An issue was discovered in the Growth extension in MediaWiki through 1.36.2. On any Wiki with the Mentor Dashboard feature enabled, users can login with a mentor account and trigger an XSS payload (such as alert) via Growthexperiments-mentor-dashboard-mentee-overview-no-js-fallback.
https://phabricator.wikimedia.org/T289063
https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/720088