SSH / PAM / Kerberos / password aging

[Nicolas Williams]

Ok, so, things are complicated.

The PAM standard insists on password aging being done after account
authorization, which comes after user authentication. Kerberos can't
authenticate users whose passwords are expired.

So PAM_KRB5 implementations tend to return PAM_SUCCESS from
pam_krb5:pam_sm_authenticate() and arrange for pam_krb5:pam_sm_acct_mgmt()
to return PAM_NEW_AUTHTOK_REQD, as required by PAM even though the user
can't be said to be authenticated at that point.

The problem with this is that by the time pam_acct_mgmt() is called in
OpenSSH userauth has been completed, so kbd-interactive is not used for
the password changing and instead the work is relegated till the TTY
session is setup. By then the login process has begun and the user even
has a UTMP entry.

To make matters worse, OpenSSH calls pam_setcred() before
pam_chauthtok(). Clearly that is wrong.

Our PAM_KRB5 module has a module option "pw_exp_in_auth" which causes it
to do the password aging prompting in pam_krb5:pam_sm_authenticate().
Using this option with the "sshd" PAM service causes password aging to
be performed over the kbd-interactive protocol. Good? Bad? I say "good."

Nico
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