The RandomGameUnit extension for MediaWiki through 1.35 was not properly escaping various title-related data. When certain varieties of games were created within MediaWiki, their names or titles could be manipulated to generate stored XSS within the RandomGameUnit extension.
https://phabricator.wikimedia.org/T266400
https://gerrit.wikimedia.org/r/q/I497d2076038f75c9eb77e0e250f2af56f5bd2bfc