MIT Kerberos for Windows failing with Windows 10 update 1803?

[Ruurd Beerstra]

Hi,

Thanks for the quick reply.
The first screenshot showed a popup from  MIT Kerberos (Title "Kerberos 
Five") saying:
Matching credential not found. Kerberos error -1765328243.
krb5_get_renewed_creds() failed.

The 2nd was the ticket manager showing just my TGT and nothing else.

I probably should have mentioned I tried setting the ccache type to 
"FILE", and that didn't work either.
I tried "API" just now (didn't know that one), the ticket manager shows 
"API: Initial default ccache" in its "Credential cache" column.
But no joy: IVT and Putty both fail to use GSSAPI, and IVT also fails on 
Kerberized telnet.
Tried reboot of my workstation: No change.

Both IVT and Putty cause the ticket-manager to pop up to obtain new 
credentials, because they do not see my TGT (and so allow me to obtain 
one).
The AllowTGTSessionKey is set in the registry, value 1.

A quick search on Credential Guard says: The Windows Defender Credential 
Guard prevents these attacks by protecting NTLM password hashes, 
Kerberos Ticket Granting Tickets, and credentials stored by applications 
as domain credentials.

And that sounds like exactly my problem. I can't find any way to turn 
that protection  off.
I don't understand why a FILE: or API: based ccache would be affected by 
this Credential Guard, but I can't get any GSSAPI or Kerberized telnet 
session to work anymore.
And up until recently (I have automated regression tests fror IVT) that 
was not a problem.

Bedtime here now, maybe more tomorrow,

     Thanks again,
     Ruurd


Op 17-6-2018 om 22:35 schreef Greg Hudson:
> On 06/17/2018 02:02 PM, Ruurd Beerstra wrote:
>> The symptoms are that I can obtain a TGT from my KDC (which ends up in
>> de LSA of Windows), but every attempt to use that TGT to obtain a
>> service ticket yields an error:
>> Matching credential not found.
>
> Unfortunately, our mailing list server doesn't pass through 
> attachments, so while I briefly saw your screenshots before moderating 
> through your message, they didn't make it to the list (and I didn't 
> keep a copy.)
>
> I believe the correct short answer is to use the "API:" ccache instead 
> of the "MSLSA:" ccache for this setup.
>
> For some time Windows has restricted access to TGT session keys in the 
> LSA, which means our libkrb5 code can't use a TGT from the LSA to get 
> service tickets.  Instead, our MSLSA ccache type requests service 
> tickets via Windows, but that only works if the realm is set up in the 
> LSA configuration.  Since you are using an MIT krb5 KDC, I am guessing 
> that it is not set up in the LSA configuration, so we fall back to 
> trying to get service tickets using the TGT.
>
> The TGT session key restriction can be overridden by the registry 
> value HKLM\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos 
> AllowTGTSessionKey, which I believe our installer sets.  I would not 
> be surprised if update 1803 disables this registry value so that the 
> restriction is always in effect.
>
> I have also heard that Microsoft plans to disable access to service 
> ticket session keys from userspace, effectively preventing KfW from 
> using the LSA ccache.  I don't know if that restriction is present in 
> update 1803, and I believe it only applies if Credential Guard is 
> enabled.  (I don't know what determines whether Credential Guard is 
> enabled.)  We could conceivably work around this restriction in the 
> GSSAPI library by getting context establishment tokens via SSPI 
> instead of via our krb5 code, but I can't make any promises as to when 
> that might be implemented.  I don't believe this is the restriction at 
> issue in your test setup anyway.
>