- A
Configure the instances' security group to allow traffic from the ALB's security group.
This ensures only traffic that passes through the ALB can reach the instances.
- B
Configure the instances' security group to allow traffic from the client's source IP addresses.
Why wrong: This would allow direct traffic bypassing the ALB.
- C
Configure a network ACL on the subnet to allow traffic from the ALB's private IP addresses.
Why wrong: Network ACLs are stateless and would require managing IP changes.
- D
Assign an IAM role to the instances that allows traffic only from the ALB.
Why wrong: IAM roles do not control network traffic.
Question 52 of 1,748
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 hosts a web application on EC2 instances behind an Application Load Balancer. The security team wants to ensure that only traffic from the ALB can reach the EC2 instances. Which configuration should be applied?
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
✓
Configure the instances' security group to allow traffic from the ALB's security group.
Option A is correct because security groups support referencing other security groups as a source. By configuring the EC2 instances' security group to allow inbound traffic from the ALB's security group, only traffic originating from the ALB (which uses the ALB's security group) is permitted. This ensures that traffic from any other source, including direct internet traffic, is blocked at the instance level.
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.
- ✓
Configure the instances' security group to allow traffic from the ALB's security group.
Why this is correct
This ensures only traffic that passes through the ALB can reach the instances.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Configure the instances' security group to allow traffic from the client's source IP addresses.
Why it's wrong here
This would allow direct traffic bypassing the ALB.
- ✗
Configure a network ACL on the subnet to allow traffic from the ALB's private IP addresses.
Why it's wrong here
Network ACLs are stateless and would require managing IP changes.
- ✗
Assign an IAM role to the instances that allows traffic only from the ALB.
Why it's wrong here
IAM roles do not control network traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse network ACLs (stateless, IP-based) with security groups (stateful, group-based) and mistakenly choose option C, not realizing that security group referencing is the correct and more secure method for this scenario.
Detailed technical explanation
How to think about this question
Security group referencing works by using the ENI (Elastic Network Interface) of the ALB as the source, which is dynamically updated as the ALB scales. This approach is more resilient than using IP-based rules because ALB IP addresses can change due to scaling or replacement events. Under the hood, AWS translates the security group reference into the private IP addresses of the ALB's ENIs, ensuring only traffic from those specific interfaces is allowed.
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 SCS-C02 question test?
Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure the instances' security group to allow traffic from the ALB's security group. — Option A is correct because security groups support referencing other security groups as a source. By configuring the EC2 instances' security group to allow inbound traffic from the ALB's security group, only traffic originating from the ALB (which uses the ALB's security group) is permitted. This ensures that traffic from any other source, including direct internet traffic, is blocked at the instance level.
What should I do if I get this SCS-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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