What does "defense in depth" entail regarding security controls?

"Defense in depth" is a security strategy that aims to provide multiple layers of security controls to protect assets from a range of potential threats. This approach recognizes that no single security measure is sufficient to address all potential vulnerabilities; therefore, it employs various complementary measures that work together to create a more robust defense.

The correct understanding lies in the concept of using multiple differing security controls that protect the same assets, which is the essence of the "defense in depth" strategy. By layering different types of controls—such as firewalls, intrusion detection systems, encryption, and access controls—organizations can reduce the likelihood of a successful attack. If one layer fails or is bypassed, additional layers can still provide protection, thereby enhancing overall security.

In contrast, focusing solely on a singular defensive measure at the network perimeter would leave other potential attack vectors unprotected and increase the risk of a breach. Encrypting data during transmission is a crucial security practice, but it is only one component of a comprehensive security strategy. Similarly, isolating applications from the internet is an effective technique, but it does not encompass the full range of measures that "defense in depth" entails.