    public function validate($string, $config, $context) {

        // regular pre-processing
        $string = $this->parseCDATA($string);
        if ($string === '') return false;

        // munge rgb() decl if necessary
        $string = $this->mungeRgb($string);

        // assumes URI doesn't have spaces in it
        $bits = explode(' ', $string); // bits to process

        $caught = array();
        $caught['color']    = false;
        $caught['image']    = false;
        $caught['repeat']   = false;
        $caught['attachment'] = false;
        $caught['position'] = false;

        $i = 0; // number of catches
        $none = false;

        foreach ($bits as $bit) {
            if ($bit === '') continue;
            foreach ($caught as $key => $status) {
                if ($key != 'position') {
                    if ($status !== false) continue;
                    $r = $this->info['background-' . $key]->validate($bit, $config, $context);
                } else {
                    $r = $bit;
                }
                if ($r === false) continue;
                if ($key == 'position') {
                    if ($caught[$key] === false) $caught[$key] = '';
                    $caught[$key] .= $r . ' ';
                } else {
                    $caught[$key] = $r;
                }
                $i++;
                break;
            }
        }

        if (!$i) return false;
        if ($caught['position'] !== false) {
            $caught['position'] = $this->info['background-position']->
                validate($caught['position'], $config, $context);
        }

        $ret = array();
        foreach ($caught as $value) {
            if ($value === false) continue;
            $ret[] = $value;
        }

        if (empty($ret)) return false;
        return implode(' ', $ret);

    }
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.