The answer is to update the KMS key policy to explicitly grant the IAM role permission to use the key, ideally with a kms:ViaService condition for S3. This is correct because a customer managed KMS key acts as its own identity-based firewall; even if an IAM role has kms:Decrypt and kms:GenerateDataKey permissions in its IAM policy, the key policy must separately allow that role to invoke those actions, or the key will deny the request. On the SAA-C03 exam, this scenario tests your understanding that KMS key policies are the primary authorization mechanism for customer managed keys, and that an AccessDenied from KMS—not from S3—points to a key policy gap, not an IAM policy gap. A common trap is assuming IAM permissions alone suffice for KMS operations; they do not unless the key policy includes a blanket delegation to the account. Memory tip: “Key policy is the gatekeeper—IAM policies can knock, but only the key policy opens the door.”
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
{
"bucket": "arn:aws:s3:::secure-reports-prod",
"object_encryption": "SSE-KMS",
"role_policy": {
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": ["s3:GetObject", "s3:PutObject"],
"Resource": "arn:aws:s3:::secure-reports-prod/*"
},
{
"Effect": "Allow",
"Action": ["kms:Decrypt", "kms:GenerateDataKey"],
"Resource": "arn:aws:kms:us-east-1:111122223333:key/abcd-1234"
}
]
},
"cloudtrail_event": {
"eventSource": "kms.amazonaws.com",
"eventName": "Decrypt",
"errorCode": "AccessDenied",
"errorMessage": "The key policy does not allow this principal to use the specified KMS key"
},
"key_policy_summary": "Only the account root principal is allowed; no application roles are listed"
}
Based on the exhibit, an application in the same AWS account can upload and read objects in an S3 bucket encrypted with a customer managed KMS key, but GetObject fails with an AccessDenied error from AWS KMS. The IAM role already has s3:GetObject, s3:PutObject, kms:Decrypt, and kms:GenerateDataKey permissions. What change most directly fixes the issue while preserving least privilege?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
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 to allow the application role to use the key, ideally with a kms:ViaService condition for S3.
The error is an AccessDenied from AWS KMS, not from S3, which means the IAM role has the required S3 permissions (s3:GetObject) and KMS API permissions (kms:Decrypt), but the KMS key policy does not explicitly grant the role access to the key. Since customer managed KMS keys require a key policy to grant IAM principals permission to use the key (IAM policies alone are insufficient unless the key policy delegates such authority), updating the key policy to allow the role with a kms:ViaService condition for S3 directly resolves the KMS-side denial while preserving least privilege.
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.
✗
Add an S3 bucket ACL that grants the application role full control over objects.
Why it's wrong here
Object ACLs do not authorize KMS decryption, so they cannot fix a KMS key policy denial.
✓
Update the KMS key policy to allow the application role to use the key, ideally with a kms:ViaService condition for S3.
Why this is correct
KMS key policy must explicitly trust the principal. Adding a role-scoped statement with kms:ViaService keeps access limited to S3 use only.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Replace the customer managed key with the AWS managed S3 key so IAM permissions become sufficient.
Why it's wrong here
AWS managed keys change the encryption model and do not preserve the same key-level control requirements.
✗
Add an S3 bucket policy that grants s3:GetObject and s3:PutObject to the role for all objects.
Why it's wrong here
Bucket policy controls S3 access, but the failure is specifically in KMS authorization during decrypt.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates see 'AccessDenied' and assume the S3 bucket policy or ACL is missing, when the error message explicitly states it is from AWS KMS, meaning the fix must be at the KMS key policy level, not the S3 resource policy.
Detailed technical explanation
How to think about this question
Customer managed KMS keys have a resource-based key policy that acts as the primary access control mechanism; IAM policies alone cannot grant access unless the key policy explicitly allows it (e.g., via a statement with 'Principal': { 'AWS': 'arn:aws:iam::account-id:root' } to enable IAM-based access). The kms:ViaService condition restricts key usage to requests originating from a specific AWS service (e.g., s3.amazonaws.com), which ensures the role can only use the key through S3 operations, reducing the risk of key misuse. In practice, this error often occurs when developers add KMS API permissions to an IAM role but forget to update the key policy, leading to a confusing 'AccessDenied' that appears to be from S3 but is actually a KMS authorization failure.
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 to allow the application role to use the key, ideally with a kms:ViaService condition for S3. — The error is an AccessDenied from AWS KMS, not from S3, which means the IAM role has the required S3 permissions (s3:GetObject) and KMS API permissions (kms:Decrypt), but the KMS key policy does not explicitly grant the role access to the key. Since customer managed KMS keys require a key policy to grant IAM principals permission to use the key (IAM policies alone are insufficient unless the key policy delegates such authority), updating the key policy to allow the role with a kms:ViaService condition for S3 directly resolves the KMS-side denial while preserving least privilege.
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.
Are there clue words in this question I should notice?
Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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