CV0-004 Security Practice Question
An organization uses AWS and wants to control inbound traffic to its EC2 instances. They need a solution that automatically allows response traffic for any permitted inbound request. Which of the following should they use?
⚠ Common exam trap
The trap is confusing security groups with network ACLs; candidates often pick NACLs because they sound like firewalls, but only security groups are stateful and automatically allow return traffic for permitted inbound requests.
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
✓
Security groups
AWS security groups are stateful virtual firewalls that automatically allow return traffic for any permitted inbound request, regardless of outbound rules. This stateful behavior means that if an inbound request is allowed, the response is automatically permitted, satisfying the requirement. Network ACLs, by contrast, are stateless and require explicit rules for both directions.
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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DDoS protection
Why it's wrong here
DDoS protection absorbs or mitigates volumetric and protocol attacks against availability; it does not evaluate per-connection inbound rules or track connection state to permit response traffic. It would be the correct choice when the requirement is defending against distributed denial-of-service floods rather than controlling instance traffic.
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Web Application Firewall
Why it's wrong here
A Web Application Firewall inspects HTTP/S requests at layer 7 and blocks application-layer exploits; it does not manage stateful connection tracking for arbitrary inbound traffic to EC2 instances. It would be correct when the requirement is filtering malicious web requests, such as SQL injection or cross-site scripting.
- ✓
Security groups
Why this is correct
Security groups are stateful: they automatically permit return traffic for any inbound connection you allow, so no separate outbound rule is needed. This satisfies the requirement that response traffic be allowed automatically. Network ACLs are stateless and would require explicit inbound and outbound rules.
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Network ACLs
Why it's wrong here
Network ACLs are stateless, so response traffic must be explicitly permitted by separate inbound and outbound rules; they do not automatically allow return traffic. They suit subnet-level stateless filtering, whereas the scenario requires stateful security groups that track connections and permit responses automatically.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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