Subject: RE: disallow requests naming principal as a service

From: "Moore, Patrick" <[email-masked]>
Date: Thu, 21 Mar 2002 19:03:11 -0700

This is the reason another flag is needed so that a user is allowed as
a service, but only via the ENC-TKT-IN-SKEY option, so that you can
authenticate to users as a service, but without exposing a 
conventional TGS_REP to attack on a user's key.

This was added to DCE servers a while back per OSF RFC 91.0  (see
http://www.opengroup.org/tech/rfc/rfc91.0.html )

Some of us would really like to see it in MIT KDC.  I'm hoping 
that this is the purpose of a flag mentioned in clarifications.00:
"KDC_ERR_MUST_USE_USER2USER  27   Server principal valid for user2user only"

Pat Moore
[email-masked]
Sandia National Labs

> -----Original Message-----
> From: Sam Hartman [mailto:[email-masked]]
> Sent: Thursday, March 21, 2002 6:41 PM
> To: Douglas E. Engert
> Cc: Matt Crawford; [email-masked]
> Subject: Re: disallow requests naming principal as a service
> 
> 
> >>>>> "Douglas" == Douglas E Engert <[email-masked]> writes:
> 
>     Douglas> I am changing the title on this, as it is not a comment
>     Douglas> on the minutes, but a suggestion.
> 
>     Douglas> Its a good suggestion, and we should have it in the
>     Douglas> clarifications as an implementation note.
> 
> This seems worth noting, but I'd point out that it does disallow some
> usage of Kerberos.  In particular, it means I cannot store my key on a
> machine and have people authenticate to me as a service.
> 
> In most cases this is OK.
> 
>

---

From: Matt Crawford <[email-masked]>
Date: Thu, 21 Mar 2002 20:19:25 -0600

> conventional TGS_REP to attack on a user's key.

It was pointed out to me in private mail (I don't know why, the
sender isn't usually shy) that DISALLOW_SVR in MIT's KDC does stop
user-to-user.  Since there already exists a separate
DISALLOW_DUP_SKEY flag this surprised me, but it seems to be true.
I suggest that the MIT krb5 has all the flags it needs, it just
doesn't use them properly.  Still, they probably have to invent a new
one to avoid breaking some existing deployment somewhere ......

---

From: Nicolas Williams <[email-masked]>
Date: Fri, 22 Mar 2002 16:44:12 -0500

[Cc'ing [email-masked]]

Sounds like kdc/kdc_util.c:validate_tgs_req():

  1306      /* Server must be allowed to be a service */
  1307      if (isflagset(server.attributes, KRB5_KDB_DISALLOW_SVR)) {
  1308          *status = "SERVER NOT ALLOWED";
  1309          return(KDC_ERR_S_PRINCIPAL_UNKNOWN);
  1310      }

should be changed to:

  1306      /* Server must be allowed to be a service */
  1307      if (!isflagset(request->kdc_options, KDC_OPT_ENC_TKT_IN_SKEY) &&
  1308          isflagset(server.attributes, KRB5_KDB_DISALLOW_SVR)) {
  1309          *status = "SERVER NOT ALLOWED";
  1310          return(KDC_ERR_S_PRINCIPAL_UNKNOWN);
  1311      }

so as to match the handling of DUP SKEY:

  1293      /* Server must allow DUP SKEY requests */
  1294      if (isflagset(request->kdc_options, KDC_OPT_ENC_TKT_IN_SKEY) &&
  1295          isflagset(server.attributes, KRB5_KDB_DISALLOW_DUP_SKEY)) {
  1296          *status = "DUP_SKEY DISALLOWED";
  1297          return(KDC_ERR_POLICY);
  1298      }

Someone should file a bug report :)

Cheers,

Nico

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