MediaWiki before 1.27.4, 1.28.x before 1.28.3, and 1.29.x before 1.29.2, when a private wiki is configured, provides different error messages for failed login attempts depending on whether the username exists, which allows remote attackers to enumerate account names and conduct brute-force attacks via a series of requests.
https://www.debian.org/security/2017/dsa-4036
https://lists.wikimedia.org/pipermail/mediawiki-announce/2017-November/000216.html