The answer is that the IAM policy is missing the kms:Encrypt permission. While the policy correctly includes s3:PutObject for writing to the bucket and kms:Decrypt plus kms:GenerateDataKey for reading, writing encrypted objects with SSE-KMS requires the kms:Encrypt action to authorize the S3 service to encrypt the new object using the specified KMS key. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of how S3 SSE-KMS interacts with Glue ETL jobs—a common trap is assuming that kms:GenerateDataKey alone covers both read and write operations, but writing encrypted data explicitly needs kms:Encrypt. Remember the memory tip: "Read needs Decrypt, Write needs Encrypt" to avoid this access denied error when configuring IAM permissions for Glue ETL writing to SSE-KMS S3.
MLS-C01 Data Engineering Practice Question
This MLS-C01 practice question tests your understanding of data engineering. 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.
Refer to the exhibit. A data engineer is creating an IAM policy for an AWS Glue ETL job that reads encrypted objects from an S3 bucket, transforms them, and writes the results back to the same bucket. The bucket uses SSE-KMS encryption with the KMS key specified. The ETL job is failing with an "Access Denied" error when trying to write data. What is the likely cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The policy is missing the kms:Encrypt permission
Option C is correct because the policy grants s3:PutObject, which is needed to write, but the KMS permissions include kms:Decrypt and kms:GenerateDataKey, which are sufficient for reading and writing with SSE-KMS. The issue is that the job role must also have kms:Encrypt to write encrypted objects. Option A is wrong because the policy includes s3:PutObject. Option B is wrong because the policy includes both KMS actions needed for reading. Option D is wrong because there is no s3:PutObject condition missing.
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.
✗
The policy is missing the kms:Decrypt permission
Why it's wrong here
kms:Decrypt is included.
✗
The policy is missing the s3:PutObjectAcl permission
Why it's wrong here
ACL permissions are not required for this operation.
✗
The policy is missing the s3:PutObject permission
Why it's wrong here
The policy includes s3:PutObject.
✓
The policy is missing the kms:Encrypt permission
Why this is correct
Writing with SSE-KMS requires kms:Encrypt.
Related concept
Standard ACLs match source addresses.
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: The policy is missing the kms:Encrypt permission — Option C is correct because the policy grants s3:PutObject, which is needed to write, but the KMS permissions include kms:Decrypt and kms:GenerateDataKey, which are sufficient for reading and writing with SSE-KMS. The issue is that the job role must also have kms:Encrypt to write encrypted objects. Option A is wrong because the policy includes s3:PutObject. Option B is wrong because the policy includes both KMS actions needed for reading. Option D is wrong because there is no s3:PutObject condition missing.
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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Question Discussion
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