- 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.
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.
When this WOULD be correct
This option would be correct if the question stated that NACLs are stateful and the security group was blocking return traffic. For example: 'An application uses a stateful firewall (like a security group) and clients see timeouts; what is the fix?'
- ✗
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.
When this WOULD be correct
In a scenario where an ALB target group health check is failing because the health check path or port is not configured correctly (e.g., the application listens on port 8080 but health check targets port 80), updating the health check to the correct port would resolve the issue.
- ✗
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.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The SAA-C03 exam frequently reuses these exact scenarios with slightly different constraints.
✓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.Correct answer▾
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.
✗NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.Wrong answer — click to see why▾
Why this is wrong here
NACLs are stateless, not stateful; they do not automatically track connections. The issue is missing outbound ephemeral port rules in the NACL, not security group inbound rules for client source ports.
★ When this WOULD be the correct answer
This option would be correct if the question stated that NACLs are stateful and the security group was blocking return traffic. For example: 'An application uses a stateful firewall (like a security group) and clients see timeouts; what is the fix?'
Why candidates choose this
Candidates may confuse NACLs with security groups, assuming NACLs are stateful like security groups, and think the fix involves adding inbound rules to the security group for client ports.
✗The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.Wrong answer — click to see why▾
Why this is wrong here
The question describes connection timeouts due to missing outbound NACL rules for ephemeral ports, not health check failures. ALB health checks are not mentioned as failing, and changing the health check port does not address the stateless NACL issue.
★ When this WOULD be the correct answer
In a scenario where an ALB target group health check is failing because the health check path or port is not configured correctly (e.g., the application listens on port 8080 but health check targets port 80), updating the health check to the correct port would resolve the issue.
Why candidates choose this
Candidates may confuse connectivity issues with health check misconfigurations, especially when ALB is involved, and assume that adjusting health check settings can fix general traffic flow problems.
✗Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.Wrong answer — click to see why▾
Why this is wrong here
The issue is not about internet connectivity; instances are in private subnets but the ALB is in a public subnet and handles internet-facing traffic. A NAT gateway is for outbound internet access from private instances, not for fixing return traffic blocked by a stateless NACL.
★ When this WOULD be the correct answer
In a scenario where EC2 instances in private subnets need to download updates from the internet, and the outbound NACL blocks ephemeral ports, adding a NAT gateway would be correct to allow outbound traffic and its return traffic.
Why candidates choose this
Candidates may confuse network connectivity issues with internet access requirements, assuming private subnets always need a NAT gateway for any traffic flow, even when the traffic is internal to the VPC via an ALB.
Analysis generated from the official SAA-C03blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Visual reference
What to study next
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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.
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Last reviewed: Jun 11, 2026
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