The correct answer is a bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption-aws-kms-key-id' header matching the desired KMS key ID. This works because S3 bucket policies can use the s3:x-amz-server-side-encryption-aws-kms-key-id condition key to enforce that only objects encrypted with a specific customer-managed KMS key are accepted, effectively blocking any unencrypted uploads or those using a different key. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of combining S3 bucket policies with KMS to meet security compliance for data lakes, a common requirement for ML workloads handling sensitive data. A frequent trap is confusing the condition key for SSE-KMS with the simpler s3:x-amz-server-side-encryption key used for SSE-S3, or forgetting that the policy must explicitly deny when the required header is missing. Memory tip: think “Deny unless KMS key ID matches” — the policy says “Deny” when the header is absent or wrong, not “Allow” when it’s present.
MLS-C01 Data Engineering Practice Question
This MLS-C01 practice question tests your understanding of data engineering. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 data engineering team is designing a data lake on Amazon S3. They need to enforce encryption at rest for all data stored in the bucket. The security policy requires that the encryption keys be managed by the organization using AWS Key Management Service (KMS), and that the bucket must deny uploads of unencrypted objects. Which bucket policy should be applied?
A bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption' header with value 'AES256'
Why wrong: This enforces SSE-S3, not KMS.
B
A bucket policy that denies PutObject if the 'x-amz-server-side-encryption' header is not present
Why wrong: This would allow any encryption (including SSE-S3), not specifically KMS.
C
A bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption-aws-kms-key-id' header matching the desired KMS key ID
This enforces the use of a specific KMS key.
D
Enable default encryption on the bucket with AWS-KMS
Why wrong: Default encryption does not prevent uploads without encryption; it only encrypts objects that are not already encrypted.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption-aws-kms-key-id' header matching the desired KMS key ID
To enforce encryption, a bucket policy can deny PutObject if the object is not encrypted with the required KMS key. The condition 's3:x-amz-server-side-encryption-aws-kms-key-id' checks the key ID. Option B correctly denies requests that do not include the required key. Option A is incomplete; Option C uses the wrong condition; Option D uses SSE-S3 instead of KMS.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
A bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption' header with value 'AES256'
Why it's wrong here
This enforces SSE-S3, not KMS.
✗
A bucket policy that denies PutObject if the 'x-amz-server-side-encryption' header is not present
Why it's wrong here
This would allow any encryption (including SSE-S3), not specifically KMS.
✓
A bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption-aws-kms-key-id' header matching the desired KMS key ID
Why this is correct
This enforces the use of a specific KMS key.
Related concept
Standard ACLs match source addresses.
✗
Enable default encryption on the bucket with AWS-KMS
Why it's wrong here
Default encryption does not prevent uploads without encryption; it only encrypts objects that are not already encrypted.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related MLS-C01 ACL questions on filtering logic and placement.
Data Engineering — This question tests Data Engineering — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: A bucket policy that denies PutObject unless the request includes the 'x-amz-server-side-encryption-aws-kms-key-id' header matching the desired KMS key ID — To enforce encryption, a bucket policy can deny PutObject if the object is not encrypted with the required KMS key. The condition 's3:x-amz-server-side-encryption-aws-kms-key-id' checks the key ID. Option B correctly denies requests that do not include the required key. Option A is incomplete; Option C uses the wrong condition; Option D uses SSE-S3 instead of KMS.
What should I do if I get this MLS-C01 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related MLS-C01 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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