Mastering Linux Security and Hardening

1. Linux security requires a multi-layered approach, starting with user account management

Security is one of those things that's best done in layers. Security-in-depth, we call it.

User accounts are the first line of defense. Properly managing user accounts is crucial for maintaining system security. This includes:

• Using strong password policies
• Implementing password aging and account expiration
• Limiting root access and using sudo for privileged operations
• Regularly auditing user accounts and permissions

Additionally, consider implementing two-factor authentication for sensitive systems and restricting access to critical directories. Remember, the principle of least privilege should guide your user management strategy.

2. Implement discretionary and mandatory access controls to protect files and directories

Discretionary Access Control allows users to control who can access their own files and directories. But, what if your company needs to have more administrative control over who accesses what? For this, we need some sort of Mandatory Access Control or MAC.

Access controls are vital for data protection. Linux provides two main types of access controls:

• Discretionary Access Control (DAC):

  • Uses file permissions (read, write, execute)
  • Allows users to control access to their own files
  • Implemented through chmod, chown, and ACLs

• Mandatory Access Control (MAC):

  • Provides system-wide policies enforced by the kernel
  • Examples include SELinux and AppArmor
  • Offers finer-grained control and better protection against certain attacks

Implementing both DAC and MAC provides a robust defense against unauthorized access and potential security breaches.

3. Set up robust firewalls and intrusion detection systems to safeguard your network

On any given corporate network, you will find a firewall appliance separating the internet from the demilitarized zone (DMZ), where your internet-facing servers are kept. You will also find a firewall appliance between the DMZ and the internal LAN, and firewall software installed on each individual server and client.

Firewalls and IDS are crucial network defenses. A comprehensive network security strategy should include:

• Host-based firewalls (e.g., iptables, firewalld)
• Network-based firewalls for perimeter defense
• Intrusion Detection Systems (IDS) like Snort
• Regular firewall rule audits and updates

Configure firewalls to follow the principle of least privilege, allowing only necessary traffic. Use tools like Security Onion for easier IDS deployment and management. Remember to monitor logs and alerts from these systems to quickly identify and respond to potential threats.

4. Use encryption to protect data at rest and in transit

Encryption is important for safeguarding sensitive data both at rest and in transit.

Encryption is essential for data protection. Implement encryption strategies for:

• Data at rest:

  • Full disk encryption (e.g., LUKS)
  • File-level encryption (e.g., eCryptfs)
  • Encrypted containers (e.g., VeraCrypt)

• Data in transit:

  • SSL/TLS for web services
  • SSH for remote administration
  • VPNs for secure remote access

Additionally, use tools like GPG for encrypting individual files and secure communication. Properly manage encryption keys and consider implementing a key management system for larger environments.

5. Regularly scan for vulnerabilities and conduct security audits

You'll want to know when that happens, so you'll want to have a good Network Intrusion Detection System (NIDS) in place.

Proactive scanning is key to maintaining security. Implement a regular schedule for:

• Vulnerability scans using tools like OpenVAS
• Web application scans with Nikto
• System audits using Lynis
• Network-wide scans with Nmap

Analyze scan results promptly and prioritize addressing critical vulnerabilities. Consider using automated tools to schedule and report on regular scans. Keep scanning tools and vulnerability databases up-to-date to ensure you're protected against the latest threats.

6. Harden systems using automated tools and security frameworks

OpenSCAP is a set of free open source software tools that can be used to implement SCAP.

Automation enhances security consistency. Leverage tools and frameworks for system hardening:

• OpenSCAP for applying security policies
• Ansible, Puppet, or Chef for configuration management
• CIS Benchmarks for best-practice guidelines
• Custom scripts for environment-specific hardening

Implement a consistent hardening process across all systems, including:

• Disabling unnecessary services
• Applying security patches promptly
• Configuring secure defaults for applications
• Implementing logging and monitoring

Regularly review and update your hardening procedures to address new threats and vulnerabilities.

7. Stay vigilant with ongoing monitoring and incident response planning

Even with its numerous weaknesses, GPG is still one of the best ways to share encrypted files and emails.

Constant vigilance is crucial for security. Develop a comprehensive monitoring and response strategy:

• Implement centralized logging (e.g., ELK stack)
• Set up real-time alerting for suspicious activities
• Develop and regularly test an incident response plan
• Conduct regular security awareness training for staff

Use tools like fail2ban to automatically block potential threats. Regularly review logs and conduct security drills to ensure your team is prepared to respond to incidents quickly and effectively. Remember, security is an ongoing process, not a one-time task.

Last updated: August 19, 2024