An issue was discovered in the UnlinkedWikibase extension in MediaWiki before 1.39.6, 1.40.x before 1.40.2, and 1.41.x before 1.41.1. XSS can occur through an interface message. Error messages (in the $err var) are not escaped before being passed to Html::rawElement() in the getError() function in the Hooks class.
An issue was discovered in WikibaseLexeme in MediaWiki before 1.39.6, 1.40.x before 1.40.2, and 1.41.x before 1.41.1. Loading Special:MergeLexemes will (attempt to) make an edit that merges the from-id to the to-id, even if the request was not a POST request, and even if it does not contain an edit token.
An issue was discovered in includes/specials/SpecialMovePage.php in MediaWiki before 1.39.7, 1.40.x before 1.40.3, and 1.41.x before 1.41.1. If a user with the necessary rights to move the page opens Special:MovePage for a page with tens of thousands of subpages, then the page will exceed the maximum request time, leading to a denial of service.
An issue was discovered in includes/CommentFormatter/CommentParser.php in MediaWiki before 1.39.7, 1.40.x before 1.40.3, and 1.41.x before 1.41.1. XSS can occur because of mishandling of the 0x1b character, as demonstrated by Special:RecentChanges#%1b0000000.
An issue was discovered in the Phonos extension in MediaWiki before 1.40.2. PhonosButton.js allows i18n-based XSS via the phonos-purge-needed-error message.
An issue was discovered in the GlobalBlocking extension in MediaWiki before 1.40.2. For a Special:GlobalBlock?uselang=x-xss URI, i18n-based XSS can occur via the parentheses message. This affects subtitle links in buildSubtitleLinks.
An issue was discovered in the CampaignEvents 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. The Special:EventDetails page allows XSS via the x-xss language setting for internationalization (i18n).
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.