krb5_change_password rejected on some Windows 2025

[Arnaud Bouchez]

Hello,

Windows Server 2025-based Active Directory domain controllers appear to 
regress in handling KPASSWD protocol. Namely, a password change request 
is being processed and a password of an Active Directory account has 
been changed but the response produced by the domain controller is 
Kerberos error with code 0, explicitly not allowed by the RFC3244 
describing Microsoft KPASSWD protocol.

Source:
https://lists.samba.org/archive/cifs-protocol/2024-December/004390.html

The `krb5_change_password()` function reject is_error + 
KRB5_KPASSWD_SUCCESS as expected by the RFC, but it fails on this 
buggy/featured Windows Server 2025 AD.

If I understand correctly, we could remove the following lines in chpw.c :

krb5int_rd_chpw_rep(krb5_context context, krb5_auth_context auth_context,
....
     /* Successful replies must not come from errors. */
     if (is_error && result_code == KRB5_KPASSWD_SUCCESS) {
         ret = KRB5KRB_AP_ERR_MODIFIED;
         goto cleanup;
     }

So that the API would work on those AD version.

Additional information about this MS/Windows issue is available at:
https://gitlab.freedesktop.org/realmd/adcli/-/issues/40

The official server-side fix is not there yet, and it may help 
mitigating it on the client side.

Thanks for your attention.
Arnaud