Description of problem:
When sssd is configured to use Kerberos for authentication
(auth_provider = krb5 in a domain section) any password is accepted as
valid under the following conditions:

 - the system is offline, i.e. the KDC configured with the option
   krb5_kdcip cannot be reached
 - the user who tries to authenticate has a valid TGT for the Kerberos
   realm configured with to option krb5_realm in his credential cache
   file

Short term fix:
 - disable Kerberos authentication and use alternatives if possible


Version-Release number of selected component (if applicable):


How reproducible:
Every time

Steps to Reproduce:
1. Configure an SSSD system to authenticate against a Kerberos KDC
2. Log in as a user while SSSD is online.
3. Verify that the user has a valid TGT for the Kerberos realm.
4. Unplug the network cable.
5. Lock the display.
6. The user will be able to unlock the display with any password.
  
Additionally, this can make a system network-vulnerable, as if the SSSD is in the offline state (either the KDC or the identity server is unreachable), an attacker can log into any account that has a valid TGT.

Actual results:
Passwords are not properly verified against the cached credentials. Any password will be treated as a valid authentication.

Expected results:
Authentication should succeed with only the valid credentials.

Additional info:
The only current workaround for this problem is to disable Kerberos authentication on affected machines until a patched binary is available.