DOP-C02 Security and Compliance Practice Question
A company runs a web application on EC2 instances behind an Application Load Balancer (ALB). The security team requires that all traffic to the ALB must be encrypted (HTTPS) and that the ALB must only accept traffic from CloudFront. The DevOps engineer has configured CloudFront with an origin pointing to the ALB, and the ALB has a listener on port 443 with a valid SSL certificate. The engineer also added a security group rule to the ALB that allows HTTPS traffic only from CloudFront's IP ranges. However, users are reporting intermittent 503 errors. The engineer checks CloudFront logs and sees that some requests are failing with 'Origin Connect Error'. What is the most 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The security group rule is using an outdated list of CloudFront IP ranges, and CloudFront has added new IP ranges that are being blocked.
The most likely cause is that the security group rule is using an outdated list of CloudFront IP ranges. CloudFront's IP ranges are dynamic and change over time; AWS publishes these ranges but they are not static. When a new range is added that is not in the security group rule, requests from those IPs are blocked, leading to 'Origin Connect Error' and intermittent 503 errors. The correct approach is to use a custom header (e.g., X-Origin-Verify) from CloudFront and validate it at the ALB, rather than relying on IP-based security groups. Option A (WAF blocking) would typically result in 403 Forbidden, not 503. Option C (SSL certificate) would cause handshake failures with a different error. Option D (idle timeout) would cause consistent timeout errors, not intermittent ones.
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 ALB has a Web Application Firewall (WAF) that is blocking requests from CloudFront.
Why it's wrong here
A WAF blocking requests would typically return a 403 Forbidden, not a 503 Origin Connect Error.
- ✓
The security group rule is using an outdated list of CloudFront IP ranges, and CloudFront has added new IP ranges that are being blocked.
Why this is correct
Correct. The security group rule using an outdated list of CloudFront IP ranges can cause intermittent 503 errors when CloudFront adds new IPs that are blocked.
- ✗
The SSL certificate on the ALB is not trusted by CloudFront, causing handshake failures.
Why it's wrong here
SSL certificate handshake failures would result in different errors, not 503 Origin Connect Error.
- ✗
The ALB idle timeout is set too low, causing CloudFront to close connections prematurely.
Why it's wrong here
An excessively low idle timeout would cause consistent timeout errors, not intermittent 503s.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DOP-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 DOP-C02 exam.