The answer is that the NAT Gateway’s security group is blocking outbound traffic to the internet. While the security group correctly allows inbound HTTPS from the private subnet CIDR, it must also permit outbound HTTPS to 0.0.0.0/0 for the return traffic from the internet to reach the NAT Gateway; without this outbound rule, the gateway drops the response packets, and the EC2 instance never receives the patches. On the AWS Certified Security Specialty SCS-C02 exam, this question tests your understanding that a NAT Gateway, like any resource in a VPC, requires both inbound and outbound security group rules to function—a common trap is focusing only on inbound rules and forgetting that stateful security groups still need explicit outbound rules for the gateway’s own traffic. Remember the mnemonic: “NAT needs out to get back in.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The instance is associated with a security group that allows outbound HTTPS (port 443) to 0.0.0.0/0. The private subnet route table has a default route (0.0.0.0/0) pointing to a NAT Gateway in the public subnet. The NAT Gateway's security group allows inbound HTTPS from the private subnet CIDR. However, the instance cannot download patches. What is the most likely cause?
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 NAT Gateway's security group is blocking outbound traffic to the internet
NATGateway's security group must allow inbound HTTPS from the private subnet CIDR. The problem statement says it does, so that is not the issue. The most likely issue is that the NAT Gateway's security group must also allow outbound HTTPS to 0.0.0.0/0 for the return traffic. Option D is correct. Option A is wrong because NACL is stateless but default NACL allows all. Option B is wrong because the security group allows outbound. Option C is wrong because the route is present.
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 for the private subnet is blocking outbound traffic
Why it's wrong here
Default NACL allows all outbound; custom NACL would need explicit allow.
✓
The NAT Gateway's security group is blocking outbound traffic to the internet
Why this is correct
NATGateway's security group must allow outbound HTTPS to 0.0.0.0/0.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group attached to the EC2 instance is blocking inbound traffic
Why it's wrong here
Security group is stateful; outbound rule allows return traffic.
Common exam traps
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 NAT Gateway's security group is blocking outbound traffic to the internet — NAT Gateway's security group must allow inbound HTTPS from the private subnet CIDR. The problem statement says it does, so that is not the issue. The most likely issue is that the NAT Gateway's security group must also allow outbound HTTPS to 0.0.0.0/0 for the return traffic. Option D is correct. Option A is wrong because NACL is stateless but default NACL allows all. Option B is wrong because the security group allows outbound. Option C is wrong because the route is present.
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
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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 company has a VPC with public and private subnets. The private subnets need to access the internet for software updates. The security engineer has set up a NAT gateway in a public subnet and updated the route tables accordingly. However, instances in the private subnets cannot reach the internet. The engineer checks the security group for the NAT gateway and finds that it allows all outbound traffic. What is the most likely cause of the issue?
hard
A.The route table for the private subnet does not have a default route (0.0.0.0/0) pointing to the NAT gateway.
B.The NAT gateway does not have an Elastic IP address assigned.
✓ C.The security group for the NAT gateway does not allow inbound traffic from the private subnets.
D.The network ACL for the private subnet does not allow inbound HTTP/HTTPS traffic.
Why C: Option D is correct because the NAT gateway's security group must allow inbound traffic from the private subnets to receive return traffic. Option A is incorrect because the NAT gateway automatically responds. Option B is incorrect because network ACLs are stateless and require inbound rules for return traffic. Option C is incorrect because the private subnets' route table must have a route to the NAT gateway.
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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