- A
Allow inbound HTTP traffic from the ALB's public IP address on the EC2 instances' security group.
Why wrong: The ALB's public IP can change, and this does not account for the ALB's private IP.
- B
Allow inbound HTTP traffic from the VPC CIDR range on the EC2 instances' security group.
Why wrong: This allows traffic from any resource in the VPC, not just the ALB.
- C
Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group.
This restricts traffic to only the ALB.
- D
Allow inbound HTTP traffic from 0.0.0.0/0 on the EC2 instances' security group.
Why wrong: This allows traffic from any source, not just the ALB.
Question 397 of 1,748
Restrict EC2 Traffic to ALB Only with Security Group Referencing
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 security engineer is designing a multi-tier web application. The application uses an Application Load Balancer (ALB) to distribute traffic to EC2 instances in private subnets. The engineer needs to ensure that the EC2 instances only accept traffic from the ALB and not from any other source. Which security group configuration should the engineer use?
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
✓
Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group.
Option C is correct because it uses a security group reference to allow inbound HTTP traffic from the ALB's security group. This ensures that only traffic originating from the ALB (which has that security group attached) can reach the EC2 instances, regardless of the ALB's IP addresses or changes in the VPC CIDR. This is the recommended AWS best practice for securing traffic between an ALB and backend instances.
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.
- ✗
Allow inbound HTTP traffic from the ALB's public IP address on the EC2 instances' security group.
Why it's wrong here
The ALB's public IP can change, and this does not account for the ALB's private IP.
- ✗
Allow inbound HTTP traffic from the VPC CIDR range on the EC2 instances' security group.
Why it's wrong here
This allows traffic from any resource in the VPC, not just the ALB.
- ✓
Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group.
Why this is correct
This restricts traffic to only the ALB.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Allow inbound HTTP traffic from 0.0.0.0/0 on the EC2 instances' security group.
Why it's wrong here
This allows traffic from any source, not just the ALB.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think they need to use the ALB's public IP address (Option A) or the VPC CIDR (Option B) as the source, not realizing that security group referencing is the correct and more secure method for allowing traffic from an ALB to backend instances.
Detailed technical explanation
How to think about this question
When you reference a security group as a source in another security group rule, AWS evaluates the rule based on the network interfaces' primary private IP addresses that are associated with the source security group. This means the ALB's elastic network interfaces (ENIs) in the VPC are the actual source, not the ALB's public IPs. This approach remains valid even if the ALB scales or its IP addresses change, because the security group reference is resolved dynamically by AWS.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Visual reference
What to study next
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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: Allow inbound HTTP traffic from the ALB's security group on the EC2 instances' security group. — Option C is correct because it uses a security group reference to allow inbound HTTP traffic from the ALB's security group. This ensures that only traffic originating from the ALB (which has that security group attached) can reach the EC2 instances, regardless of the ALB's IP addresses or changes in the VPC CIDR. This is the recommended AWS best practice for securing traffic between an ALB and backend instances.
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.
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
2 more ways this is tested on SCS-C02
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 uses an Application Load Balancer (ALB) to distribute traffic to a fleet of EC2 instances in private subnets. The security team wants to ensure that only the ALB can communicate with the EC2 instances. Which security group configuration should be applied to the EC2 instances?
medium- A.Allow inbound HTTP traffic from the EC2 instances' own security group
- B.Allow inbound HTTP traffic from 0.0.0.0/0
- C.Allow inbound HTTP traffic from the VPC CIDR block
- ✓ D.Allow inbound HTTP traffic from the ALB's security group
Why D: Option D is correct because security groups can reference other security groups as a source, allowing traffic only from resources associated with that security group. By specifying the ALB's security group as the source for inbound HTTP traffic, the EC2 instances will only accept traffic originating from the ALB, effectively restricting all other inbound traffic. This is a best practice for securing backend instances behind a load balancer.
Variation 2. A company is using an Application Load Balancer (ALB) to distribute traffic to a set of EC2 instances in private subnets. The security team wants to ensure that only traffic from the ALB can reach the EC2 instances. Which security group configuration should be applied to the EC2 instances?
easy- ✓ A.Allow inbound HTTP/HTTPS from the security group attached to the ALB.
- B.Configure the network ACL to allow traffic from the ALB's private IP addresses.
- C.Allow inbound HTTP/HTTPS from 0.0.0.0/0.
- D.Allow inbound HTTP/HTTPS from the VPC CIDR block.
Why A: Option A is correct because referencing the security group of the Application Load Balancer as the source in the inbound rule ensures that only traffic coming from the ALB can reach the EC2 instances. Option B is incorrect because network ACLs are stateless and cannot reference security groups; they also operate at the subnet level, not at the instance level. Option C is incorrect because allowing traffic from 0.0.0.0/0 would expose the instances to the internet. Option D is incorrect because allowing traffic from the VPC CIDR would permit any instance in the VPC to access the EC2 instances, not just the ALB.
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Last reviewed: Jul 4, 2026
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