SCS-C02 Infrastructure Security Practice Question
A security engineer is investigating a potential breach. The engineer notices that an EC2 instance's security group allows inbound SSH (port 22) from 0.0.0.0/0. The instance is in a public subnet and has a public IP address. However, the engineer finds that SSH access is only possible from a specific IP address. What is the most likely explanation?
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
✓
The network ACL denies inbound SSH from all IPs except the specific IP
A network ACL (NACL) can override security group rules if it denies inbound traffic. Option A is wrong because if the NACL allowed SSH from anywhere, then the restriction to a specific IP would not occur. Option B is wrong because if the NACL allowed traffic, it wouldn't explain the restriction. Option C is wrong because security group rules are evaluated; the rule allowing all traffic would still apply.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The network ACL allows inbound SSH from 0.0.0.0/0
Why it's wrong here
If the network ACL allowed inbound SSH from 0.0.0.0/0, it would permit all IPs, not restrict to a specific IP. The restriction must come from a deny rule in the NACL or another layer.
- ✗
The security group rule is being overridden by a more restrictive security group attached to the same instance
Why it's wrong here
Security groups are additive; a more restrictive security group does not override a permissive one. All rules are evaluated together, and any allow rule is sufficient to permit traffic.
- ✗
The security group rule is being evaluated but the instance's operating system firewall is blocking SSH
Why it's wrong here
While the OS firewall could block SSH, this is not the most likely explanation in AWS because network ACLs are a common mechanism for such restrictions and operate at the subnet level.
- ✓
The network ACL denies inbound SSH from all IPs except the specific IP
Why this is correct
Network ACLs are stateless and can explicitly deny traffic. If the NACL denies SSH from all IPs except the specific one, it would override the permissive security group rule, explaining why only that IP can connect.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services 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 SCS-C02 exam.