Before jumping into Hardening and Securing OpenSSH, we must first understand what are the benefits as well as the consequences in case the OpenSSH server is not hardened.
OpenSSH is the default gateway into most Unix-based systems, used by system administrators, developers, and automation tools to remotely access and control machines. While OpenSSH itself is a well-audited and secure tool, leaving it configured with default settings exposes critical vulnerabilities and attack surfaces that malicious actors can exploit. This is why OpenSSH hardening is not optional — it is essential.
Why Harden OpenSSH?
By default, OpenSSH is configured to allow broad and flexible access, which is ideal for initial setup but dangerous in a production environment. Without applying security best practices, attackers can:
- Attempt brute-force attacks to guess usernames and passwords.
- Leverage weak or outdated cryptographic algorithms to intercept or alter sessions.
- Exploit known vulnerabilities in older OpenSSH versions.
- Pivot into internal networks once a single server is compromised.
A poorly secured SSH configuration is a prime target for cybercriminals and automated bots scanning the internet for entry points. In cloud-based infrastructure or critical environments, one misconfigured SSH port can lead to privilege escalation, data theft, or a complete system compromise.
Goals of Hardening
The goal of OpenSSH hardening is to:
- Minimize exposure to threats.
- Limit attack surface and access vectors.
- Enforce strong authentication mechanisms.
- Protect confidentiality and integrity of remote sessions.
- Comply with industry standards such as CIS Benchmarks, NIST, or ISO 27001.
Through proper hardening techniques, administrators can dramatically reduce the risk of unauthorized access while maintaining usability for legitimate users.
Benefits of Hardening OpenSSH
- Enhanced system security with reduced chances of breach.
- Improved compliance with security frameworks and regulations.
- Lower risk of brute-force and dictionary attacks.
- Better control and monitoring over access to critical systems.
- Peace of mind in knowing your first line of remote access defense is fortified.
Let’s dive into Hardening and Securing OpenSSH which is the crucial part that every System Administrator must know.
1. Backup the Configuration File
Before making any changes, backup your SSH configuration:
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
2. Use SSH Protocol Version 2 Only
Ensure only the more secure SSH protocol version is used.
Protocol 2
3. Disable Root Login
To prevent brute-force attacks targeting the root user:
PermitRootLogin no
Tip: Create a separate user with sudo privileges instead.
4. Limit SSH Access to Specific Users
Only allow certain users or groups to log in via SSH.
AllowUsers youruser anotheruser
# OR
AllowGroups sshusers
5. Change the Default SSH Port
Changing the default port (22) reduces random attack attempts:
Port 2222
Make sure the new port is allowed in your firewall settings.
6. Use Key-Based Authentication Instead of Passwords
Generate a key pair on the client and copy the public key to the server:
ssh-keygen
ssh-copy-id user@server
Then, disable password authentication:
PasswordAuthentication no
7. Disable Empty Passwords
Ensure users without passwords cannot log in:
PermitEmptyPasswords no
8. Enable SSH Rate Limiting (Fail2Ban)
Use Fail2Ban to block IPs after a certain number of failed login attempts:
sudo apt install fail2ban
Fail2Ban will monitor log files and ban suspicious IPs automatically.
9. Set Idle Timeout Interval
Automatically disconnect idle users to prevent session hijacking:
ClientAliveInterval 300
ClientAliveCountMax 0
(Disconnects idle sessions after 5 minutes.)
10. Use a Strong Cipher and MACs
Customize the encryption algorithms used:
Ciphers aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512,hmac-sha2-256
11. Enable Two-Factor Authentication (2FA)
You can use Google Authenticator or Duo Security:
sudo apt install libpam-google-authenticator
Then configure PAM and SSHD accordingly.
12. Restrict Port Forwarding and Tunneling (if not needed)
AllowTcpForwarding no
X11Forwarding no
PermitTunnel no
13. Use TCP Wrappers or Firewall for IP Whitelisting
Limit access to certain IPs using /etc/hosts.allow and /etc/hosts.deny or with UFW/iptables.
Example using UFW:
sudo ufw allow from 192.168.1.0/24 to any port 2222 proto tcp
14. Monitor SSH Logs Regularly
SSH logs are usually found in:
/var/log/auth.log (Debian/Ubuntu)
/var/log/secure (RHEL/CentOS/Fedora)
Use tools like logwatch, auditd, or SIEM solutions for alerts.
15. Disable SSH Access for System Users
Prevent non-login accounts (like nobody, www-data, etc.) from SSH access:
Ensure /etc/passwd has:
'nologin' or '/bin/false' for such accounts
16. Upgrade OpenSSH Regularly
Always keep your OpenSSH server updated to patch vulnerabilities:
sudo apt update && sudo apt upgrade
Summary of Key Directives for /etc/ssh/sshd_config
Port 2222
Protocol 2
PermitRootLogin no
AllowUsers youruser
PasswordAuthentication no
PermitEmptyPasswords no
ClientAliveInterval 300
ClientAliveCountMax 0
Ciphers aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512,hmac-sha2-256
AllowTcpForwarding no
X11Forwarding no
Benefits of Hardening OpenSSH
- Enhanced system security with reduced chances of breach.
- Improved compliance with security frameworks and regulations.
- Lower risk of brute-force and dictionary attacks.
- Better control and monitoring over access to critical systems.
- Peace of mind in knowing your first line of remote access defense is fortified.
Conclusion
Securing your systems begins with protecting the entry points — and in most Unix-based environments, that means OpenSSH. While OpenSSH is inherently robust and reliable, its power can quickly become a liability if left with default settings or misconfigurations.
Through proper hardening techniques — such as disabling root logins, enforcing key-based authentication, restricting access, and disabling weak ciphers — system administrators can significantly reduce the risk of unauthorized access, brute-force attacks, and data breaches.
Hardening OpenSSH is not just a security best practice; it is a critical step toward building a resilient infrastructure. It ensures that your remote access is not only efficient but also airtight against evolving cyber threats. In today’s threat landscape, overlooking SSH hardening is an invitation for compromise.
Take the time to review and tighten your OpenSSH configurations. A few changes today could prevent a major incident tomorrow.