Defense in Depth (also known as Castle Approach) is an information assurance (IA) concept in which multiple layers of security controls (defense) are placed throughout an information technology (IT) system. Its intent is to provide redundancy in the event a security control fails or a vulnerability is exploited that can cover aspects of personnel, procedural, technical and physical for the duration of the system’s life cycle.

The idea behind the defense in depth approach is to defend a system against any particular attack using several independent methods.  It is a layering tactic, conceived by the National Security Agency (NSA) as a comprehensive approach to information and electronic security.

Defense in depth is originally a military strategy that seeks to delay, rather than prevent, the advance of an attacker by yielding space in order to buy time. The placement of protection mechanisms, procedures and policies is intended to increase the dependability of an IT system where multiple layers of defense prevent espionage and direct attacks against critical systems. In terms of computer network defense, defense in depth measures should not only prevent security breaches, but also buy an organization time to detect and respond to an attack, thereby reducing and mitigating the consequences of a breach.

Using more than one of the following layers constitutes defense in depth:

  • Anti virus software
  • Authentication and password security
  • Biometrics
  • Encryption
  • Firewalls (hardware or software)
  • Hashing passwords
  • Logging and auditing
  • Multi-factor authentication
  • Packet filters
  • Vulnerability scanners
  • Physical security (e.g. deadbolt locks)
  • Timed access control
  • Internet Security Awareness Training
  • Virtual private network (VPN)