What does "security in depth (SID)" refer to?

The concept of "security in depth (SID)" refers to a layered security approach that employs multiple levels of protection across a system or network. This strategy recognizes that no single security measure is foolproof, and therefore it combines various security controls to address potential risks that could be exploited by an attacker. By implementing a series of defensive mechanisms at multiple layers—such as physical barriers, technical controls, and administrative policies—organizations can create a more resilient security posture.

This layered approach offers several benefits, including the ability to mitigate risks at different levels and to provide redundancy; if one layer fails, others remain in place to protect the system. It emphasizes the idea of not putting all security efforts into a single solution but spreading them across different areas, which enhances overall security effectiveness.