[18938] in Kerberos_V5_Development
Re: [PATCH] Fix SPNEGO interoperability with servers implementing
daemon@ATHENA.MIT.EDU (David Woodhouse)
Mon Aug 4 19:22:47 2014
Message-ID: <1407194550.28796.20.camel@infradead.org>
From: David Woodhouse <dwmw2@infradead.org>
To: Nico Williams <nico@cryptonector.com>
Date: Tue, 05 Aug 2014 00:22:30 +0100
In-Reply-To: <CAK3OfOhwH4eZogJvhA-ufu8zfb+hZbdQCQ290sHyVtE9Dkk_BQ@mail.gmail.com>
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Cc: "krbdev@mit.edu" <krbdev@mit.edu>, Simo Sorce <simo@redhat.com>
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On Mon, 2014-08-04 at 18:02 -0500, Nico Williams wrote:
> On Mon, Aug 4, 2014 at 5:53 PM, David Woodhouse <dwmw2@infradead.org> wro=
te:
> > It's not clear that there's *any* way a client can safely infer from th=
e
> > NTLMSSP exchange that a server really *is* one of the RFC2478ish Window=
s
> > versions. Any feature we use to detect a 'newer' server can be disabled
> > by the attacker=C2=B9, AFAICT. I can't see a better option than just to=
allow
> > fallback without REQUEST-MIC but *only* to NTLMSSP. And perhaps only if
> > enabled by a krb5.conf option? Either abusing allow_weak_crypto or
> > adding something more appropriate...
>=20
> That seems reasonable to me, yes.
> If you're willing to fallback on a mechanism that can't do integrity
> protection then you can't have downgrade detection.
That was the patch I posted at 14:10:16 GMT, albeit without the
krb5.conf bit. Which, on reflection, seems like the wrong thing to do
anyway; this is SPNEGO code not krb5 so krb5.conf isn't the right place
to tweak its behaviour.
> Of course, if you have credentials for a mechanism like Kerberos that
> can tell you a priori that you stand a good chance of succeeding, and
> it does tell you that, then you shouldn't offer any other mechanisms
> weaker than Kerberos.=20
Hm, I frequently need to fall back to NTLMSSP when I *do* have Kerberos
credentials. SPN mismatches are all too common. And since the Windows
clients will silently fall back to NTLMSSP, the admins neither notice
nor care.
We get a slightly higher hit rate for Kerberos in our environment by
enabling 'rdns-=3Dtrue' but that has its own issues...
--=20
dwmw2
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