- A
NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
Stateless NACLs require both inbound and outbound rules. Missing outbound for ephemeral ports will block return traffic even if SG rules are correct.
- B
NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.
Why wrong: NACLs are stateless; connection tracking occurs at the security group level only. Adding SG inbound rules won’t unblock outbound traffic blocked by the NACL.
- C
The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.
Why wrong: Health checks may affect routing, but the symptom points to NACL blocking return packets. Changing health check ports won’t address NACL stateless behavior.
- D
Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.
Why wrong: NAT affects internet egress, not east-west return paths inside the VPC subnet. The observed NACL rule gap is within the subnet path.
Quick Answer
The answer is that the missing outbound NACL rule for ephemeral ports causes connection timeouts because NACLs are stateless. Unlike security groups, which automatically track and allow return traffic, a stateless NACL requires explicit rules for both inbound and outbound traffic. Even though the security group correctly allows inbound HTTPS from the ALB and outbound ephemeral ports, the NACL blocks the server’s response traffic since it lacks an outbound allow rule for the ephemeral port range (1024–65535). On the SAA-C03 exam, this is a classic trap: candidates often focus only on security group configurations and forget that NACLs operate independently and are evaluated first. The key distinction is that security groups are stateful, while NACLs are stateless—a missing outbound rule in the NACL will silently drop return packets, leading to timeouts. Memory tip: think of NACLs as a bouncer that needs a written list for both entering and leaving the club; if the exit list is missing, no one can leave, even if the entrance list is perfect.
SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. 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.
An application runs on EC2 instances in private subnets behind an Application Load Balancer (ALB). Security groups allow inbound HTTPS (443) from the ALB’s security group to the instance security group, and outbound from instances is set to allow ephemeral ports.
Despite this, clients see connection timeouts. After reviewing network ACLs, you find the NACL associated with the instance subnet has an inbound allow for destination port 443, but it does not have a corresponding outbound allow for ephemeral ports.
What is the most likely reason the traffic fails, and what should be updated?
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 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
NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
Network ACLs are stateless, meaning they do not automatically allow return traffic. Even though the security group allows inbound HTTPS from the ALB, the NACL blocks the response traffic because it lacks an outbound rule for ephemeral ports (typically 1024-65535). Since NACLs are evaluated before security groups, a missing outbound allow rule causes the connection to time out.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
Why this is correct
Stateless NACLs require both inbound and outbound rules. Missing outbound for ephemeral ports will block return traffic even if SG rules are correct.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.
Why it's wrong here
NACLs are stateless; connection tracking occurs at the security group level only. Adding SG inbound rules won’t unblock outbound traffic blocked by the NACL.
- ✗
The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.
Why it's wrong here
Health checks may affect routing, but the symptom points to NACL blocking return packets. Changing health check ports won’t address NACL stateless behavior.
- ✗
Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.
Why it's wrong here
NAT affects internet egress, not east-west return paths inside the VPC subnet. The observed NACL rule gap is within the subnet path.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume security groups alone handle all traffic filtering, forgetting that NACLs are stateless and require explicit outbound rules for return traffic, especially for ephemeral ports.
Detailed technical explanation
How to think about this question
Network ACLs operate at the subnet level and are evaluated in order by rule number, with no connection tracking. For TCP traffic, the response packets use a random ephemeral source port (typically 1024-65535) and must be explicitly allowed in the outbound NACL rule. In contrast, security groups are stateful and automatically allow return traffic, but they cannot override a blocked NACL because NACLs are evaluated first for inbound and last for outbound traffic.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
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FAQ
Questions learners often ask
What does this SAA-C03 question test?
Design Secure Architectures — This question tests Design Secure Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL. — Network ACLs are stateless, meaning they do not automatically allow return traffic. Even though the security group allows inbound HTTPS from the ALB, the NACL blocks the response traffic because it lacks an outbound rule for ephemeral ports (typically 1024-65535). Since NACLs are evaluated before security groups, a missing outbound allow rule causes the connection to time out.
What should I do if I get this SAA-C03 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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 →
Same concept, more angles
1 more ways this is tested on SAA-C03
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. An application runs on EC2 instances in private subnets behind an Application Load Balancer (ALB). Security groups allow inbound HTTPS (443) from the ALB’s security group to the instance security group, and outbound from instances is set to allow ephemeral ports. Despite this, clients see connection timeouts. After reviewing network ACLs, you find the NACL associated with the instance subnet has an inbound allow for destination port 443, but it does not have a corresponding outbound allow for ephemeral ports. What is the most likely reason the traffic fails, and what should be updated?
medium- ✓ A.NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
- B.NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.
- C.The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.
- D.Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.
Why A: Network ACLs are stateless, meaning they do not automatically allow return traffic. Even though the security group allows inbound HTTPS from the ALB, the NACL blocks the return traffic because it lacks an outbound rule for ephemeral ports (typically 1024-65535). Since NACLs are evaluated before security groups, a missing outbound allow rule causes the response packets to be dropped, resulting in connection timeouts.
Last reviewed: Jun 11, 2026
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