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ISC2 CC Practice Question: Is configuring a firewall to allow web traffic…
A security engineer is configuring a firewall to allow web traffic but block all other inbound connections. The firewall is set to deny all traffic by default and only allow specific ports. Which security principle is being applied?
⚠ Common exam trap
ISC2 often tests the distinction between 'default deny' (a firewall ACL posture) and 'least privilege' (a user/process access control model), causing candidates to confuse network-level traffic filtering with user-level permissions.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
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Default deny
Default deny. The scenario describes a firewall configured to deny all traffic by default and then explicitly allow only specific ports (e.g., TCP 80/443 for web traffic). This directly implements the default deny security principle, where any traffic not explicitly permitted is blocked. This contrasts with a default allow posture, which would permit all traffic unless explicitly denied.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Default deny
Why this is correct
Default deny enforces an implicit drop for all traffic not explicitly permitted, so only the specified web ports are reachable. This directly satisfies the stem's constraint of blocking every other inbound connection while allowing web traffic, since the firewall's baseline posture rejects anything unmatched.
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Defense in depth
Why it's wrong here
Defense in depth layers multiple independent controls; a single firewall with a default-deny rule is one control, not layered defence. It is tempting because firewalls often form one layer, and defense in depth is correct when several overlapping safeguards protect the same asset.
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Fail-safe
Why it's wrong here
Fail-safe describes a system defaulting to a safe state on failure, not the allowlist rule itself. It is tempting because deny-by-default firewalls resemble fail-safe behaviour, and fail-safe is the correct principle when a component must revert to a secure condition after malfunction.
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Least privilege
Why it's wrong here
Least privilege restricts each identity to the minimum permissions needed, governing user and service access rights rather than inbound port filtering. It is tempting because both limit access, and least privilege is correct when scoping IAM roles or service accounts to specific actions.
Go deeper
Related to this question
Key term
TCP
TCP is a connection-oriented transport layer protocol that ensures reliable, ordered, and error-checked delivery of data between applications over IP networks.
Key term
Firewall
A firewall is a network security system that monitors and controls incoming and outgoing traffic based on predetermined security rules to protect trusted internal networks from untrusted external networks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CC practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CC exam.