SSH-Hardening

Based on https://stribika.github.io/2015/01/04/secure-secure-shell.html (with a stripped down list of Ciphers and MACs)

Server configuration

Remove all HostKey directives in /etc/ssh/sshd_config, then append at the bottom:

# SSH hardening, see https://wiki.fsinf.at/wiki/SSH-Hardening

# Disable SSHv1
Protocol 2

# Only allow Public Key Authentication
PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no

# Only allow users in the "users" group (no system users!).
# NOTE: See /etc/adduser.conf (EXTRA_GROUPS and ADD_EXTRA_GROUPS) to 
# automatically add new non-system users to a group.
AllowGroups users
 
# Don't forget to remove HostKey directives above
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

... and execute:

cd /etc/ssh/
rm ssh_host_*
ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f ssh_host_ed25519_key -N ""

restart server

When restarting, current session is not affected, make sure you keep it open, in case you've done something wrong:

systemctl restart sshd

Client configuration

On top of your ~/.ssh/config, add:

Host *
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160

Note that we add diffie-hellman-group-exchange-sha256 in comparison to the server config - this is for comparability with Ubuntu 12.04 (remove it if you don't care).

If possible, you can also regenerate your own SSH keys:

ssh-keygen -t ed25519 -o -a 100
ssh-keygen -t rsa -b 4096 -o -a 100

WARNING: This overwrites your old keys if you're not careful. If you don't add a new key to your servers before removing the old ones, you're locked out.

Sources

• https://cipherli.st/
• old, from 2015: https://stribika.github.io/2015/01/04/secure-secure-shell.html