[SSSD] sssd-ad uses wrong key to verify tgt at login time

Jurjen Bokma j.bokma at rug.nl
Sat Jun 7 11:20:24 UTC 2014


On 06/06/2014 10:24 AM, Sumit Bose wrote:
> On Thu, Jun 05, 2014 at 04:54:50PM +0200, Jurjen Bokma wrote:
>> Hi,
>>
>> I believe the closing of
>> https://fedorahosted.org/sssd/ticket/1871#comment:2
>> to be wrong. Jakub Hrozek asked me to bring this up here after I brought it to his attention through
>> https://bugs.launchpad.net/bugs/1274543
>>
>> When it logs in to the AD server, sssd uses the first key in the keytab. In my case, that first key is of a 'join account': a special purpose AD account that has permission to join machines to the domain, and nothing else. In particular, it lacks the permissions to list users. Hence it is unfit to be used by sssd for lookups. As soon as I remove the offending key, sssd works fine, by using another key, which happens to work. When I put it back in the keytab (as the first key), sssd stops working again. I second Jakub in the opinion that sssd should select the 'right' principal to use: ldap_sasl_authid or shortname$@realm.
>> I believe the cause of the problem to be in src/providers/krb5/krb5_child.c in the loop immediately following the comment:
>> /* We look for the first entry from our realm or take the last one */
>> Indeed, if I make that loop skip the first key found, everything works as expected, whether the ADJoiner key is in the keytab or not.
>>
>> This is just an ad-hoc fix for my case of course.
> Thank you for the report. I reopened
> https://fedorahosted.org/sssd/ticket/1871 and it would be nice if you
> can add your use-case to the ticket. The key point here is that your
Thanks for reopening! I pleaded my case in Trac.
> first principal cannot be used as a service principal and hence cannot
> be used for validation.
>
> Another workaround would be to put the principal at some other place in
> the keytab or to disable validation be setting 'krb5_validate = false'
> in sssd.conf.
Thank you for the suggestions. Yet another workaround is to have the 
application (i.c. msktutil) that put the offending key in the keytab use 
another keytab entirely. That's what I did. My latest solution is even 
nicer: have the join account on a puppet server, and never even use it 
from the client. So I'm not even affected by the bug any more.
But more applications use the system keytab by default, reconfiguring 
each to use another keytab is tedious. Moreover, when an offending key 
is first in the keytab, sssd fails silently unless logging is 
sufficiently verbose,  and the message is rather cryptic. The admin 
doesn't suspect keytab problems because the proper key *is* in the 
keytab too, and will only grow suspicious when he starts sniffing the 
network connection for Kerberos traffic.
IMHO, the key point is that the first key will often work, but the 
assumption that it'll *always* work is just plain wrong. Please add code 
to pick the right principal.

Best Regards
Jurjen




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