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Re: HTTP Digest Integrity: Another look, in light of recent attacks
From: Dan Kaminsky <dan () doxpara com>
Date: Thu, 7 Jan 2010 12:15:10 +0100

On Thu, Jan 7, 2010 at 3:17 AM, Timothy D. Morgan <tmorgan () vsecurity com>wrote:

Hi Dan,

Thanks for taking the time to read it.

I haven't been wildly impressed by Digest as implemented in

Heh, no doubt.  When you look into it, it's quite sad how incomplete
and inconsistent many implementations are.

The problem is that browser developers have basically been locked out of the
UI experience for some time, so there hasn't been much push to improve.
Passwords in forms is the gold standard.  Microsoft tried to address some of
this with Cardspace...didn't work out all that well.

but it's a legitimate point that Digest has of at least *some* of the URI
embedded into it, so the TLS reneg attack can be somewhat mitigated by
leveraging that.  Empirically though, this is going to be a big pain in
butt, not least of which is the dramatic change to the user experience.

Yes, there are some serious limitations to the user interface with
Digest auth.  I have some ideas for that, which may be cooked up in a
future paper.  Stay tuned.

Happy to review when it's available.

The level of mitigation right now against TLS renegotiation attacks
may be contestable.  In fact I'd love to hear of any exploits which
workaround digest auth restrictions.  Mostly though, I just wanted
to throw it out there as food for thought and to give people a
possible option if their hair was still on fire after hearing of this
latest bug.

What's neat about your stuff is that the GET request becomes somewhat
inviolate.  What sucks is that the attacker can still play games shifting to
unauthenticated content, making a Range-Request, changing the Host to
something else behind the same load balancer, etc.

Ultimately, we need to fix TLS, and that's so amazingly hard.

I've seen people try to do similar challenge-response protocols in
JavaScript, but I've never taken the time to think carefully about how
much benefit that provides.  Hashing request bodies might be useful
against TLS renegotiation, but I'm not sure how verification of
responses would work.  I guess with lots of AJAX and a lack of
checking on the first response.  Seems like a lot of work though.

Well, there hasn't been a benefit before, because the right answer was
always "just use TLS".  Now TLS has its issues.

You're absolutely correct that it's an impossible pain in the butt.  WS-*
kinda does it though already.


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