The answer is that the IAM policy is missing the s3:ListBucket action on the bucket itself. While the policy correctly grants s3:GetObject and s3:PutObject on the objects within the bucket, the Lambda function cannot complete the write operation because it lacks permission to list the bucket’s contents. The s3:ListBucket action is required for the function to verify the target path or check for existing objects before writing, and without it, AWS S3 returns an Access Denied error even though the PutObject permission is present. On the AWS Certified Data Engineer Associate DEA-01 exam, this scenario tests your understanding of how S3 actions are scoped—object-level permissions like GetObject and PutObject apply to the object ARN, while bucket-level actions like ListBucket must be applied to the bucket ARN. A common trap is assuming PutObject alone is sufficient for writes, but S3 often requires ListBucket for operations like multipart uploads or path validation. Memory tip: “List before Put”—always check that ListBucket is included when troubleshooting Lambda S3 write access denied.
DEA-C01 Data Ingestion and Transformation Practice Question
This DEA-C01 practice question tests your understanding of data ingestion and transformation. 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 engineer attached this IAM policy to a Lambda function used to transform data in S3. The function is unable to write output to the bucket. What is the most likely reason?
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 policy does not allow the s3:ListBucket action on the bucket.
The policy allows GetObject and PutObject on objects, but not the s3:ListBucket action required to check existence or list objects. The function likely needs ListBucket to write or verify.
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 resource ARN is missing the bucket-level ARN.
Why it's wrong here
The resource is correct for objects; bucket-level actions need a separate statement.
✗
The policy does not allow the s3:DeleteObject action.
Why it's wrong here
DeleteObject is not needed for writing.
✓
The policy does not allow the s3:ListBucket action on the bucket.
Why this is correct
To write objects, the function needs ListBucket permission on the bucket itself.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The policy does not allow the s3:PutObjectAcl action.
Why it's wrong here
PutObjectAcl is not required for default writes.
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 DEA-C01 ACL questions on filtering logic and placement.
Data Ingestion and Transformation — This question tests Data Ingestion and Transformation — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The policy does not allow the s3:ListBucket action on the bucket. — The policy allows GetObject and PutObject on objects, but not the s3:ListBucket action required to check existence or list objects. The function likely needs ListBucket to write or verify.
What should I do if I get this DEA-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 DEA-C01 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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