Mediawiki before 1.28.1 / 1.27.2 / 1.23.16 contains a flaw, allowing a sysops to undelete pages, although the page is protected against it.
https://security-tracker.debian.org/tracker/CVE-2017-0369
https://phabricator.wikimedia.org/T108138
https://lists.wikimedia.org/pipermail/mediawiki-announce/2017-April/000207.html