On Thu, Jul 12, 2018 at 11:26:30AM +0300, Heikki Linnakangas wrote:
> On 12/07/18 07:14, Michael Paquier wrote:
> >On Wed, Jul 11, 2018 at 03:01:03PM +0300, Heikki Linnakangas wrote:
> >>I started digging into this more closely, and ran into a little problem. If
> >>channel binding is not used, the client sends a flag to the server to
> >>indicate if it's because the client doesn't support channel binding, or
> >>because it thought that the server doesn't support it. This is the
> >>gs2-cbind-flag. How should the flag be set, if the server supports channel
> >>binding type A, while client supports only type B? The purpose of the flag
> >>is to detect downgrade attacks, where a man-in-the-middle removes the PLUS
> >>variants from the list of supported mechanisms. If we treat incompatible
> >>channel binding types as "client doesn't support channel binding", then the
> >>downgrade attack becomes possible (the attacker can replace the legit PLUS
> >>variants with bogus channel binding types that the client surely doesn't
> >>support). If we treat it as "server doesn't support channel binding", then
> >>we won't downgrade to the non-channel-binding variant, in the legitimate
> >>case that the client and server both support channel binding, but with
> >>incompatible types.
> >>
> >>What we'd really want, is to include the full list of server's supported
> >>mechanisms as part of the exchange, not just a boolean "y/n" flag. But
> >>that's not what the spec says :-(.
> >
> >Let's not break the spec :) I understand from it that the client is in
> >charge of the choice, so we are rather free to choose the reaction the
> >client should have. In the second phase of the exchange, the client
> >communicates back to the server the channel binding it has decided to
> >choose, it is not up to the server to pick up one if the client thinks
> >that it can use multiple ones.
>
> The case where the client and the server both support the same channel
> binding type, we're OK. The problematic case is when e.g. the server only
> supports tls-unique and the client only supports tls-server-end-point. What
> we would (usually) like to happen, is to fall back to not using channel
> binding. But it's not clear how to make that work, and still protect from
> downgrade attacks. If the client responds "y", meaning "the client supports
> channel binding, but it looks like the server doesn't", then the server will
> reject the authentication, because it did actually support channel binding.
> Just not the same one that the client did. If the client could reply "y",
> and also echo back the server's list of supported channel bindings in the
> same message, that would solve the problem. But the spec doesn't do that.
----------------------------
I know this is an academic question now, but I am not sure this is true.
A man-in-the-middle attacker could say they don't support channel
binding to the real client and real server and pretend to be the real
server. What would work is to hash the secret in with the supported
channel binding list. This is how TLS works --- all previous messages
are combined with the secret into a transmitted hash to prevent a MITM
from changing it.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
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