Language: PHP
Label: safe
CWE: CWE-79 \u2014 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
The product 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.

Code:
    public function transform($attr, $config, $context)
    {
        if (!isset($attr['href'])) {
            return $attr;
        }

        // XXX Kind of inefficient
        $url = $this->parser->parse($attr['href']);
        $scheme = $url->getSchemeObj($config, $context);

        if ($scheme->browsable && !$url->isLocal($config, $context)) {
            if (isset($attr['rel'])) {
                $rels = explode(' ', $attr['rel']);
                if (!in_array('nofollow', $rels)) {
                    $rels[] = 'nofollow';
                }
                $attr['rel'] = implode(' ', $rels);
            } else {
                $attr['rel'] = 'nofollow';
            }
        }
        return $attr;
    }