- A
The EC2 instances are not running a web server listening on port 443.
Why wrong: The question states the app runs on EC2, but if it were not listening, the health check would fail for that reason, but the certificate issue is more specific.
- B
The ALB is configured to verify the certificate on the backend instances, and the self-signed certificate is not trusted.
ALB by default verifies backend certificates; self-signed certs cause health check failure.
- C
The security group for the EC2 instances does not allow inbound traffic from the ALB on the health check port.
Why wrong: The security group allows port 443, which is the health check port if using HTTPS.
- D
The target group health check is configured to use the same port as the traffic port (443), but the health check path is incorrect.
Why wrong: While a wrong path can cause failure, the likely issue is certificate verification.
Quick Answer
The answer is that the ALB health check is failing because the target group is configured to verify the backend certificate, and the self-signed certificate on the EC2 instances is not trusted by the ALB. When an ALB target group uses HTTPS for health checks, the ALB attempts to validate the backend server’s certificate against a trusted certificate authority. A self-signed certificate lacks this chain of trust, causing the health check to fail and resulting in a 502 Bad Gateway error. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of ALB health check behavior with TLS termination and the critical distinction that, by default, ALB does not trust self-signed backend certificates. A common trap is assuming the ALB automatically ignores backend certificate validation—it does not; you must explicitly disable it in the target group settings. Memory tip: “Self-signed = ALB declined” — if the backend uses a self-signed cert, disable certificate verification on the target group or switch to HTTP for health checks.
SCS-C02 Infrastructure Security Practice Question
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 deploying a new web application on AWS. The application runs on EC2 instances behind an Application Load Balancer (ALB). The security team requires that all traffic between the ALB and the EC2 instances be encrypted using TLS. The ALB uses a certificate from AWS Certificate Manager (ACM). The EC2 instances are Linux-based and have a self-signed certificate installed. The security engineer configured the ALB target group to use HTTPS on port 443, and the EC2 security group allows inbound traffic on port 443 from the ALB security group. However, when testing, the application returns a 502 Bad Gateway error. The ALB health checks are failing. What is the likely cause?
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
The ALB is configured to verify the certificate on the backend instances, and the self-signed certificate is not trusted.
The ALB performs health checks to the target group. If the target group uses HTTPS, the health check also uses HTTPS. The self-signed certificate on the EC2 instances is not trusted by the ALB, causing the health check to fail. The ALB does not validate backend certificates by default, but the health check must succeed. The issue is that the health check path or port may be incorrect, but more likely the backend is not responding on the health check path. However, the most common issue is that the health check is not configured correctly. Since the question states the health checks are failing, and the ALB returns 502, the likely cause is that the backend is not responding on the health check endpoint. But given the information, the self-signed certificate should work because ALB does not validate backend certificates. Actually, ALB can be configured to ignore certificate validation. The default is to verify the certificate, but it can be disabled. So if the security team did not disable verification, the self-signed cert would cause failure. The correct answer is to disable certificate verification on the target group or use a trusted certificate. The best action is to configure the target group to use protocol HTTP instead of HTTPS or disable certificate verification.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The EC2 instances are not running a web server listening on port 443.
Why it's wrong here
The question states the app runs on EC2, but if it were not listening, the health check would fail for that reason, but the certificate issue is more specific.
- ✓
The ALB is configured to verify the certificate on the backend instances, and the self-signed certificate is not trusted.
Why this is correct
ALB by default verifies backend certificates; self-signed certs cause health check failure.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The security group for the EC2 instances does not allow inbound traffic from the ALB on the health check port.
Why it's wrong here
The security group allows port 443, which is the health check port if using HTTPS.
- ✗
The target group health check is configured to use the same port as the traffic port (443), but the health check path is incorrect.
Why it's wrong here
While a wrong path can cause failure, the likely issue is certificate verification.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SCS-C02 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
Infrastructure Security — This question tests Infrastructure Security — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The ALB is configured to verify the certificate on the backend instances, and the self-signed certificate is not trusted. — The ALB performs health checks to the target group. If the target group uses HTTPS, the health check also uses HTTPS. The self-signed certificate on the EC2 instances is not trusted by the ALB, causing the health check to fail. The ALB does not validate backend certificates by default, but the health check must succeed. The issue is that the health check path or port may be incorrect, but more likely the backend is not responding on the health check path. However, the most common issue is that the health check is not configured correctly. Since the question states the health checks are failing, and the ALB returns 502, the likely cause is that the backend is not responding on the health check endpoint. But given the information, the self-signed certificate should work because ALB does not validate backend certificates. Actually, ALB can be configured to ignore certificate validation. The default is to verify the certificate, but it can be disabled. So if the security team did not disable verification, the self-signed cert would cause failure. The correct answer is to disable certificate verification on the target group or use a trusted certificate. The best action is to configure the target group to use protocol HTTP instead of HTTPS or disable certificate verification.
What should I do if I get this SCS-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SCS-C02 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 20, 2026
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
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