Definition
Defense in depth is a cybersecurity strategy that employs multiple layers of security controls to protect assets. Instead of relying on a single perimeter, it assumes breaches will occur and aims to contain them by creating redundant defenses. This approach ensures that if one security measure fails, others are still in place to prevent or mitigate a security incident. In practice, it involves implementing a combination of physical, technical, and administrative safeguards throughout an organization's infrastructure. This layered security is crucial for minimizing the impact of attacks and maintaining operational resilience. For example, a secure web application might employ firewalls, intrusion detection systems, input validation, and regular security patching. A common misconception is that defense in depth is solely about technical controls, but it also encompasses policies and user training. This concept is fundamental to security domains tested on certifications like CompTIA Security+, CySA+, and CISSP.