On Wed, May 16, 2018 at 09:09:22PM -0400, Bruce Momjian wrote:
> > > FYI, I think the server could also require channel binding for SCRAM. We
> > > already have scram-sha-256 in pg_hba.conf, and I think
> > > scram-sha-256-plus would be reasonable.
> >
> > Noted as well. There is of course the question of v10 libpq versions
> > which don't support channel binding, but if an admin is willing to set
> > up scram-sha-256-plus in pg_hba.conf then he can request his users to
> > update his drivers/libs as well.
>
> Yes, I don't see a way around it. Once you accept that someone in the
> middle can change what you request undetected, then you can't do
> fallback. Imagine a man-in-the-middle with TLS where the
> man-in-the-middle allows the two end-points to negotiate the shared
> secret, but the man-in-the-middle forces a weak cipher. This is what is
> happening with Postgres when the man-in-the-middle forces a weaker
> authentication method.
Technically, you can do automatic fallback if the fallback has
man-in-the-middle and downgrade protection. Technically, because TLS is
already active when we start authentication negotiation, we don't need
downgrade protection as long as we have man-in-the-middle protection.
Unfortunately, only SCRAM with channel binding and sslmode=verify-full
have man-in-the-middle protection. Therefore, downgrading from SCRAM
with channel binding to SCRAM without channel binding, MD5, or
'password' is only safe if sslmode=verify-full is enabled. Technically
MD5, or 'password' has weak or non-existent replay protection, but if we
are requiring TLS, then that doesn't really matter, assuming we have
man-in-the-middle protection.
I don't know if falling back from SCRAM with channel binding to a lesser
authentication methods only if sslmode=verify-full is enabled is really
helpful to anyone since it requires certificate installation.
TLS has similar downgrade issues:
http://www.educatedguesswork.org/2012/07/problems_with_secure_upgrade_t.html
but many of its downgrade options have downgrade protection, and you
don't lose man-in-the-middle protection by downgrading.
Man-in-the-middle protection via certificate checking happens
independent of the TLS version being used, which is not the case for
Postgres authentication downgrade options.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
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