disallow requests naming principal as a service

Nicolas Williams Nicolas.Williams at ubsw.com
Wed Mar 27 12:45:01 EST 2002

On Wed, Mar 27, 2002 at 09:29:39AM -0800, John Brezak wrote:
> inline [JBrezak]
> So it's not far-fetched to allow negotiation of traditional Kerberos vs.
> U2U at the KDC exchange. It is generally the case that Kerberos clients
> fetch service tickets before attempting to connect to the services,
> after all. That said, negotiation of U2U at the KDC exchange looks
> really rickety - at the very least it would be nice to have
> authenticated plaintext.
> [JBrezak] Is there a reason that all krb-errors from a TGS-REQ cannot
> contain a "TD-CHECKSUM" element in the e-data that is a checksum of the
> krb-error signed with the TGT session key in the corresponding TGS-REQ?
> It would also need to include the TGS-REQ nonce.

Aside from the fact that some here don't want TypedData used (right? or
do I need to go ingest some caffeine?)... the problem would be that the
TD checksum element would have to contain a checksum for a structure
that contains it itself (and you don't want to leave other TD data
unsigned either). So now we have to specify that to calculate the
checksum you must first marshal a KRB-ERROR with the checksum TD all
nulled out, then replace those nulls with the checksum. Ick.

The authenticated plaintext stuff in extensions is better.

Besides, should we be extending Green? Or clarifying it?

> I'd rather that the app know a priori. If it can't know then I don't
> mind it negotiating U2U vs. traditional AP exchange at the TGS exchange.
> Like so:
>  - get service ticket
>     - if success, connect to service and go from there
>     - if failure, connect to service anyways and see if it will do U2U,
>       if so, then get a U2U service ticket and come back to the service
> [JBrezak]
> 	- if failure, from KDC because it won't do u2u for that
> principal

Ah, yes. A TGS spoofer or MITM can make you waste exchanges. You really
need extensions :)

> [JBrezak] So you wind up with a wasted C->S exchange and a wasted
> TGS-REQ. If the KDC told you at the beginning that it is willing to do
> u2u, that is avoided.

Is it? Not unless the TGS KRB-ERROR response is signed Otherwise it can
always be spoofed.


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