Mediawiki before 1.28.1 / 1.27.2 / 1.23.16 has a flaw where Special:UserLogin?returnto=interwiki:foo will redirect to external sites.
https://security-tracker.debian.org/tracker/CVE-2017-0363
https://phabricator.wikimedia.org/T109140
https://lists.wikimedia.org/pipermail/mediawiki-announce/2017-April/000207.html