In MediaWiki before 1.35.1, the messages userrights-expiry-current and userrights-expiry-none can contain raw HTML. XSS can happen when a user visits Special:UserRights but does not have rights to change all userrights, and the table on the left side has unchangeable groups in it. (The right column with the changeable groups is not affected and is escaped correctly.)
MediaWiki before 1.35.1 blocks legitimate attempts to hide log entries in some situations. If one sets MediaWiki:Mainpage to Special:MyLanguage/Main Page, visits a log entry on Special:Log, and toggles the "Change visibility of selected log entries" checkbox (or a tags checkbox) next to it, there is a redirection to the main page's action=historysubmit (instead of the desired behavior in which a revision-deletion form appears).
An issue was discovered in MediaWiki before 1.35.1. Missing users (accounts that don't exist) and hidden users (accounts that have been explicitly hidden due to being abusive, or similar) that the viewer cannot see are handled differently, exposing sensitive information about the hidden status to unprivileged viewers. This exists on various code paths.
The PollNY extension for MediaWiki through 1.35 allows XSS via an answer option for a poll question, entered during Special:CreatePoll or Special:UpdatePoll.
The RandomGameUnit extension for MediaWiki through 1.35 was not properly escaping various title-related data. When certain varieties of games were created within MediaWiki, their names or titles could be manipulated to generate stored XSS within the RandomGameUnit extension.
The FileImporter extension in MediaWiki through 1.35.0 was not properly attributing various user actions to a specific user's IP address. Instead, for various actions, it would report the IP address of an internal Wikimedia Foundation server by omitting X-Forwarded-For data. This resulted in an inability to properly audit and attribute various user actions performed via the FileImporter extension.
XSS exists in the MobileFrontend extension for MediaWiki before 1.34.4 because section.line is mishandled during regex section line replacement from PageGateway. Using crafted HTML, an attacker can elicit an XSS attack via jQuery's parseHTML method, which can cause image callbacks to fire even without the element being appended to the DOM.
An issue was discovered in the FileImporter extension for MediaWiki before 1.34.4. An attacker can import a file even when the target page is protected against "page creation" and the attacker should not be able to create it. This occurs because of a mishandled distinction between an upload restriction and a create restriction. An attacker cannot leverage this to overwrite anything, but can leverage this to force a wiki to have a page with a disallowed title.