Defense in Depth

Defense in depth is the concept of using multiple layers of security to raise the cost and complexity of a successful attack.

To carry out a successful attack on a system, an attacker would need to find not just a single exploitable vulnerability, but would need to exploit vulnerabilities in each layer of defense that protects an asset.

For example, if a system is on a network protected only by a firewall, the attacker needs to circumvent only the firewall to gain unauthorized access. However, if there is an additional layer of defense such as a user name/password authentication requirement, the attacker needs to find a way to circumvent both the firewall and the user name/password authentication.