Re: Re: Custom session tokens and XSS

[Mark Reardon <riscorp () mindspring com>]

The reason to log a session out when a bad token is received is so that if someone steals the token (somehow), then the 
real user's next use of their token will cause the attacker to get logged out.

It opens a denial of service capability but closes another hole.

Mark

-------Original Message-------
From: Ingo Struck <ingo () ingostruck de>
Sent: 08/14/03 04:52 AM
To: PortSwigger <mail () portswigger net>
Subject: Re: Custom session tokens and XSS

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Hi...

Well, maybe it's a problem of terminology... ;o)
You talk of a
  "custom session token in a hidden form field".
- From that I would conclude that this token remains constant over the
complete session. If it is like that, i.e. constant, then it makes no 
difference if you submit the session id (SID) with hidden form fields or
any
other mechanism. If the victim gets tricked into logging in on that SID
(be it with a scripted POST request or not), then the attacker has
effectively
exactly the same chance to do whatever he wants with that SID.

- From what you write further on, I conclude that you're talking of a
 "transaction token" that changes on a per-request basis rather than a 
"session token".
If you meant that, then you're of course right:
Even if you trick somebody to login with a valid transaction token,
then that token changes with the next response / request cycle and
the attacker has no clue about that next token.
Implementing that is a good thing against the simple "session foisting",
but still open to mitm attacks.

You say, that if the request does not contain a valid transaction token,
then the user is passed to the login page (which implies that the old
session
will be invalidated). I wouldn't implement it like that but rather pass a
403 
back. Logging off the user on invalid requests opens up the chance for
attackers to log off other users with invalid requests. You should rather  

simply drop illegal requests


Kind regards

Ingo

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ingo () ingostruck de
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Mark Reardon
Reardon Information Security Corporation
(404) 444-0041