    public function validate($uri_string, $config, $context) {
        // parse the URI out of the string and then pass it onto
        // the parent object

        $uri_string = $this->parseCDATA($uri_string);
        if (strpos($uri_string, 'url(') !== 0) return false;
        $uri_string = substr($uri_string, 4);
        $new_length = strlen($uri_string) - 1;
        if ($uri_string[$new_length] != ')') return false;
        $uri = trim(substr($uri_string, 0, $new_length));

        if (!empty($uri) && ($uri[0] == "'" || $uri[0] == '"')) {
            $quote = $uri[0];
            $new_length = strlen($uri) - 1;
            if ($uri[$new_length] !== $quote) return false;
            $uri = substr($uri, 1, $new_length - 1);
        }

        $uri = $this->expandCSSEscape($uri);

        $result = parent::validate($uri, $config, $context);

        if ($result === false) return false;

        // extra sanity check; should have been done by URI
        $result = str_replace(array('"', "\\", "\n", "\x0c", "\r"), "", $result);

        // suspicious characters are ()'; we're going to percent encode
        // them for safety.
        $result = str_replace(array('(', ')', "'"), array('%28', '%29', '%27'), $result);

        // there's an extra bug where ampersands lose their escaping on
        // an innerHTML cycle, so a very unlucky query parameter could
        // then change the meaning of the URL.  Unfortunately, there's
        // not much we can do about that...

        return "url(\"$result\")";

    }
The software 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.