도도의 초록누리

XSS에 대해서 소개한다. 스크립트 변조에 관한 것이다.

너무 어렵게만 생각할 필요가 전혀 없다.

1. 코드로 살펴보는 XSS Filiter

XSS 필터에 대해서 간단하게 이해하는 방법이다.

class Security{

/*
 * XSS filter
 *
 * This was built from numerous sources
 * (thanks all, sorry I didn't track to credit you)
 *
 * It was tested against *most* exploits here: http://ha.ckers.org/xss.html
 * WARNING: Some weren't tested!!!
 * Those include the Actionscript and SSI samples, or any newer than Jan 2011
 *
 *
 * TO-DO: compare to SymphonyCMS filter:
 * https://github.com/symphonycms/xssfilter/blob/master/extension.driver.php
 * (Symphony's is probably faster than my hack)
 */

        public function xss_clean($data)
        {
               // Fix &entity\n;
              $data = str_replace(array('&','<','>'), array('&','<','>'), $data);
              $data = preg_replace('/(&#*\w+)[\x00-\x20]+;/u', '$1;', $data);
              $data = preg_replace('/(&#x*[0-9A-F]+);*/iu', '$1;', $data);
              $data = html_entity_decode($data, ENT_COMPAT, 'UTF-8');

              // Remove any attribute starting with "on" or xmlns
              $data = preg_replace('#(<[^>]+?[\x00-\x20"\'])(?:on|xmlns)[^>]*+>#iu', '$1>', $data);

              // Remove javascript: and vbscript: protocols
              $data = preg_replace('#([a-z]*)[\x00-\x20]*=[\x00-\x20]*([`\'"]*)[\x00-\x20]*j[\x00-\x20]*a[\x00-\x20]*v[\x00-\x20]*a[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2nojavascript...', $data);

             $data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*v[\x00-\x20]*b[\x00-\x20]*s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:#iu', '$1=$2novbscript...', $data);

             $data = preg_replace('#([a-z]*)[\x00-\x20]*=([\'"]*)[\x00-\x20]*-moz-binding[\x00-\x20]*:#u', '$1=$2nomozbinding...', $data);

            // Only works in IE: <span style="width: expression(alert('Ping!'));"></span>
            $data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?expression[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
            $data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?behaviour[\x00-\x20]*\([^>]*+>#i', '$1>', $data);
            $data = preg_replace('#(<[^>]+?)style[\x00-\x20]*=[\x00-\x20]*[`\'"]*.*?s[\x00-\x20]*c[\x00-\x20]*r[\x00-\x20]*i[\x00-\x20]*p[\x00-\x20]*t[\x00-\x20]*:*[^>]*+>#iu', '$1>', $data);

            // Remove namespaced elements (we do not need them)
            $data = preg_replace('#</*\w+:\w[^>]*+>#i', '', $data);

            do
            {
                 // Remove really unwanted tags
                 $old_data = $data;
                 $data = preg_replace('#</*(?:applet|b(?:ase|gsound|link)|embed|frame(?:set)?|i(?:frame|layer)|l(?:ayer|ink)|meta|object|s(?:cript|tyle)|title|xml)[^>]*+>#i', '', $data);
            }

            while ($old_data !== $data);

           // we are done...
           return $data;
     }

}

[첨부(Attachment)]

xssFilter.7z

XSS 공격이라는 것은 일반 문자를 UTF8로 변조해서 "<", ">", "applet" 등으로 접근하는 방법도 있을 수 있고. 스크립트 내에 공격하는 기법이다.

2. 참고자료(Reference)

1. XSS Filiter, https://github.com/symphonycms/xssfilter, Accessed by 2018-08-04