The answer is to update the KMS key policy or add a grant so AppServerRole can use the key for decrypt and data key operations. This is correct because when using S3 SSE-KMS with a customer-managed key, the IAM role accessing the encrypted objects must have explicit permission to perform `kms:Decrypt` and `kms:GenerateDataKey` operations on that specific KMS key; without these, the application fails even if S3 bucket policies allow read access. On the SAA-C03 exam, this scenario tests your understanding that customer-managed KMS keys require separate key policy or grant permissions—a common trap is assuming the IAM role’s S3 permissions alone are sufficient. Remember the memory tip: “S3 decrypts the envelope, but KMS holds the master key—so the role must ask KMS for permission first.”
SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. 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.
Exhibit
Application log excerpt:
2026-04-11T09:14:22Z ERROR S3 GetObject failed: AccessDenied
2026-04-11T09:14:22Z ERROR KMS Decrypt failed for key arn:aws:kms:us-east-1:111122223333:key/abcd-1234
Current setup:
- S3 bucket default encryption: SSE-KMS
- EC2 application role: AppServerRole
- Bucket policy allows s3:GetObject for AppServerRole
- KMS key policy currently allows only the account root principal
- No direct KMS permissions are attached to AppServerRole
Based on the exhibit, what is the most appropriate change to restore application access while keeping encryption at rest with customer-managed KMS controls?
Application log excerpt:
2026-04-11T09:14:22Z ERROR S3 GetObject failed: AccessDenied
2026-04-11T09:14:22Z ERROR KMS Decrypt failed for key arn:aws:kms:us-east-1:111122223333:key/abcd-1234
Current setup:
- S3 bucket default encryption: SSE-KMS
- EC2 application role: AppServerRole
- Bucket policy allows s3:GetObject for AppServerRole
- KMS key policy currently allows only the account root principal
- No direct KMS permissions are attached to AppServerRole
A
Change the bucket to SSE-S3 so the application no longer depends on KMS permissions.
Why wrong: This would remove KMS from the path, but it also removes customer-managed key control, which the requirement explicitly wants to keep.
B
Update the KMS key policy or add a grant so AppServerRole can use the key for decrypt and data key operations.
For SSE-KMS objects, the caller needs permission to use the KMS key as well as S3 permissions. The role already has S3 access, but KMS is denying Decrypt because the key policy does not allow the role. Adding the role through the key policy or a grant, together with the needed KMS actions, resolves the failure while preserving customer-managed encryption.
C
Move the EC2 instance into the same Availability Zone as the S3 bucket to reduce encryption errors.
Why wrong: S3 is a regional service and does not depend on the instance being in a specific Availability Zone for KMS access.
D
Attach AmazonS3FullAccess to the application role so S3 can bypass KMS authorization.
Why wrong: S3 permissions do not bypass KMS authorization for SSE-KMS objects. The decrypt call is still evaluated by KMS.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Update the KMS key policy or add a grant so AppServerRole can use the key for decrypt and data key operations.
The application is failing because AppServerRole lacks the necessary permissions to use the customer-managed KMS key for decrypting S3 objects. By updating the KMS key policy or adding a grant to allow the role to perform `kms:Decrypt` and `kms:GenerateDataKey` operations, you restore access while maintaining encryption at rest with customer-managed KMS controls.
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.
✗
Change the bucket to SSE-S3 so the application no longer depends on KMS permissions.
Why it's wrong here
This would remove KMS from the path, but it also removes customer-managed key control, which the requirement explicitly wants to keep.
✓
Update the KMS key policy or add a grant so AppServerRole can use the key for decrypt and data key operations.
Why this is correct
For SSE-KMS objects, the caller needs permission to use the KMS key as well as S3 permissions. The role already has S3 access, but KMS is denying Decrypt because the key policy does not allow the role. Adding the role through the key policy or a grant, together with the needed KMS actions, resolves the failure while preserving customer-managed encryption.
Related concept
Read the scenario before looking for a memorised answer.
✗
Move the EC2 instance into the same Availability Zone as the S3 bucket to reduce encryption errors.
Why it's wrong here
S3 is a regional service and does not depend on the instance being in a specific Availability Zone for KMS access.
✗
Attach AmazonS3FullAccess to the application role so S3 can bypass KMS authorization.
Why it's wrong here
S3 permissions do not bypass KMS authorization for SSE-KMS objects. The decrypt call is still evaluated by KMS.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume S3 bucket policies alone control access to encrypted objects, forgetting that SSE-KMS requires separate KMS key permissions that must be explicitly granted to the IAM role or user.
Detailed technical explanation
How to think about this question
When using SSE-KMS, S3 calls KMS to decrypt objects on behalf of the requester, requiring the IAM role to have `kms:Decrypt` and `kms:GenerateDataKey` permissions on the specific KMS key. The KMS key policy can be updated to grant these actions to the AppServerRole, or a KMS grant can be created for temporary, scoped access. Without these permissions, S3 returns a 403 Access Denied error even if the S3 bucket policy allows access.
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.
Design Secure Architectures — This question tests Design Secure Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Update the KMS key policy or add a grant so AppServerRole can use the key for decrypt and data key operations. — The application is failing because AppServerRole lacks the necessary permissions to use the customer-managed KMS key for decrypting S3 objects. By updating the KMS key policy or adding a grant to allow the role to perform `kms:Decrypt` and `kms:GenerateDataKey` operations, you restore access while maintaining encryption at rest with customer-managed KMS controls.
What should I do if I get this SAA-C03 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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