In MediaWiki through 1.37, XSS can occur in Wikibase because an external identifier property can have a URL format that includes a $1 formatter substitution marker, and the javascript: URL scheme (among others) can be used.
{ "severity": "Medium", "cpes": [ "cpe:2.3:a:mediawiki:mediawiki:*:*:*:*:*:*:*:*" ] }