ApiParse in MediaWiki before 1.23.15, 1.26.x before 1.26.4, and 1.27.x before 1.27.1 allows remote attackers to bypass intended per-title read restrictions via a parse action to api.php.
https://phabricator.wikimedia.org/T115333
https://lists.wikimedia.org/pipermail/mediawiki-announce/2016-August/000195.html