openssh ignores locked account using public key authentication

Kyle Robinson ky.robinson at gmail.com
Tue Oct 4 16:53:47 MDT 2005


On 10/4/05, Lonnie Olson <fungus at aros.net> wrote:
> SSH will authenticate using a number of different methods.  pubkey,
> keyboard-interactive, and password.
>
> public key authentication uses PAM to do no more than look up the
> home directory of the user.  It actually might not use PAM at all and
> just access the file directly.  Locking an account has no effect on
> this form of auth.

Locking an account can actually effect publickey authentication as
long as UsePAM is set to yes in the sshd_config and the linux PAM
function pam_acct_mgmt() is working as it should be (as the Solaris
PAM function does).  In this scenario sshd will authenticate the user
with publickey and will then call the pam_acct_mgmt and
pam_open_session functions that should fail based on the locked
account (this is how it works on Solaris).  If UsePAM is set to no
these functions wouldn't be called and a locked account would be
allowed by publickey.

The best thing anyone can do for SSH security is disable every
non-essential and duplicate authentication method.  A typical
configuration I'll use is (auth section only):
# I don't leave things that might matter commented because the
compiled defaults might
# not be what we expect.
RSAAuthentication no
PubkeyAuthentication yes
HostbasedAuthenticaiton no
# Just because PasswordAuthentication is set to no doesn't mean users
can't authenticate
# by way of passwords, this is being done by PAM.  Why would we want two methods
# doing the same thing.  Just let PAM do it and do it right/better.
PasswordAuthentication no
# Challenge response means exactly that.  This is dialog driven
authentication.  If
# something in PAM wants something other than a static "passwd:" this
will allow PAM
# to ask for it.  For example: "Enter user's new password:" "enter token:"
ChallengeResponseAuthentiation yes
# Kerberos is cool, but in SSH, GSSAPI is cooler
KerberosAuthentication no
KerberosOrLocalPasswd no
GSSAPIAuthentication yes
# And of course, our good friend PAM.
UsePAM yes



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