What is the principle of defense in depth and provide examples across layers?

• A. Use a single firewall at the network edge.
• B. It means using multiple overlapping controls across layers.
• C. Rely on antivirus software only
• D. It relies on encryption for all data.

Answer: (B)

Defense in depth means building multiple, overlapping security measures across different layers so no single failure breaks the protection. The strength comes from redundancy and diversity: if one control is bypassed or fails, others still block or detect the threat. For example, you combine physical security (locked doors, access badges) with network protections (firewalls, network segmentation) and endpoint controls (antivirus, EDR). Add application safeguards (input validation, secure coding practices) and data protections (encryption in transit and at rest). Control the identities with multi-factor authentication and back everything up with monitoring and incident response (logs, SIEM, anomaly detection). Relying on a single measure—like a lone firewall, antivirus only, or encryption alone—leaves gaps that attackers can exploit, whereas multiple overlapping controls across layers create a more resilient defense.