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Re: HTTP Digest Integrity: Another look, in light of recent attacks
From: "Timothy D\. Morgan" <tmorgan () vsecurity com>
Date: Wed, 6 Jan 2010 18:17:23 -0800


Hi Dan,

Thanks for taking the time to read it.

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

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

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 the
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.

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.


Ultimately, far and away the most common forms of auth are cookie based,
with hidden variables being a close second.  In both of these the password
is accessible to the DOM.  So the raw material is there to add an integrity
layer to at least sensitive HTTPS transactions (everything is worthless for
HTTP).  But an advantage of your approach is that it applies generically to
all browser/site communication, including Javascript containers like <script
src> and <link rel=stylesheet>.  There's no way to register a hook that gets
triggered whenever a site hits a particular URI within a domain, to add the
validator, in JS.  It just happens in Digest.


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.


Regards,
tim

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