The answer is that the Lambda function is not setting the x-amz-server-side-encryption header to AES256 when writing to the public/ prefix. This is the most likely cause of the IAM policy condition SSE-S3 access denied error because the Deny statement in the policy explicitly blocks PutObject requests that do not include the required encryption header. When the function writes to example-bucket/public/, it must match the condition key s3:x-amz-server-side-encryption with the value AES256; otherwise, the Deny rule activates and the request fails. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how IAM policy conditions interact with S3 server-side encryption headers, a common trap where candidates overlook that the Deny effect is triggered by a missing header, not by an explicit block on the action. A helpful memory tip is "Deny defaults to blocking anything that doesn't match the condition," so always ensure your application explicitly sets the encryption header when a Deny condition is present.
DEA-C01 Data Ingestion and Transformation Practice Question
This DEA-C01 practice question tests your understanding of data ingestion and transformation. 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 configuring an IAM policy for a Lambda function that writes transformed data to S3. The function writes to both 'example-bucket/data/' and 'example-bucket/public/'. The policy is intended to enforce server-side encryption with SSE-S3 for all objects written to the 'public/' prefix, while allowing all operations on other prefixes. However, the Lambda function is failing with an AccessDenied error when writing to 'example-bucket/public/'. 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 Lambda function is not setting the 'x-amz-server-side-encryption' header to 'AES256' when writing to 'public/'.
Option A is correct because the Deny statement uses a condition that denies PutObject if the encryption is not AES256. If the Lambda function does not set SSE-S3, the condition fails and the request is denied. Option B is wrong because the policy allows PutObject on all resources. Option C is wrong because DeleteObject is not denied. Option D is wrong because GetObject is allowed.
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 denies DeleteObject on 'public/'.
Why it's wrong here
DeleteObject is allowed.
✗
The policy denies PutObject on 'public/' unconditionally.
Why it's wrong here
The Deny is conditional on encryption.
✗
The policy does not allow GetObject for 'public/'.
Why it's wrong here
GetObject is allowed.
✓
The Lambda function is not setting the 'x-amz-server-side-encryption' header to 'AES256' when writing to 'public/'.
Why this is correct
The Deny condition requires AES256 encryption.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 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 Lambda function is not setting the 'x-amz-server-side-encryption' header to 'AES256' when writing to 'public/'. — Option A is correct because the Deny statement uses a condition that denies PutObject if the encryption is not AES256. If the Lambda function does not set SSE-S3, the condition fails and the request is denied. Option B is wrong because the policy allows PutObject on all resources. Option C is wrong because DeleteObject is not denied. Option D is wrong because GetObject is allowed.
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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