An issue was discovered in the CheckUser extension in MediaWiki before 1.35.14, 1.36.x through 1.39.x before 1.39.6, and 1.40.x before 1.40.2. XSS can occur via message definitions. e.g., in SpecialCheckUserLog.Referenceshttps://phabricator.wikimedia.org/T347708https://gerrit.wikimedia.org/r/c/mediawiki/extensions/CheckUser/+/989179
