Checking the transited list of a kerberos ticket in a transitive cross-realm trust situation...

[Stefan Metzmacher]

Hi,

I'm currently researching on how I can implement S4U2Self in
Samba's winbindd in order to get the PAC with the full
Windows authorization token in a reliable way for any user
within an active directory forest as well across transitive
forest trusts.

The only thing that should be required is a service (computer) account
in the primary domain/realm.

But in practice I'm facing several problems:

Heimdal (at least the copy of ~ 1.5 within Samba)
doesn't support S4U2Self for cross-realm trusts.

MIT (tested with 1.14.3) supports S4U2Self for
cross-realm trusts, which are in simple hierarchy.
Otherwise it complains and returns KRB5KRB_AP_ERR_ILL_CR_TKT.
That can be fixed if I add the correct magic to the [capaths] section
of krb5.conf.

The problem happens when you have 2 tree root domains within an
active directory forest together with a forest trust.

In my case I have a forest called W4EDOM-L4.BASE with a single domain
and a forest called BLA.BASE with a 2nd domain BLA2.BASE.

So trust path between W4EDOM-L4.BASE and BLA2.BASE goes via BLA.BASE.

In an active directory environment domain members just delegate
authentication to the domain controllers, so they trust
their DCs to do the correct things, e.g. applying SID-Filtering
for the PAC within the tickets.

So the service can just verify the PAC was correctly signed by
a KDC of it's own realm and everything else shouldn't matter,
it doesn't have to know about the full trust topology!

While thinking about this I can't see any value in checking the
transited list of the ticket. As that list is always under the
control of the KDC that issued the ticket. And the service
trusts it's own KDC anyway, as well as any KDC in the trust
chain trusts the next hop. The only reason for this list
might be debugging.

The thing is that KDC's should apply some policies
of which client realms can come over which direct trust.
As KDC's have some knowledge about the trust topology.
This is basically what the SID-Filtering in active directory
is for, it prevents DCs from other domains/realms to impersonate
principals of the local realm.

Is there any reason to keep the krb5_check_transited() (in Heimdal)
and krb5_check_transited_list() (in MIT) is their current form?

If a KDC checks something it should be checking the PA-TGS-REQ,
and verify the client realm is allowed to transit via the
realm of the (cross-realm) tgt. But checking the transited field
of the ticket seems pointless to me.

If there's however a good reason to keep the checks for non
active directory realms, I'd propose to add something like
gss_set_cred_option(GSS_KRB5_CRED_NO_TRANSIT_CHECK_X)
to Heimdal and MIT in order to allow applications to avoid
the pointless checks.

Comments on this would be highly appreciated!

If you're not so familiar with active directory domains,
please have a look at:
https://www.samba.org/~metze/presentations/2017/SambaXP/StefanMetzmacher_sambaxp2017_windows_authentication-rev1-handout.pdf

Thanks!
metze

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