Cross-site scripting (XSS) vulnerability in MediaWiki before 1.23.12, 1.24.x before 1.24.5, 1.25.x before 1.25.4, and 1.26.x before 1.26.1, when is configured with a relative URL, allows remote authenticated users to inject arbitrary web script or HTML via wikitext, as demonstrated by a wikilink to a page named "javascript:alert('XSS!')."