Harden Ubuntu 24.04 Server with the ssh audit recommendations

ssh-hardening-ubuntu-24-04.sh

# SSH hardening (run as root). From https://www.sshaudit.com/hardening_guides.html#ubuntu_24_04_lts
#!/bin/bash
set -eux pipefail

# Re-generate the ED25519 and RSA keys
sudo rm -f /etc/ssh/ssh_host_*
sudo ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""


# Remove small Diffie-Hellman moduli
sudo awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe || true
sudo mv /etc/ssh/moduli.safe /etc/ssh/moduli || true

# Restrict supported key exchange, cipher, and MAC algorithms (as per sshaudit Ubuntu 24.04 guide)
sudo tee /etc/ssh/sshd_config.d/ssh-audit_hardening.conf >/dev/null << 'EOF_CRYPTO'
# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com hardening guide.
KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,gss-group16-sha512-,diffie-hellman-group16-sha512

Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr

MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com

RequiredRSASize 3072

HostKeyAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-

HostbasedAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

PubkeyAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256
EOF_CRYPTO

# Restart OpenSSH server
sudo service ssh restart

# Implement connection rate throttling
sudo iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set || true
sudo iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP || true
sudo ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set || true
sudo ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP || true

sudo DEBIAN_FRONTEND=noninteractive apt install -q -y netfilter-persistent iptables-persistent || true
sudo service netfilter-persistent save || true