Mediawiki before 1.28.1 / 1.27.2 contains an unsafe use of temporary directory, where having LocalisationCache directory default to system tmp directory is insecure.
https://security-tracker.debian.org/tracker/CVE-2017-0367
https://phabricator.wikimedia.org/T161453
https://lists.wikimedia.org/pipermail/mediawiki-announce/2017-April/000207.html