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 sanitize parameters when calling the cURL library, which allows remote attackers to read arbitrary files via an @ (at sign) character in unspecified POST array parameters.
https://phabricator.wikimedia.org/T118032
https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-December/000186.html