The answer is the KMS key policy does not grant the Flow Logs service principal permission to use the key. When troubleshooting cross-account VPC flow logs, the most overlooked detail is that the customer-managed KMS key in the security account must explicitly allow the Flow Logs service principal from the source accounts to perform the `kms:GenerateDataKey` and `kms:Decrypt` actions, even if the S3 bucket policy is correctly configured. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding that cross-account encryption requires a two-layer permission model: the bucket policy handles write access, but the key policy controls encryption rights, and the default key policy only grants access to the owning account. A common trap is assuming the bucket policy alone suffices or that CloudTrail is involved. Remember the memory tip: "Key first, bucket second" — always verify the KMS key policy before the S3 bucket policy when flow logs fail to deliver cross-account.
SCS-C02 Security Logging and Monitoring Practice Question
This SCS-C02 practice question tests your understanding of security logging and monitoring. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 uses AWS Organizations to manage multiple accounts. The security team needs to implement a centralized logging solution where all VPC Flow Logs from all accounts are sent to a central S3 bucket in the security account. The flow logs must be encrypted with a customer-managed KMS key (CMK) that is owned by the security account. The security engineer has enabled VPC Flow Logs in each account and configured the destination to be the central S3 bucket. However, the flow logs are not being delivered. The engineer checks the S3 bucket policy and confirms that it grants the required permissions to the Flow Logs service principal. What is the MOST likely cause of the failure?
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 KMS key policy does not grant the Flow Logs service principal permission to use the key.
Option C is correct because the KMS key policy must grant the Flow Logs service principal permission to use the key for encryption. Option A is wrong because VPC Flow Logs can write to cross-account buckets. Option B is wrong because the bucket policy is already set. Option D is wrong because CloudTrail is not involved in Flow Logs delivery.
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 VPC Flow Logs service does not support cross-account delivery.
Why it's wrong here
Cross-account delivery is supported.
✓
The KMS key policy does not grant the Flow Logs service principal permission to use the key.
Why this is correct
The key policy must allow the service principal to encrypt.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
CloudTrail must be enabled in the source account for Flow Logs to work.
Why it's wrong here
CloudTrail is not required for Flow Logs.
✗
The S3 bucket policy is missing a condition for source account.
Why it's wrong here
The bucket policy is correct according to the scenario.
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.
Trap categories for this question
Scenario analysis trap
The bucket policy is correct according to the scenario.
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 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.
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.
Security Logging and Monitoring — This question tests Security Logging and Monitoring — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The KMS key policy does not grant the Flow Logs service principal permission to use the key. — Option C is correct because the KMS key policy must grant the Flow Logs service principal permission to use the key for encryption. Option A is wrong because VPC Flow Logs can write to cross-account buckets. Option B is wrong because the bucket policy is already set. Option D is wrong because CloudTrail is not involved in Flow Logs delivery.
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.
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?
Static NAT maps one inside address to one outside address.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company uses AWS Organizations with multiple accounts. The security team wants to centralize VPC Flow Logs from all accounts into a single S3 bucket in the security account. The flow logs are created in the member accounts and sent to the centralized bucket. However, the security team notices that flow logs from some member accounts are not being delivered. What is the most likely cause?
medium
A.The member accounts need an IAM role with permissions to write to the centralized bucket.
B.CloudTrail must be enabled in each member account before VPC Flow Logs can be sent to a centralized bucket.
✓ C.The S3 bucket policy does not grant write permissions to the member accounts.
D.VPC Flow Logs cannot be aggregated across multiple AWS accounts.
Why C: Option B is correct because VPC Flow Logs require a resource-based policy on the destination bucket to allow the member accounts to write logs. Option A is wrong because flow logs can be aggregated across accounts. Option C is wrong because CloudTrail is not used for VPC Flow Logs. Option D is wrong because VPC Flow Logs do not require IAM roles on member accounts.
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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.
Question Discussion
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