The answer is to apply a bucket policy that denies s3:PutObject unless the x-amz-server-side-encryption header is set to AES256. This works because the condition key s3:x-amz-server-side-encryption evaluates the encryption header value on each upload request, and the Deny effect overrides any Allow, ensuring that only SSE-S3 (AES256) is accepted for future writes. Existing SSE-KMS objects remain readable since the policy only targets write operations, not read or decrypt actions. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of using bucket policies to enforce encryption standards at the API level, a common alternative to default encryption settings. A frequent trap is assuming default encryption alone blocks non-compliant uploads, but it only applies when no encryption header is specified—a bucket policy with a Deny is required to reject explicit SSE-KMS headers. Memory tip: think “Deny on Put unless AES256” to lock writes, while reads stay open.
SCS-C02 Data Protection Practice Question
This SCS-C02 practice question tests your understanding of data protection. 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 company stores sensitive data in Amazon S3 and wants to ensure that all objects are encrypted at rest. The security team has enabled default encryption on the S3 bucket using SSE-S3. However, an audit reveals that some objects are stored with SSE-KMS. How can the company enforce that only SSE-S3 is used for all future uploads, while still allowing existing SSE-KMS objects to be read?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Apply a bucket policy that denies s3:PutObject unless the x-amz-server-side-encryption header is AES256.
Option C is correct because it uses a bucket policy to deny s3:PutObject unless the x-amz-server-side-encryption header is set to AES256, which is the value for SSE-S3. This enforces that all future uploads use SSE-S3, while existing SSE-KMS objects remain readable because the policy only applies to write operations. The condition key s3:x-amz-server-side-encryption checks the encryption header value, and the Deny effect overrides any Allow, ensuring compliance.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Configure a bucket policy that denies s3:PutObject with s3:x-amz-server-side-encryption-aws:kms.
Why it's wrong here
This would also deny reading objects encrypted with SSE-KMS if the policy is applied to all actions.
✗
Use an S3 Lifecycle policy to transition existing SSE-KMS objects to SSE-S3.
Why it's wrong here
Lifecycle policies do not change encryption of existing objects; they only transition storage class.
✓
Apply a bucket policy that denies s3:PutObject unless the x-amz-server-side-encryption header is AES256.
Why this is correct
This policy enforces SSE-S3 for uploads without affecting reads of existing objects.
Related concept
Read the scenario before looking for a memorised answer.
✗
Disable SSE-KMS in the AWS KMS key policy to prevent its use.
Why it's wrong here
Disabling the key would make existing SSE-KMS objects unreadable.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the condition key s3:x-amz-server-side-encryption with s3:x-amz-server-side-encryption-aws:kms, or think that default encryption can be overridden by a bucket policy without explicitly denying non-compliant uploads.
Detailed technical explanation
How to think about this question
The bucket policy condition s3:x-amz-server-side-encryption uses the value 'AES256' for SSE-S3 and 'aws:kms' for SSE-KMS. The Deny effect with a NotIpAddress or StringNotEquals condition ensures that any PutObject request without the correct header is rejected. Under the hood, S3 evaluates bucket policies before applying default encryption, so a Deny on the header overrides the bucket’s default encryption setting, forcing clients to explicitly specify SSE-S3.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Data Protection — This question tests Data Protection — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Apply a bucket policy that denies s3:PutObject unless the x-amz-server-side-encryption header is AES256. — Option C is correct because it uses a bucket policy to deny s3:PutObject unless the x-amz-server-side-encryption header is set to AES256, which is the value for SSE-S3. This enforces that all future uploads use SSE-S3, while existing SSE-KMS objects remain readable because the policy only applies to write operations. The condition key s3:x-amz-server-side-encryption checks the encryption header value, and the Deny effect overrides any Allow, ensuring compliance.
What should I do if I get this SCS-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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