SSH Sudo – Sudo Authentication When Using SSH Key Auth with Keepass and Putty

key-authenticationputtysshsudo

I an using Putty to login via SSH to various Linux machines (mostly CentOS and Ubuntu). I use SSH key authentication, whereas the key ist stored in Keepass 2.x and handed over to Putty via tha Keeagent plugin (basically a replacement for pageant). Works fine so far.

But when logged in, using sudo requires me to enter the password. Is there a way around it? So, when logged in via SSH key, no password should be needed for sudo?

There is this very similar question from 6 years ago: sudo: don't ask password when logged in with ssh key
. The given answer is not realy an answer but a work around to use ssh key login and log in with root directly. There are various reasons not to do that.

Best Answer

If you can use SSH agent forwarding, there actually is a way: pam_ssh_agent_auth.so (source here) is a PAM module that can do what you ask. It's available in Debian and Ubuntu as package libpam-ssh-agent-auth and as CentOS package pam_ssh_agent_auth.

# Debian/Ubuntu:
apt update; apt install libpam-ssh-agent-auth
# CentoOS
yum install pam_ssh_agent_auth

Security Considerations

You should evaluate the risks of using SSH agent forwarding, as the developer says:

There are caveats of course, ssh-agent forwarding has it’s own security risks which must be carefully considered for your environment. In cases where there are not untrustworthy intermediate servers, and you wish to retain traceability, accountability, and required authentication for privileged command invocation, the benefits should outweigh the risks.

If you make sure your KeeAgent has the option Always require confirmation when client program requests to use key set, this even offers you a degree of protection against someone else with root access on the remote host: if you get a SSH key request confirmation dialog with no obvious reason, you'll know that someone is trying to abuse your SSH agent connection.

If you also make sure you'll always lock your KeePass and/or workstation screen when you step away from it, I think this should offer pretty good security; it's certainly more secure than using NOPASSWD in sudoers. It is also better than allowing root logins with ssh keys only and adding everyone who is allowed to log in as root to root's authorized_keys file because it maintains the advantages of sudo.

Usage

To use it, you basically add this as the first auth line in /etc/pam.d/sudo:

auth    sufficient      pam_ssh_agent_auth.so file=/etc/security/authorized_keys

A simple sed command to do this (adds it to the second line, since the first is a comment):

sed -i '2 i\auth    sufficient      pam_ssh_agent_auth.so file=/etc/security/authorized_keys' /etc/pam.d/sudo

Then add the public SSH keys of users that should be authorized to use SSH-authenticated sudo to /etc/security/authorized_keys in the usual single-line OpenSSH compatible format.

Then configure sudoers to preserve the environment variable SSH_AUTH_SOCK by editing the sudoers file (use visudo). Add this line to the section with the other Defaults.

Defaults env_keep += "SSH_AUTH_SOCK"

Then, you need to make sure that your ssh client allows agent forwarding. In PuTTY you need to check this:

Putty Settings for agent forwarding

While testing this, don't forget to terminate your sudo sessions with sudo -k.

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