Citizen is a MediaWiki skin that makes extensions part of the cohesive experience. The page `MediaWiki:Tagline` has its contents used unescaped, so custom HTML (including Javascript) can be injected by someone with the ability to edit the MediaWiki namespace (typically those with the `editinterface` permission, or sysops). This vulnerability is fixed in 2.16.0.
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.