SSH Hardening and Audit Best Practices

The single most impactful SSH hardening step is setting PasswordAuthentication no in /etc/ssh/sshd_config. This forces all users to authenticate with public keys, eliminating the risk from weak or reused passwords and brute force attacks. Ensure at least one authorized key is installed before disabling password authentication to avoid locking yourself out.

The AllowUsers and AllowGroups directives restrict SSH access to named users or group members. For example, AllowGroups sshusers permits only members of the sshusers group. This provides defense in depth: even if an attacker knows a username and obtains credentials, SSH access is denied unless the account is explicitly permitted.