Defense in depth is a security strategy in which multiple security tools, mechanisms, and policies are deployed in tandem on the assumption that if one fails, another will hold. Rather than, say, relying solely on a firewall to keep hackers out of a corporate network, an organization would also deploy endpoint security software and intrusion detection systems (IDS) to spot any attacker who manages to slip past that firewall. The intention isn’t to deploy different tools to face different specific threats: rather, a defense in depth strategy assumes that an attacker manages to defeat or bypass one tool, then other tools will pick up the slack and fight back in a different way.
