Cross-site scripting (XSS) vulnerability in MediaWiki before 1.19.24, 1.2x before 1.23.9, and 1.24.x before 1.24.2, when using HHVM, allows remote attackers to inject arbitrary web script or HTML via an invalid parameter in a wddx format request to api.php, which is not properly handled in an error message, related to unsafe calls to wddx_serialize_value.