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Security Fundamentals & Crypto · Foundational

Implicit Deny

Default rule that blocks any traffic or access not explicitly permitted.

Definition

Implicit deny (default deny) is placed at the end of firewall rule sets and ACLs, so anything that does not match an allow rule is dropped. It embodies least privilege for network access. Many platforms make it invisible, so logging requires an explicit deny-all rule.

Example

Cisco ACLs end with an invisible 'deny ip any any'.

Exam tip

If no rule matches, traffic is denied; add an explicit deny-all rule with logging to see it.

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