MediaWiki before 1.23.11, 1.24.x before 1.24.4, and 1.25.x before 1.25.3 uses the thumbnail ImageMagick command line argument, which allows remote attackers to obtain the installation path by reading the metadata of a PNG thumbnail file.
https://phabricator.wikimedia.org/T108616
https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-October/000181.html