SSH provides ubiquitous secure remote access to Linux and Unix systems. However, poor SSH security can expose your servers to attacks like brute force credential stuffing. In this article, we’ll explore best practices for hardening SSH security on your Linux systems.
Why SSH Security Matters
Most internet-facing Linux servers have SSH exposed for administrative access. This access must be properly secured as compromising SSH can allow attackers to gain full control of your servers.
Threats to vulnerable SSH include:
- Brute force attacks trying common or leaked credentials
- Session hijacking via Man-in-the-Middle attacks
- Exploiting weak encryption algorithms
- Bypassing authentication to gain entry
Hardening SSH security is critical to prevent unauthorized remote access and safeguard your infrastructure.
Create Dedicated User Accounts
Don’t enable SSH access for the root user. Instead, create individual user accounts with sudo privileges for each administrator. Avoid shared or generic logins.
This allows better auditing of who accessed systems. Accounts can also be restricted to specific source IP ranges for least privilege SSH access.
Leverage SSH Keys Over Passwords
Disable password-based SSH authentication which is prone to brute forcing. Require administrators to use SSH keys to prove their identity.
SSH key pairs provide much stronger cryptographic protection. Keys should use at least 2048 bit RSA or higher encryption.
Update SSH Configuration
Review your /etc/ssh/sshd_config file and apply recommended hardening like:
- Disallowing root login
- Limiting access to admin subnet
- Disabling X11 forwarding
- Enabling strict mode
- Restricting ciphers and protocols
Restart SSH daemon after making changes. Stay updated on DNS configuration hardening guidance.
Utilize Two-Factor Authentication
For additional protection, require administrators to use two-factor authentication when connecting via SSH.
Tools like Duo Security, YubiKeys, and Google Authenticator can provide 2FA codes during SSH logins for robust identity verification.
Monitor and Limit Failed Attempts
Use centralized logging to monitor SSH connection attempts – both successful and failed. Alert on unusual activity like brute force attacks.
Limit max failed attempts via MaxAuthTries in sshd_config and utilize tools like Fail2ban to block brute forcing.
Conclusion
Properly securing SSH is mandatory for any servers exposed to untrusted networks like the public internet. Following these best practices for access controls, encryption, monitoring, and more can help harden your Linux environment against SSH attacks.
For additional layers of security, consider using a cloud VPS provider that offers managed firewall services to protect SSH server access.