Quick Answer
The answer is default deny. This is correct because the scenario describes a firewall that blocks all inbound traffic by default and then explicitly permits only web traffic on ports 80 and 443, which is the essence of the default deny security principle—anything not explicitly allowed is automatically rejected. On the ISC2 Certified in Cybersecurity CC exam, this concept tests your understanding of access control postures, often contrasting default deny with default allow; a common trap is confusing it with least privilege, though default deny is a specific implementation of that broader principle. A reliable memory tip is to think of a bouncer at a club: a default deny bouncer turns everyone away unless their name is on the VIP list, while a default allow bouncer lets everyone in until they cause trouble.
This CC practice question tests your understanding of security principles. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Default deny
Correct. The firewall denies everything by default.
Defense in depth
Why wrong: Defense in depth is about multiple layers, not default deny.
Fail-safe
Why wrong: Fail-safe ensures system remains secure on failure, not default deny.
Least privilege
Why wrong: Least privilege applies to user permissions, not network traffic.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Default deny
The correct answer is A, 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Default deny
Why this is correct
Correct. The firewall denies everything by default.
Related concept
Read the scenario before looking for a memorised answer.
Defense in depth
Why it's wrong here
Defense in depth is about multiple layers, not default deny.
Fail-safe
Why it's wrong here
Fail-safe ensures system remains secure on failure, not default deny.
Least privilege
Why it's wrong here
Least privilege applies to user permissions, not network traffic.
Common exam traps
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.
Detailed technical explanation
In practice, a default deny firewall rule is implemented as an implicit deny at the end of the access control list (ACL). For example, on a Cisco ASA or IOS router, the ACL ends with an implicit 'deny ip any any', meaning any packet not matching a prior permit statement is dropped. This is critical for preventing unauthorized inbound connections, such as blocking SSH (TCP 22) or RDP (TCP 3389) unless explicitly allowed. Real-world scenarios often involve misconfigurations where an overly permissive rule (e.g., 'permit ip any any') is placed before the deny, effectively bypassing the default deny posture.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
What to study next
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FAQ
Security Principles — This question tests Security Principles — Read the scenario before looking for a memorised answer..
The correct answer is: Default deny — The correct answer is A, 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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.
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