The sanitizeLocalUrl function in TYPO3 6.x before 6.2.15, 7.x before 7.4.0, 4.5.40, and earlier allows remote authenticated users to bypass the XSS filter and conduct cross-site scripting (XSS) attacks via a base64 encoded data URI, as demonstrated by the (1) returnUrl parameter to show_rechis.php and the (2) redirect_url parameter to index.php.