- A
Enable KMS automatic key rotation for the customer-managed key in account B.
Why wrong: Key rotation is unrelated to authorization failures. The problem is that the key policy in account B is not allowing the cross-account principal to use kms:Decrypt for the key.
- B
Update the KMS key policy in account B to allow the instance role’s principal from account A to call kms:Decrypt and kms:CreateGrant.
Customer-managed KMS keys use resource-based key policies to control cross-account usage. Even if the IAM role in account A has kms:Decrypt permissions, the account B key policy must also allow that principal to use the key. Including kms:Decrypt (and often kms:CreateGrant) resolves cross-account mount authorization.
- C
Attach the key policy as an IAM permissions policy to the instance role in account A only; key policies are not evaluated cross-account.
Why wrong: KMS key policies are evaluated on the key itself. Without updating the key policy in account B, cross-account calls will still be denied even if IAM permissions exist in account A.
- D
Disable encryption on the EBS volume until authorization is fixed, then re-enable encryption after mount.
Why wrong: Disabling encryption violates security requirements and does not address why KMS authorization fails. Encrypted volumes and snapshots require correct KMS policies to be usable.
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. A key principle to apply: kMS key policies are resource-based policies that control access to customer-managed keys.. 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.
An application in account A needs to use an encrypted EBS volume whose snapshots were copied from account B. The EBS volume is encrypted with a customer-managed KMS key in account B. After attaching the volume, the instance fails to mount it and logs show KMS access errors (kms:Decrypt) for the instance role. The instance role in account A already has an IAM policy allowing kms:Decrypt on that key ARN, but the mount still fails. What must be updated in account B to allow the mount to succeed?
Answer choices
Why each option matters
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 in account B to allow the instance role’s principal from account A to call kms:Decrypt and kms:CreateGrant.
The instance role in account A has an IAM policy allowing kms:Decrypt on the key ARN, but cross-account KMS access requires the key policy in account B to explicitly grant the external principal (the instance role's ARN) the necessary permissions. Without a key policy statement allowing kms:Decrypt and kms:CreateGrant for the account A role, KMS will deny the decryption request, causing the mount to fail. Option B correctly identifies that the key policy in account B must be updated to authorize the cross-account principal.
Key principle: KMS key policies are resource-based policies that control access to customer-managed keys.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable KMS automatic key rotation for the customer-managed key in account B.
Why it's wrong here
Key rotation is unrelated to authorization failures. The problem is that the key policy in account B is not allowing the cross-account principal to use kms:Decrypt for the key.
- ✓
Update the KMS key policy in account B to allow the instance role’s principal from account A to call kms:Decrypt and kms:CreateGrant.
Why this is correct
Customer-managed KMS keys use resource-based key policies to control cross-account usage. Even if the IAM role in account A has kms:Decrypt permissions, the account B key policy must also allow that principal to use the key. Including kms:Decrypt (and often kms:CreateGrant) resolves cross-account mount authorization.
Related concept
KMS key policies are resource-based policies that control access to customer-managed keys.
- ✗
Attach the key policy as an IAM permissions policy to the instance role in account A only; key policies are not evaluated cross-account.
Why it's wrong here
KMS key policies are evaluated on the key itself. Without updating the key policy in account B, cross-account calls will still be denied even if IAM permissions exist in account A.
- ✗
Disable encryption on the EBS volume until authorization is fixed, then re-enable encryption after mount.
Why it's wrong here
Disabling encryption violates security requirements and does not address why KMS authorization fails. Encrypted volumes and snapshots require correct KMS policies to be usable.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume an IAM policy on the instance role is sufficient for cross-account KMS access, but KMS requires the key policy in the owning account to explicitly authorize the external principal, as IAM policies alone cannot grant cross-account permissions.
Detailed technical explanation
How to think about this question
When an EBS volume is encrypted with a customer-managed KMS key, the instance must call kms:Decrypt to derive the plaintext data key, and kms:CreateGrant to allow the EBS service to re-encrypt data during attachment. Cross-account KMS operations require the key policy to include a statement with the external IAM role ARN as the principal; IAM policies in the consuming account are insufficient because KMS evaluates key policies first for cross-account requests. This is enforced by the KMS key policy evaluation logic where, for external accounts, only the key policy is evaluated (IAM policies in the external account are not considered).
KKey Concepts to Remember
- KMS key policies are resource-based policies that control access to customer-managed keys.
- For cross-account KMS access, both the requesting IAM principal's policy and the key's policy must grant permission.
- The KMS key policy must explicitly allow the cross-account principal (e.g., instance role ARN) to use the key.
- Permissions like `kms:Decrypt` and often `kms:CreateGrant` are necessary in the key policy for cross-account EBS volume usage.
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
KMS key policies are resource-based policies that control access to customer-managed keys.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review kMS key policies are resource-based policies that control access to customer-managed keys., then practise related SAA-C03 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this SAA-C03 question test?
Design Secure Architectures — This question tests Design Secure Architectures — KMS key policies are resource-based policies that control access to customer-managed keys..
What is the correct answer to this question?
The correct answer is: Update the KMS key policy in account B to allow the instance role’s principal from account A to call kms:Decrypt and kms:CreateGrant. — The instance role in account A has an IAM policy allowing kms:Decrypt on the key ARN, but cross-account KMS access requires the key policy in account B to explicitly grant the external principal (the instance role's ARN) the necessary permissions. Without a key policy statement allowing kms:Decrypt and kms:CreateGrant for the account A role, KMS will deny the decryption request, causing the mount to fail. Option B correctly identifies that the key policy in account B must be updated to authorize the cross-account principal.
What should I do if I get this SAA-C03 question wrong?
Review kMS key policies are resource-based policies that control access to customer-managed keys., then practise related SAA-C03 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
KMS key policies are resource-based policies that control access to customer-managed keys.
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Last reviewed: Jun 11, 2026
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