I have Google 2FA set up for ssh password logins. Works fine, but if possible I'd like to tighten a small security hole: if the password given is correct, it will then ask for the 2FA token. If the password given is incorrect, though, then it will keep asking for the password. This obviously would reveal to an attacker that they have actually discovered the password, even if they can't get in immediately.
For example, if the password is hunter2:
Password: hunter2 Verification code:
Password: banana Password:
I'd like to set it up so that it asks for the 2FA code in every instance, even when the password is wrong, to close off this attack vector. I'd also like it to send me an alert if the correct password is entered along with an incorrect OTP.
Is this something I can achieve with PAM configuration or would I need to basically write my own module for it?
# PAM configuration for the Secure Shell service # Standard Un*x authentication. @include common-auth auth required pam_google_authenticator.so nullok # Disallow non-root logins when /etc/nologin exists. account required pam_nologin.so # Uncomment and edit /etc/security/access.conf if you need to set complex # access limits that are hard to express in sshd_config. # account required pam_access.so # Standard Un*x authorization. @include common-account # SELinux needs to be the first session rule. This ensures that any # lingering context has been cleared. Without this it is possible that a # module could execute code in the wrong domain. session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so close # Set the loginuid process attribute. session required pam_loginuid.so # Create a new session keyring. session optional pam_keyinit.so force revoke # Standard Un*x session setup and teardown. @include common-session # Print the message of the day upon successful login. # This includes a dynamically generated part from /run/motd.dynamic # and a static (admin-editable) part from /etc/motd. session optional pam_motd.so motd=/run/motd.dynamic session optional pam_motd.so noupdate # Print the status of the user's mailbox upon successful login. session optional pam_mail.so standard noenv #  # Set up user limits from /etc/security/limits.conf. session required pam_limits.so # Read environment variables from /etc/environment and # /etc/security/pam_env.conf. session required pam_env.so #  # In Debian 4.0 (etch), locale-related environment variables were moved to # /etc/default/locale, so read that as well. session required pam_env.so user_readenv=1 envfile=/etc/default/locale # SELinux needs to intervene at login time to ensure that the process starts # in the proper default security context. Only sessions which are intended # to run in the user's context should be run after this. session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so open # Standard Un*x password updating. @include common-password