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Re: channel-bound cookies

From: Dirk Balfanz <balfanz@google.com>
Date: Mon, 12 Mar 2012 12:02:13 -0700
Message-ID: <CADHfa2Cs+eekka_GO0TCjXX3dxxLXsrZr--13oL6PrwPSRVh6Q@mail.gmail.com>
To: Mike Hanson <mhanson@mozilla.com>
Cc: "Richard L. Barnes" <rbarnes@bbn.com>, Enduro USA Tour <endurousatour@yahoo.com>, "public-web-security@w3.org" <public-web-security@w3.org>
What Mike said. :-)

Also, note that somewhat counter-intuitively, channel-bound cookies protect
against many related-domain attacks even if the client cert that they are
bound to has broader scope than a web origin.

Imagine, for a moment, that a user-agent creates a single self-signed
certificate that it uses as a TLS client cert for all connections to all
servers (not a good idea in terms of privacy, but follow me along for this
thought experiment). The servers then set cookies on their respective
top-level domains, but channel-bind them to the user-agent's one-and-only
client cert.

So, let's say that an app app.heroku.com sets a (channel-bound) cookie on
my browser for domain .heroku.com, and that there is an attacker on
attacker.heroku.com. One attack we might be concerned about is that the
attacker simply harvests the .heroku.com cookie from my browser by luring
me to attacker.heroku.com. They won't be able to actually _use_ the cookie,
however, because the cookie is channel-bound to _my_ browser's client cert,
not to the attacker's client cert.

Another attack we might be concerned about is that
attacker.heroku.com_sets_ an .
heroku.com cookie on my user agent in order to make me log into
app.heroku.com as himself. Again, assuming that the only way the attacker
can obtain the cookies is by getting them from app.heroku.com, this means
that the cookies he has at his disposal will be channel-bound to _his_
client cert, not to my client cert - thus when my browser sends them to
app.heroku.com they won't be valid.

The TLS-OBC proposal, of course, assumes more fine-grained "scopes" for the
client certificates. The reason for that, however, is purely to prevent
tracking across unrelated domains. Related-domain attacks are already
mitigated even if we used coarse-grained client certificates and
coarse-grained (i.e., domain) cookies. I, at least, found this a little
counter-intuitive at first, since the other proposed defense it to forbid
coarse-grained cookies altogether and use origin cookies instead.

Dirk.

On Mon, Mar 12, 2012 at 10:03 AM, Mike Hanson <mhanson@mozilla.com> wrote:

> I would also note that Dirk Balfanz (the author of that draft) has written
> a more verbose / user-friendly explanation of Origin Bound Certificates and
> Channel Bound Cookies, here:
> http://www.browserauth.net/channel-bound-cookies
>
> -mh
> --
> Michael Hanson
> Mozilla Labs
>
>
>
> On Mar 12, 2012, at 7:25 AM, Richard L. Barnes wrote:
>
> There's some related work proposed in the IETF TLS working group.  The
> idea there is to bind cookies to TLS client certificates, so as long as the
> private key corresponding to the cert is only on one machine, the cookie
> can only be used on one machine.
> <http://tools.ietf.org/html/draft-balfanz-tls-obc-01>
>
> Of course, you could also just associate cookies with a client IP
> addresses on the server side...
>
>
>
> On Mar 11, 2012, at 11:53 AM, Enduro USA Tour wrote:
>
>
> I'm an independent security researcher and am interested in addressing
>
> Related Domain Cookie Attacks.  See these links for more info on the
>
> problem:
>
>
> http://security.stackexchange.com/q/12412/396  and
>
> http://stackoverflow.com/q/9636857/328397
>
>
> I would like to pitch a few approaches on addressing this vulnerability,
>
> but before I do that, is anyone aware of a solution that binds a cookie to
>
> a host, limiting the ability of the attacker to transfer or replay it on a
>
> different host?  That is essentially the vulnerability that is described in
>
> the links above.
>
>
> Before I pitch my solution, I'd like to see if you agree that the issue is
>
> relevant to this group, and of importance.
>
>
> Thanks for your time!
>
>
> Chris Lamont Mankowski
>
>
>
>
>
>
>
>
Received on Monday, 12 March 2012 19:02:43 GMT

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