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SSH (Secure Shell)

This advanced guide is crafted for system administrators and security experts who are committed to enhancing the security of their SSH (Secure Shell) servers. It encompasses a broad spectrum of hardening techniques, ranging from fundamental configuration adjustments to complex security mechanisms, with detailed explanations for each recommendation.

Enforce SSH Protocol 2​

SSH Protocol 2 is the latest version and offers improved security features over Protocol 1, including more robust encryption algorithms and host-based authentication. Enforcing its use ensures that communications are better protected.

# /etc/ssh/sshd_config
Protocol 2

Disable Root Login​

Disabling root login over SSH reduces the risk of unauthorized access by requiring users to authenticate as a non-privileged user before escalating privileges. This minimizes the potential impact of brute force attacks.

# /etc/ssh/sshd_config
PermitRootLogin no

Employ Key-Based Authentication​

Key-based authentication is more secure than password-based authentication, as it relies on cryptographic keys that are much harder to brute force. It significantly enhances the security of SSH access.

# /etc/ssh/sshd_config
PasswordAuthentication no
AuthenticationMethods publickey

Restrict SSH Access to Specific Users​

Limiting SSH access to specific users reduces the attack surface by ensuring only authorized personnel have remote access, enhancing overall system security.

# /etc/ssh/sshd_config
AllowUsers specified_user

Alter the Default SSH Port​

Changing the default SSH port (22) can help mitigate automated attacks and scans targeting default SSH ports, reducing the visibility to automated threats.

# /etc/ssh/sshd_config
Port 2222

Activate Fail2Ban​

Fail2Ban monitors login attempts to SSH and automatically bans IPs that exhibit malicious behavior, such as repeated failed login attempts, helping to protect against brute-force attacks.

Schedule SSH Key Rotation​

Regularly rotating SSH keys minimizes the risk of exposure and compromise, ensuring that access remains secure even if a key is somehow leaked or stolen.

Implement Two-Factor Authentication​

Two-factor authentication adds an extra layer of security by requiring a second form of verification beyond the SSH key, significantly reducing the risk of unauthorized access.

# /etc/ssh/sshd_config
AuthenticationMethods publickey,keyboard-interactive

Harden SSH Ciphers​

Specifying strong ciphers for SSH sessions enhances the encryption strength, protecting against cryptographic attacks that target weaker encryption algorithms.

# /etc/ssh/sshd_config
Ciphers aes256-ctr,aes192-ctr,aes128-ctr

Limit SSH Sessions​

Limiting the number of concurrent SSH sessions per user can help prevent resource exhaustion and potential denial of service attacks.

# /etc/ssh/sshd_config
MaxSessions 2

Configure Idle Timeout​

Setting an idle timeout disconnects inactive sessions, reducing the risk of session hijacking and conserving system resources.

# /etc/ssh/sshd_config
ClientAliveInterval 300
ClientAliveCountMax 0

Use Security Extensions​

Security extensions like SELinux or AppArmor add an additional layer of security, enforcing strict access controls around the SSH service and further securing the system.

Monitoring and Maintenance​

Audit SSH Access (Log Analysis)​

Regularly reviewing SSH access logs is crucial for detecting unauthorized access attempts and ensuring that access controls are effective.

Update SSH Software Regularly​

Keeping SSH server software up to date is essential for protecting against newly discovered vulnerabilities and maintaining a secure environment.