The chunked upload API (ApiUpload) in MediaWiki before 1.23.11, 1.24.x before 1.24.4, and 1.25.x before 1.25.3 does not restrict the uploaded data to the claimed file size, which allows remote authenticated users to cause a denial of service via a chunk that exceeds the file size.
https://phabricator.wikimedia.org/T91203
https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-October/000181.html