RE: Browser refresh sends username/password after log out -- URGE NT

Hi K

I've only caught the rear end of this thread, so I do apologise if I'm replicating something that has already been said or I've missed the point of your query.

As part of our past web developments, one of the most simplest methods of preventing resubmitted authentication (assuming logout has been performed!), was the inclusion of a SessionID as part of every POST/GET statement. Upon logout the unique SessionID (on SQL server) is time/date expired (also set to a default time out after x mins of no activity) and also expires regardless of activity after x hours, (forcing a new login). Thus for each POST/GET the SessionID must be unexpired in each instance.

As Ingo has said below, a unique transaction token also works very well and this was another preventative measure of resubmission acceptance that we like.

Kind Regards

Andy Talbot
IT Developer
PC Development Team

-----Original Message-----
From: Ingo Struck [mailto:ingo@ingostruck.de] Sent: Tuesday, August 05, 2003 1:57 PM
To: K Kohli
Cc: webappsec@securityfocus.com
Subject: RE: Browser refresh sends username/password after log out -- URGENT Hi...

See my comments inline.

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> 1) - 5)

I guess anybody in this lists knows that... ;o)

> 6) Can anyone explain this behaviour and how to
> avoid the resubmission of the credentials.
What you see here is a "feature" of most today's browsers. The reason for that is the idea of a history: you should be able to move back and forth in the browsers history and see exactly the "content" that you saw before. For GET reqs this is not a problem, because all necessary parameters are encoded in the URL. For POST, however, it is necessary to memorize the posted parameters to be able to reconstruct the result of a former request. So it is indeed an exact copy of the former request that is issued if you click ok in the confirmation dialog. This dialog exists exactly because it may be unwanted to "replay" that former request.

> 7) How many requests does the browser window store
> in its temporary cache.

I haven't heard of any browser with full-automatic persistent storage of submitted form data.
If you close your browser, all POSTed parameters are lost. However, most modern browsers, provide a facility to store form data permanently. Some of them seem to set that as a default, but all of them that I know ask before they store form data and allow to switch off that thing completely.

Your problem is only an issue for "shared" browsers, i.e. machines where the browser is shared by multiple users (e.g. icafes, highschool, library etc.)

The solution for your problem is two-way: 1) inform your users that it is a *VERY HIGH RISK* to use public browsers for banking anyway and advise them *NOT TO DO THAT* 2) use a technical work around for the problem.

There are two main approaches, where the first is more common and the second is more secure.

Work-Around I) (not recommended, using pop-up windows to break

                the browser's history)

You may work around the problem if you place the login dialog in a pop-up window, since that breaks the browser's history. I know many libraries doing that for their online-accounts. The mechanism is like that:

1. for the login, pop up a new browser window
2. after login finished close that new window

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To be perfectly sure that nothing went wrong, you should break the browser's history during logout too:

c) for logoff, pop up a new browser window d) close *all* other open browser window

An alternative is to run the complete application within a new pop-up window, that is then closed upon logoff. That breaks browser's history too effectively.

Of course, popping up a window is an absolute NO-GOs regarding usability, but it may be necessary as a security work-around.

The main drawback of this solution is, that you need to enable scripting, which by itself is a security problem.

Work-Around II) (recommended, works with transaction tokens)

Add a per-request token for each submitted form. The mechanism is like that:

1. for each new session generate a random number in the lower half of the number range (e.g. 0 up to 1/2 MAX_INT), store that number in the session, it is your CURRENT TRANSACTION
2. for each response that leads to a form using POST, encrypt the CURRENT TRANSACTION and send it as a hidden field within the form that needs input, this is your TRANSACTION TOKEN
3. for each request that contains a TRANSACTION TOKEN, decrypt that token, parse it to a number and compare the result to the CURRENT TRANSACTION. If they are both equal, then you received a correct transaction so you
4. increase the CURRENT TRANSACTION in the session (this is vital!) ii) process the request Else the transaction is illegal and you can ignore the request.

You could create completely random transaction tokens as well (saves the effort
of encrypting the numbers), but then you run into the probability that you replicate some old transaction, if you RNG is poor.  

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Hope that helped and kind regards

Ingo Struck

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