The (1) Special:Preferences and (2) Special:UserLogin pages in MediaWiki before 1.19.20, 1.22.x before 1.22.12 and 1.23.x before 1.23.5 allows remote authenticated users to conduct cross-site scripting (XSS) attacks or have unspecified other impact via crafted CSS, as demonstrated by modifying MediaWiki:Common.css.