MediaWiki before 1.23.12, 1.24.x before 1.24.5, 1.25.x before 1.25.4, and 1.26.x before 1.26.1 do not properly normalize IP addresses containing zero-padded octets, which might allow remote attackers to bypass intended access restrictions by using an IP address that was not supposed to have been allowed.
https://phabricator.wikimedia.org/T97897
https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-December/000186.html