SSH

sudo nano /etc/ssh/sshd_config
Port XX #any different to 22, IMPORTANT open port before!
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
MaxAuthTries 3
LoginGraceTime 30

#Refrescar 
systemctl daemon-reload
systemctl restart ssh.socket

Vulnerabilidades sysctl

sudo nano /etc/sysctl.d/99-hardening.conf

# ─────────── Networking ───────────
# Evitar source routing (protección contra ataques de red)
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0

# Reverse path filtering (prevención spoofing)
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

# No enviar ICMP redirects
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0

# Logear paquetes martianos
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.log_martians = 1

# Evitar que respondan a pings broadcast
net.ipv4.icmp_echo_ignore_broadcasts = 1

# Habilitar TCP SYN cookies (previene SYN flood)
net.ipv4.tcp_syncookies = 1

# ─────────── IPv6 hardening ───────────
net.ipv6.conf.all.accept_ra = 0
net.ipv6.conf.default.accept_ra = 0
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0

# ─────────── Kernel security ───────────
# Deshabilitar IP forwarding (si no es router)
net.ipv4.ip_forward = 0
net.ipv6.conf.all.forwarding = 0

# ─────────── Otros ───────────
# Logging y audit
kernel.dmesg_restrict = 1
kernel.kptr_restrict = 2

Si en dockershield

net.ipv4.conf.all.secure_redirects is set to '1' (recommended: '0')

net.ipv4.conf.default.secure_redirects is set to '1' (recommended: '0')

y

sysctl net.ipv4.ip_forward

devuelve

net.ipv4.ip_forward = 1

No hacer caso

sudo sysctl --system

Autenticación

Eliminar NOPASSWD

Revisa NOPASSWD en tus usuarios y quítalo según corresponda.

sudo visudo -f /etc/sudoers.d/90-cloud-init-users

Cambia ubuntu ALL=(ALL) NOPASSWD:ALL

Por:

ubuntu ALL=(ALL) ALL

fail2ban

sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
sudo nano /etc/fail2ban/jail.local

[sshd]
enabled = true
mode = aggressive
port = ssh
logpath = %(sshd_log)s
backend = %(sshd_backend)s

maxretry = 3
findtime = 10m
bantime = -1

sudo fail2ban-client reload
sudo systemctl enable fail2ban