Kerberos KRB_AP_REQ message - Server name verification required ?

[Vipul Mehta]

Got it. Even if sname is encrypted, it won't make any difference as it can
be modified and re-encrypted as the key is equal.
Signature also won't help for the same reason. So, it is clear that
responsibility lies on AD admin to use unique passwords for accounts.

On Sun, Mar 21, 2021 at 10:29 AM Benjamin Kaduk <kaduk at mit.edu> wrote:

> On Fri, Mar 19, 2021 at 11:47:49PM +0530, Vipul Mehta wrote:
> > Hi,
> >
> > Suppose there are two servers A and B running under different kerberos
> > service principals.
> > If both the service principals have same password and kvno then kerberos
> > long term encryption key will be same for both. Seems to be the case for
> > windows KDC.
> >
> > In such case, a client having service ticket for A tries to authenticate
> > with that ticket with server B, should it work ? It is working fine in
> JDK
> > implementation.
> >
> > https://tools.ietf.org/html/rfc1510#page-21 : in RFC it is not clear
> > whether server should validate server principal in the service ticket
> when
> > KRB_AP_REQ message is received. Looks like just decryption with key is
> > sufficient along with some other validations but i don't find server name
> > validation explicitly mentioned.
>
> I note that RFC 1510 has been obsoleted by RFC 4120 (but
> https://tools.ietf.org/html/rfc4120#section-3.2.3 contains essentially the
> same text that you reference).
>
> My understanding is that the RFC authors assumed that different services
> would have different keys, so the scenario you describe would not occur
> (though, as you know, the situation does occur quite often in Active
> Directory environments).  Since the Ticket sname is in the unencrypted part
> of the ticket, there is no value in validating its contents, as the Ticket
> could be re-encoded with an arbitrary sname value.  It is, in essence, just
> a hint for locating the proper key, in the same that the realm is (and the
> realm is explicitly discussed as serving this role in the referenced text).
>
> -Ben
>


-- 
Regards,
Vipul