Defense in Depth

Defense in depth (also called layered defense) is one of the major concept of security. It is used to avoid single point of failure or compromise.

Layered defense is putting a series of challenges a user has to overcome before they got to something of value. That can take the form of input validation, a process check, output validation and of course access permissions.

But the checks must be layered, i.e. there shouldn’t be only one check to protect the key to the kingdom.

Layers of defense can be put on hardware, network, database, and virtual machines.

A principle behind defense in depth is the segmentation and separation of concerns, or security zones.

Security zones can be physical (constrained environment or user interface) or logical (pre-selected items on a menu, separation of environments, virtual machines etc).

Diversity of defense is another way to provide defense in depth: through geographic diversity (redundancy in a separate country), distributed computing, supply chain diversity (not relying on only one vendor).

Technical diversity is good when using multiple vendor but it can creates compatibility and support issues.

Key points: Defense in depth ensures that layers of protection are used to protect key assets. Controls are in serial, not in parallel, and acts as hurdles that an attacker must overcome.