Subject: Re: comments on krb-dns-locate draft

From: Jeffrey Altman <[email-masked]>
Date: Tue, 23 Nov 99 19:10:40 EST

> This is the key point, so I will elaborate. Suppose I have a server
> principal name host/www.foo.com in my realm R1.FOO.COM. An attacker adds
> the same server principal name into the realm R2.FOO.COM which has a trust
> relationship with R1.FOO.COM. Furthermore, the attacker knows the key for
> the principal in R2.FOO.COM. When a client in the R1.FOO.COM realm makes a
> TGS request for host/www.foo.com, it first makes a DNS query but the
> attacker changes the DNS TXT reply to R2.FOO.COM. The client requests realm
> R2.FOO.COM in the TGS request and obtains a ticket to the attacker's
> server. Everything seems fine to the client, mutual authentication works,
> etc. But the client is not talking with the legitimate server, rather it is
> talking with the attacker's server that it thinks is the legitimate server.
> So the atttacker has defeated the mutual authentication.

There is no new attack here.  As Ken H. said earlier, if the attacker
has the password for the service there is no way for the client to 
know that it does not have the correct server.  Even when using config
files to specify the REALMs and KDCs it is possible to spoof the DNS
record for the www.foo.com in order to redirect the client to the
attacker's server.

    Jeffrey Altman * Sr.Software Designer * Kermit-95 for Win32 and OS/2
                 The Kermit Project * Columbia University
              612 West 115th St #716 * New York, NY * 10025
  http://www.kermit-project.org/k95.html * [email-masked]

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From: Ken Raeburn <[email-masked]>
Date: 23 Nov 1999 21:27:57 -0500

Okay, I think there's pretty widespread agreement: There's a DNS
spoofing attack here, and false TXT records are one avenue of exploit,
as are false CNAME records.  Jonathan's original message, referring
specifically to TXT records and in the context of this proposal, has
got us focussing on whether DNS spoofing is a *new* attack, and I
don't think that's really relevant.  I suggest we focus on the real
problem, which is this whole class of attack, and which is what
Jonathan's and Michael's draft appears to be aimed at.

Their view of the problem appears to be, how do we prevent the
insecurity of DNS from compromising the security Kerberos is trying to
provide?  Thus, the avoidance of DNS in the cases where spoofing
results in more than a denial of service.

I see it differently: Can we do something about the insecurity of DNS,
so that Kerberos (and other security-critical functions) *can* rely on
it?  Consequently, I don't view it as a Kerberos protocol issue.
There are already proposals in the works for secure DNS.  Can we make
use of none of this work?

Ken