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.
MediaWiki before 1.36.2 allows a denial of service (resource consumption because of lengthy query processing time). ApiQueryBacklinks (action=query&list=backlinks) can cause a full table scan.
MediaWiki before 1.36.2 allows a denial of service (resource consumption because of lengthy query processing time). Visiting Special:Contributions can sometimes result in a long running SQL query because PoolCounter protection is mishandled.