Re: Negotiating the SCRAM channel binding type

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 guess we should err on the side of caution, and fail the authentication in
>> that case. That's unfortunate, but it's better than not negotiating at all.
>> At least with the negotiation, the authentication will work if there is any
>> mutually supported channel binding type.
>
> I think that in this case the client should throw an error
> unconditionally if it wants to use channel A but server supports only B.
> It is always possible for the client to adjust the channel binding type
> it wants to use by using the connection parameter scram_channel_binding,
> or to recompile. If there is incompatibility between channel binding
> types, it would actually mean that the client and the server are not
> compiled with the same SSL implementation, would that actually work? Say
> a server has only tls-unique with gnu's library and the client uses JDBC
> which only has tls-server-end-point..

We would like to fall back to not using channel binding at all in that
scenario, assuming the configuration doesn't require channel binding.
But yeah, rejecting the connection seems to be the best we can do, given
what the protocol is.

> So, to keep things simple, it seems to me that we should just make the
> server send the list, and then check at client-level if the list sent by
> server includes what the client wants to use, complaining if the option
> is not present.

Yep.

It seems that all implementations can support tls-server-end-point,
after all, so I'm not too worried about this anymore. The spec says that
it's the default, but I don't actually see any advantage to using it
over tls-server-end-point. I think the main reason for tls-unique to
exist is that it doesn't require the server to have a TLS certificate,
but PostgreSQL requires that anyway.

Actually, I wonder if we should just rip out the tls-unique support,
after all? We can add it back at a later version, if there is a need for
it, but I don't think we will. With security-related code, simpler is
better.

- Heikki