Operating System Host Firewalls

Sample Answer

Using host firewalls is a valid component of a defense-in-depth strategy, but they rarely offer sufficient security alone. Evaluating host firewall capabilities against established best practices is crucial for effective network security.

🛡️ Firewall Guidelines and Best Practices

Here are five typical firewall guidelines and best practices:

Deny All, Permit by Exception (Implicit Deny): This is the fundamental security principle for firewalls. The default rule set should be to deny all traffic unless explicitly allowed by a specific rule. This minimizes the attack surface by ensuring that only necessary and validated services can communicate.

Regular Rule Set Review and Cleanup: Firewall rules are prone to "rule creep"—the accumulation of outdated, redundant, or overly permissive rules over time. Rules should be reviewed quarterly to remove obsolete rules for decommissioned applications or services. This improves performance and reduces configuration complexity and security gaps.

Principle of Least Privilege (PoLP): Firewall rules should be designed to grant the minimum permissions necessary for an application or user to function. This means defining rules with specific source and destination IP addresses, protocols, and ports, rather than using broad "Any" settings.

Change Management Process: All modifications to firewall rules must be documented, tested, approved by management, and scheduled for implementation during low-impact times. This prevents unauthorized changes, accidental security lapses, and ensures an audit trail for compliance.

Centralized Logging and Monitoring: All firewall events (accepts, denies, policy violations) must be sent to a centralized logging system (like a SIEM) for real-time analysis and long-term retention. This enables quick detection of security incidents, provides forensic data, and helps identify misuse or targeted attacks.

Most Important Best Practices

I consider the following two practices to be the most important:

Deny All, Permit by Exception (Implicit Deny): This is the single most critical configuration principle. If a firewall fails to deny implicitly, an attacker only needs to find one hole in the allow list. By denying all, the burden of proof shifts to the service to justify its access, preventing accidental or undiscovered open ports.

Regular Rule Set Review and Cleanup: In complex organizations, outdated rules become a primary source of vulnerability. A rule created five years ago for a temporary project often stays active, creating an unnecessary pathway into the network that security teams forget to monitor. Cleaning these rules actively and regularly is essential for maintaining an accurate attack surface profile.

💻 Host Firewall Evaluation

I will evaluate the Windows 10 Firewall (Windows Defender Firewall with Advanced Security) and the Linux iptables/nftables host firewall.