- A
Add a rule to the target instances' security group to allow inbound traffic from 0.0.0.0/0 on the health check port.
Why wrong: This would allow all inbound traffic, which is insecure and not necessary.
- B
Add a rule to the target instances' security group to allow inbound traffic from the VPC CIDR on the health check port.
ALB health checks come from the VPC CIDR.
- C
Add a rule to the ALB's security group to allow inbound traffic from the internet on the health check port.
Why wrong: Health checks originate from the VPC, not the internet.
- D
Add a rule to the ALB's security group to allow outbound traffic to the target instances on the health check port.
Why wrong: Security groups are stateful; outbound traffic is automatically allowed if inbound is allowed.
Fix ALB Health Check Failures: Allow VPC CIDR in Security Group
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 is using an Application Load Balancer (ALB) to distribute traffic to EC2 instances in a VPC. The Security Engineer notices that the ALB health checks are failing. Which configuration change should the Engineer make to resolve the issue?
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
Add a rule to the target instances' security group to allow inbound traffic from the VPC CIDR on the health check port.
The ALB health checks originate from private IP addresses within the VPC subnet, not from the internet. Therefore, the target instances' security group must allow inbound traffic from the VPC CIDR (or the ALB subnet CIDR) on the health check port. Option B correctly configures this, enabling the ALB to successfully reach the health check endpoint on the EC2 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.
- ✗
Add a rule to the target instances' security group to allow inbound traffic from 0.0.0.0/0 on the health check port.
Why it's wrong here
This would allow all inbound traffic, which is insecure and not necessary.
- ✓
Add a rule to the target instances' security group to allow inbound traffic from the VPC CIDR on the health check port.
Why this is correct
ALB health checks come from the VPC CIDR.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add a rule to the ALB's security group to allow inbound traffic from the internet on the health check port.
Why it's wrong here
Health checks originate from the VPC, not the internet.
- ✗
Add a rule to the ALB's security group to allow outbound traffic to the target instances on the health check port.
Why it's wrong here
Security groups are stateful; outbound traffic is automatically allowed if inbound is allowed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates mistakenly think ALB health checks come from the internet or that modifying the ALB's security group inbound rules will fix the issue, when in fact the target instances' security group must permit inbound traffic from the VPC CIDR on the health check port.
Detailed technical explanation
How to think about this question
ALB health checks are sent from the ALB's elastic network interfaces (ENIs) in the VPC subnets, using the private IP addresses of those ENIs. The health check traffic uses the HTTP or HTTPS protocol (or TCP) on the configured port, and the target must respond with a 200 OK status. In a multi-VPC or peered setup, you must allow the ALB's subnet CIDR, not just the VPC CIDR, to ensure connectivity.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: Add a rule to the target instances' security group to allow inbound traffic from the VPC CIDR on the health check port. — The ALB health checks originate from private IP addresses within the VPC subnet, not from the internet. Therefore, the target instances' security group must allow inbound traffic from the VPC CIDR (or the ALB subnet CIDR) on the health check port. Option B correctly configures this, enabling the ALB to successfully reach the health check endpoint on the EC2 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.
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Last reviewed: Jul 4, 2026
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