Terminology Index

Glossary

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32

Default Deny

A security principle of blocking everything by default and explicitly allowing only what is needed, applied to firewalls, access control, and network policy. It fails safe, since anything not expressly permitted is denied.

Default deny inverts the risky 'allow unless blocked' posture: the baseline is to refuse, and each permitted flow or access must be explicitly added. In networking it means firewall and policy rules permit only required traffic and drop the rest; in access control it means no access unless granted. This fail-safe stance dramatically shrinks what an attacker can reach and is a cornerstone of segmentation and least privilege.

Introduced in: Networking Foundations

Examples

  • A firewall that drops all traffic except explicitly allowed flows.
  • Network policies permitting only required service-to-service connections.
  • Access defaulting to none until a permission is explicitly granted.