SAA-C03 Design Secure Architectures Practice Question
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?
⚠ Common exam trap
Test-takers frequently 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.
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.
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.
- ✗
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.