This DBS-C01 practice question tests your understanding of database security. 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.
A security engineer is troubleshooting an issue where an application using IAM role 'app-role' with a trust policy to assume the 'app-user' user is unable to decrypt an RDS database that uses a customer-managed KMS key. The above key policy is attached to the KMS key. What is the likely cause of the failure?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The principal in the policy is 'app-user', but the application uses an IAM role that does not have the necessary permissions.
Option C is correct because the KMS key policy specifies the principal as 'app-user', which is an IAM user. However, the application uses an IAM role 'app-role' to make the decryption requests. Since the principal in the key policy does not include the role or its associated permissions, the KMS key policy denies access to the role, causing the decryption failure. The trust policy on the role allows the user to assume it, but that does not grant the role any KMS permissions unless explicitly added.
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.
✗
The 'kms:ViaService' condition requires the request to come from RDS, but the application is making KMS API calls directly.
Why it's wrong here
The decryption of RDS databases is done by RDS on behalf of the application, so the call to KMS comes from RDS, satisfying the condition.
✗
The policy has a missing 'Effect': 'Deny' statement that is blocking access.
Why it's wrong here
There is no explicit deny; the issue is that the role is not authorized.
✓
The principal in the policy is 'app-user', but the application uses an IAM role that does not have the necessary permissions.
Why this is correct
The policy grants permissions to the user 'app-user', but the application uses a role. The role needs to be added to the key policy.
Related concept
Read the scenario before looking for a memorised answer.
✗
The condition 'kms:ViaService' restricts the key to be used only with RDS, but the application is using a different service.
Why it's wrong here
The application is using RDS, so this condition should be satisfied.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the trust policy of an IAM role with the permissions granted to the role, assuming that if a user can assume a role, the role inherits the user's KMS permissions, when in fact the role must be explicitly authorized in the key policy or via an IAM policy.
Detailed technical explanation
How to think about this question
KMS key policies use a resource-based access control model where the 'Principal' element defines which IAM users or roles can use the key. When an application assumes an IAM role, the role's ARN becomes the identity for API calls, not the user that assumed it. Even if the role has a trust policy allowing the user to assume it, the role itself must be listed as a principal in the key policy or have an IAM policy granting kms:Decrypt. The 'kms:ViaService' condition key restricts requests to those made through a specific AWS service (e.g., RDS), but it does not override the principal requirement.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Database Security — This question tests Database Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The principal in the policy is 'app-user', but the application uses an IAM role that does not have the necessary permissions. — Option C is correct because the KMS key policy specifies the principal as 'app-user', which is an IAM user. However, the application uses an IAM role 'app-role' to make the decryption requests. Since the principal in the key policy does not include the role or its associated permissions, the KMS key policy denies access to the role, causing the decryption failure. The trust policy on the role allows the user to assume it, but that does not grant the role any KMS permissions unless explicitly added.
What should I do if I get this DBS-C01 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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