The answer is that the security group on the Maven repository server does not allow inbound HTTPS from the CodeBuild security group. While security groups are stateful, meaning that if you allow outbound traffic the return traffic is automatically permitted, this statefulness only applies to the same security group’s own rules—it does not automatically create inbound rules on a separate resource’s security group. The CodeBuild environment’s outbound HTTPS rule correctly initiates the connection, but the Maven repository server’s security group must explicitly permit inbound HTTPS from the CodeBuild security group to accept that traffic; without it, the server rejects the connection, resulting in the “Connection refused” error. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of the difference between stateful security groups and stateless network ACLs, as well as the common trap of checking only one side of a two-way rule. A helpful memory tip is “outbound initiates, inbound accepts”—always verify both the source’s outbound and the destination’s inbound rules when troubleshooting connectivity between VPC resources.
DOP-C02 Security and Compliance Practice Question
This DOP-C02 practice question tests your understanding of security and compliance. 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 AWS CodeBuild to build and test a Java application. The build process requires access to a private Maven repository hosted on an internal HTTPS server. The DevOps engineer has configured CodeBuild to use a VPC and placed the build environment in a private subnet. The security group for the build environment allows outbound HTTPS to the Maven repository's security group. The Maven repository server is in the same VPC but in a different private subnet. The build fails with a 'Connection refused' error when trying to download dependencies. The engineer checks the security group rules and confirms they are correct. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 on the Maven repository server does not allow inbound HTTPS from the CodeBuild security group.
The Maven repository server likely has a security group that does not allow inbound HTTPS from the CodeBuild security group. The engineer only checked outbound rule on CodeBuild side, but need inbound rule on the server side. Option A is correct. Option B is about Route53, not relevant. Option C is about NACL, which is stateless, but security group is stateful, so outbound rule alone is not enough. Option D is about the buildspec being incorrect, but the error is connection refused, which suggests network issue.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 private DNS resolution is failing because the VPC does not have Route53 Resolver enabled.
Why it's wrong here
Connection refused suggests the server is reachable but not listening.
✗
The CodeBuild buildspec is using the wrong URL for the Maven repository.
Why it's wrong here
Would cause 404 or host not found, not connection refused.
✓
The security group on the Maven repository server does not allow inbound HTTPS from the CodeBuild security group.
Why this is correct
Inbound rules are needed for the server to accept connections.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL on the private subnets is blocking outbound ephemeral ports.
Why it's wrong here
NACLs are stateless; if outbound is blocked, it would be timeout, not connection refused.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Security and Compliance — This question tests Security and Compliance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group on the Maven repository server does not allow inbound HTTPS from the CodeBuild security group. — The Maven repository server likely has a security group that does not allow inbound HTTPS from the CodeBuild security group. The engineer only checked outbound rule on CodeBuild side, but need inbound rule on the server side. Option A is correct. Option B is about Route53, not relevant. Option C is about NACL, which is stateless, but security group is stateful, so outbound rule alone is not enough. Option D is about the buildspec being incorrect, but the error is connection refused, which suggests network issue.
What should I do if I get this DOP-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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