disallow requests naming principal as a service

[Nicolas Williams]

[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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