The correct answer is that the bucket policy lacks a condition using `aws:SourceArn` or `aws:SourceAccount` to prevent the confused deputy problem. In cross-account delivery, VPC Flow Logs rely on a service-linked role in the member account to write to the central S3 bucket, but without these source conditions, the bucket policy is vulnerable to a confused deputy scenario where an unintended AWS service or account could impersonate the flow logs service and gain write access. This is a classic trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam, testing your understanding of how to secure cross-account resource policies against privilege escalation. Many candidates mistakenly focus on IAM roles or VPC endpoints, but the core issue is that the flow logs delivery service cannot prove its identity to the bucket without the source condition. Memory tip: think of the condition as a “return address” on an envelope—without it, the bucket can’t verify who sent the package.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 security engineer is designing a network security architecture for a multi-account AWS environment using AWS Organizations. The company requires that all VPC flow logs be delivered to a central S3 bucket in the security account. The security engineer has created a bucket policy that grants the necessary permissions. However, flow logs from member accounts are failing to be delivered. 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 bucket policy does not include a condition that restricts access to the flow log delivery service using aws:SourceArn or aws:SourceAccount.
Option B is correct because the VPC Flow Logs delivery service uses a service-linked role to write to the destination S3 bucket. Without a condition in the bucket policy that restricts access using `aws:SourceArn` or `aws:SourceAccount`, the policy is vulnerable to the confused deputy problem, where a different AWS service or account could potentially write to the bucket. The missing condition causes the delivery to fail because the service cannot verify that the request originated from the expected source.
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.
✗
The member accounts have not created an IAM role with permissions to write to the central bucket.
Why it's wrong here
VPC Flow Logs use a service-linked role, not an IAM role in member accounts.
✓
The bucket policy does not include a condition that restricts access to the flow log delivery service using aws:SourceArn or aws:SourceAccount.
Why this is correct
This condition is essential to prevent the confused deputy problem.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The bucket policy does not grant write access to the member accounts' root user.
Why it's wrong here
The bucket policy should grant access to the VPC Flow Logs service principal, not the member account root.
✗
The central S3 bucket has not been configured with ACLs enabled.
Why it's wrong here
ACLs are not required; bucket policy is used.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the confused deputy problem in cross-account service delivery scenarios, and the trap here is that candidates mistakenly focus on IAM roles or root user permissions instead of recognizing that the bucket policy must include a source condition to prevent unauthorized access.
Detailed technical explanation
How to think about this question
The confused deputy problem is mitigated by including the `aws:SourceArn` condition with the ARN of the flow log (e.g., `arn:aws:ec2:region:account-id:vpc-flow-log/flow-log-id`) or `aws:SourceAccount` with the member account ID. Without this, the bucket policy would allow any VPC Flow Logs service in any account to write to the bucket, which is a security risk and causes AWS to reject the policy as insufficiently scoped. In practice, the bucket policy must explicitly allow the `logs:PutObject` action for the service principal `delivery.logs.amazonaws.com` with the source condition to pass the security validation.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The bucket policy does not include a condition that restricts access to the flow log delivery service using aws:SourceArn or aws:SourceAccount. — Option B is correct because the VPC Flow Logs delivery service uses a service-linked role to write to the destination S3 bucket. Without a condition in the bucket policy that restricts access using `aws:SourceArn` or `aws:SourceAccount`, the policy is vulnerable to the confused deputy problem, where a different AWS service or account could potentially write to the bucket. The missing condition causes the delivery to fail because the service cannot verify that the request originated from the expected source.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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