Citizen is a MediaWiki skin that makes extensions part of the cohesive experience. All system messages in menu headings using the Menu.mustache template are inserted as raw HTML, allowing anybody who can edit those messages to insert arbitrary HTML into the DOM. This impacts wikis where a group has the `editinterface` but not the `editsitejs` user right. This vulnerability is fixed in 3.3.1.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.