.4, and 1.27.x before 1.27.1, when $wgBlockDisablesLogin is true, might allow remote attackers to obtain sensitive information by leveraging failure to terminate sessions when a user account is blocked.
https://phabricator.wikimedia.org/T129738
https://lists.wikimedia.org/pipermail/mediawiki-announce/2016-August/000195.html