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 allows remote authenticated users to cause a denial of service (disk consumption) via a file upload using one byte chunks.
https://phabricator.wikimedia.org/T91205
https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-October/000181.html