On Tue, Oct 3, 2017 at 1:30 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Sep 15, 2017 at 6:29 PM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> I would like to point out that per the RFC, if the client attempts a
>> SSL connection with SCRAM and that the server supports channel
>> binding, then it has to publish the SASL mechanism for channel
>> binding, aka SCRAM-PLUS. If the client tries to force the use of SCRAM
>> even if SCRAM-PLUS is specified, this is seen as a downgrade attack by
>> the server which must reject the connection. So this parameter has
>> meaning only if you try to connect to a PG10 server using a PG11
>> client (assuming that channel binding gets into PG11). If you connect
>> with a PG11 client to a PG11 server with SSL, the server publishes
>> SCRAM-PLUS, the client has to use it, hence this turns out to make
>> cbind=disable and prefer meaningless in the long-term. If the client
>> does not use SSL, then there is no channel binding, and cbind=require
>> loses its value. So cbind's fate is actually linked to sslmode.
>
> That seems problematic. What if the client supports SCRAM but not
> channel binding?
Peter has outlined here that my interpretation of the RFC was wrong on
the client side to begin with:
https://www.postgresql.org/message-id/f74525e4-6c53-c653-6860-a8cc8d7c8ad9@2ndquadrant.com
If a client does not support channel binding (it is not compiled with
SSL or the connection is done without SSL), it should not send 'y' but
'n'. It should be up to the client to decide if it wants to use
channel binding or not. libpq is also going to need some extra logic
to send 'y' when it thinks that the server should have channel binding
support. This can be done by looking at the backend version.
--
Michael
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