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Re: (ITS#5812) New option to disable SASL host canonicalization
> (1) is a vulnerability that Kerberos can protect against if you use it
> properly. It's called the Zanarotti attack, and the way that you defend
> against it is by obtaining a service ticket for a principal in a local
> keytab after initial authentication and verifying that the service
> ticket matches.
I'm sorry -- this is wrong. I overthought this. The scenario discussed
here is not the Zanarotti attack. The Zanarotti attack involves spoofing
the KDC so that you don't have to know the correct key for a principal;
you instead use a key that you know and fool the system into talking to a
KDC that uses the same key. It's an attack on login or a screen saver to
allow someone to use a bogus password to authenticate.
The protection against the scenario (1) listed here is integral into the
Kerberos protocol. The Kerberos authentication process involves
requesting a TGT from the KDC that's encrypted in your private key and
then decrypting the return with your private key. You cannot spoof
Kerberos initial authentication from a keytab or a correct password by
using DNS to redirect the authentication to a bogus KDC because the bogus
KDC won't know your correct private key and hence won't be able to return
a TGT that you can decrypt. Similarly, you can't successfully redirect a
TGS_REQ to a bogus KDC because that KDC doesn't have the private key and
can't complete the protocol exchange.
This is why kinit doesn't bother to protect against the Zanarotti attack;
there's no need, since kinit doesn't provide increased local privileges
and the Zanarotti attack is an attack only on the local system, not
against a network service.
Russ Allbery (firstname.lastname@example.org) <http://www.eyrie.org/~eagle/>