The answer is that the network ACL is overriding the security group by denying inbound SSH from all IPs except the specific one. This happens because network ACLs are stateless and act as a firewall at the subnet level, evaluating traffic before it reaches the instance-level security group. Even though the security group allows SSH from 0.0.0.0/0, the NACL’s explicit deny rule for port 22 blocks all other source IPs, effectively overriding the more permissive security group rule. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the layered defense model, where NACLs provide a subnet-wide filter that can restrict traffic even when security groups are wide open. A common trap is forgetting that security groups are stateful and only support allow rules, while NACLs support both allow and deny rules—and a deny in the NACL always wins. Memory tip: “NACL denies first, security groups allow last.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
Option D is correct because a network ACL (NACL) can override security group rules if it denies inbound traffic. Option A is wrong because security groups are stateful; the inbound rule would allow traffic. 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.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 NACL allowed all traffic, SSH from any IP would work.
✗
The security group rule is being overridden by a more restrictive security group attached to the same instance
Why it's wrong here
Multiple security groups are additive; the permissive rule would still allow SSH from all IPs.
✗
The security group rule is being evaluated but the instance's operating system firewall is blocking SSH
Why it's wrong here
The OS firewall could block SSH, but the question asks for the most likely explanation given the security group allows all traffic.
✓
The network ACL denies inbound SSH from all IPs except the specific IP
Why this is correct
NACLs are stateless and can override security group rules by denying traffic at the subnet level.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL denies inbound SSH from all IPs except the specific IP — Option D is correct because a network ACL (NACL) can override security group rules if it denies inbound traffic. Option A is wrong because security groups are stateful; the inbound rule would allow traffic. 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.
What should I do if I get this SCS-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A security engineer is troubleshooting connectivity issues between two EC2 instances in the same VPC but different subnets. Both instances have security groups that allow all traffic from each other's security group. However, traffic is still blocked. What is the most likely cause?
medium
A.The instances are in different VPCs.
✓ B.The network ACL for one or both subnets is blocking the traffic.
C.The route tables do not have a route between the subnets.
D.VPC Flow Logs are not enabled.
Why B: Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the connection. Even if security groups allow traffic, a network ACL can block it. Option A is wrong because route tables handle routing, not filtering. Option B is wrong because VPC flow logs only monitor traffic, not block it. Option D is wrong because the default VPC has a default network ACL that allows all traffic, but custom ACLs may have restrictive rules.
Last reviewed: Jun 20, 2026
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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.
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