Defense in depth

Description
The principle of defense-in-depth is that layered security mechanisms increase security of the system as a whole. If an attack causes one security mechanism to fail, other mechanisms may still provide the necessary security to protect the system. For example, it is not a good idea to totally rely on a firewall to provide security for an internal-use-only application, as firewalls can usually be circumvented by a determined attacker (even if it requires a physical attack or a social engineering attack of some sort). Other security mechanisms should be added to complement the protection that a firewall affords (e.g., surveillance cameras, and security awareness training) that address different attack vectors.

Implementing a defense-in-depth strategy can add to the complexity of an application, which runs counter to the “simplicity” principle often practiced in security. That is, one could argue that adding new protection functionality adds additional complexity that might bring new risks with it. The total risk to the system needs to be weighed. For example, an application with username/password-based authentication may not benefit from increasing the required password length from eight characters to 15 characters as the added complexity may result in users writing their passwords down, thus decreasing the overall security to the system; however, adding a smart-card requirement to authenticate to the application would enhance the security of the application by adding a complementary layer to the authentication process.

Vulnerable Administrative Interface

 * A failed authentication control to the Administrative Interface is unlikely to be enable to anonymous attack on the system as a whole if it correctly gates access to production management networks, checks for administrative user authorization, and logs all access.

Related Controls
The Principle of Defense-in-Depth does not relate to a particular control or subset of controls. It is a design principle to guide the selection of controls for an application to ensure its resilience against different forms of attack, and to reducce to probability of a single-point of failure in the security of the system.