disallow requests naming principal as a service

[Nicolas Williams]

NP. The return of KDC_ERR_MUST_USE_USER2USER can be arranged with very
little extra code, of course, but see the next message in this thread
from John Brezak. So there's an example of use of TypedData folks.

Cheers,

Nico


On Mon, Mar 25, 2002 at 06:10:22PM -0700, Moore, Patrick wrote:
> Thanks Nico, this solution looks OK to me.
> 
> It modifies the current semantics of KRB5_KDB_DISALLOW_SVR 
> a bit -- hope that's OK.  Besides allowing U2U on a princ that is
> set DISALLOW_SVR, it will not support a princ that may be a 
> normal service but must not be used with dup_skey service 
> tickets.  (I can't imagine who might want that feature -- 
> although it's for some reason supported in DCE secd's.)  
> 
> With this suggested fix, my client would need to try a U2U
> handshake upon getting a PRINC_UNKNOWN error from the MIT KDC.
> Not streamlined - but functional enough. Long term, I'd prefer
> using  KDC_ERR_MUST_USE_USER2USER and report that back to the 
> client when you see that DUP_SKEY is allowed but SVR is not. 
>   
> Pat 
>  
> > -----Original Message-----
> > From: Nicolas Williams [mailto:Nicolas.Williams at ubsw.com]
> > Sent: Friday, March 22, 2002 2:44 PM
> > To: Matt Crawford
> > Cc: Moore, Patrick; ietf-krb-wg at anl.gov; krbdev at mit.edu
> > Subject: Re: disallow requests naming principal as a service
> > 
> > 
> > [Cc'ing krbdev at mit.edu]
> > 
> > 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
> > 
> > On Thu, Mar 21, 2002 at 08:19:25PM -0600, Matt Crawford wrote:
> > > > 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.
> > > 
> > > 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 ......
> > -- 
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