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I have a Lenovo X131e running Ubuntu Server 18.04.1 LTS and I wanted to be sure that I am doing all I can to secure the connection between the Ubuntu-book and my Mac Mini at home. I will be using the Ubuntu-book to remote back into my Mac while on campus.

Ive set up public key authentication and it appears to work; however, to force the use of the key on the Mac, I needed to modify the ssh_config file to the following...

PubKeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication yes
usePam no

!! - with usePam enabled, the Mac would allow password authentication despite having PasswordAuthentication disabled.

My main question is whether disabling PAM is opening up a vulnerability elsewhere?

Thanks

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    Did you restart SSH with sudo service sshd restart when changing the configurations? Having PasswordAuthentication no disables passwords. Also try and see if you can actually SSH in with a password since SSH may still prompt for a password but the authentication fails. – Kristopher Ives Dec 30 '18 at 22:09
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No, disabling PAM disables PAM. It does not allow for any other authentication modes, nor does it open any other vulnerabilities. By default, Ubuntu is quite secure, and even allowing password authentication is rather secure, as long as you stick to high quality passwords.

PAM is the Pluggable Authentication Module for Linux, which by default handles logins, and are able to authenticate via a wide variety of methods, which includes passwords by default, but may also be extended to require for instance one time passwords.

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