DVA-C02 Security Practice Question
A developer needs to grant an IAM role in Account B read-only access to objects in an S3 bucket in Account A. The bucket is encrypted with server-side encryption using AWS KMS (SSE-KMS) with a customer managed key (CMK) in Account A. Which combination of policies is required for the cross-account access to succeed?
⚠ Common exam trap
Candidates often assume bucket and key policies alone are sufficient for cross-account access, forgetting that the requesting principal (the IAM role) must also have an IAM policy that explicitly allows the required actions.
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
✓
The bucket policy in Account A grants s3:GetObject to the role, the KMS key policy grants kms:Decrypt to the role, and the role in Account B has an IAM policy allowing s3:GetObject and kms:Decrypt
Cross-account access to an SSE-KMS encrypted S3 bucket requires three layers of permissions: the bucket policy in Account A must grant s3:GetObject to the IAM role in Account B, the KMS key policy must grant kms:Decrypt to the same role, and the role's IAM policy in Account B must allow both s3:GetObject and kms:Decrypt. Without any one of these, the request will fail due to either an S3 authorization error or a KMS decryption failure.
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 bucket policy in Account A grants s3:GetObject to the role, the KMS key policy grants kms:Decrypt to the role, and the role in Account B has an IAM policy allowing s3:GetObject and kms:Decrypt
Why this is correct
All three policies are required: bucket policy and key policy in Account A grant the necessary permissions, and the IAM role in Account B must have the corresponding IAM policy to authorize the use of those grants.
- ✗
The bucket policy in Account A grants s3:GetObject to the role, and the role in Account B has an IAM policy allowing s3:GetObject. No KMS permissions are needed because SSE-KMS uses AWS managed keys by default.
Why it's wrong here
This option is incorrect because cross-account access to S3 objects encrypted with SSE-KMS typically requires a Customer Managed Key (CMK), not an AWS managed key. AWS managed keys (like `aws/s3`) cannot be directly shared or have their key policies modified to grant cross-account decryption permissions. Therefore, explicit `kms:Decrypt` permissions on the CMK's key policy and the accessing IAM role's policy are essential for the role in Account B to decrypt the objects.
- ✗
The bucket policy in Account A grants s3:GetObject to the role, and the KMS key policy grants kms:Decrypt to the role. The role in Account B does not need additional IAM policies because the bucket and key policies provide sufficient permissions.
Why it's wrong here
This option is incorrect because AWS IAM operates on a 'two-policy' evaluation model for cross-account access. While resource-based policies (like the S3 bucket policy and KMS key policy) grant permissions to an external principal, the IAM identity (the role in Account B) must also have an identity-based policy that explicitly allows those same actions. Without an IAM policy on the role permitting `s3:GetObject` and `kms:Decrypt`, the role is not authorized to perform these actions, even if the resource policies permit it.
- ✗
Only the bucket policy in Account A needs to grant s3:GetObject to the role. KMS is not involved because the bucket is encrypted with SSE-KMS but the role can decrypt using the default KMS key.
Why it's wrong here
This option is incorrect because when S3 objects are encrypted with SSE-KMS, decryption always involves AWS KMS, making KMS permissions mandatory. Furthermore, the concept of a 'default KMS key' (referring to an AWS managed key like `aws/s3`) cannot be used for cross-account access. For an external account to decrypt objects encrypted with SSE-KMS, a Customer Managed Key (CMK) must be used, and explicit `kms:Decrypt` permissions must be granted in both the CMK's key policy and the accessing IAM role's policy.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DVA-C02 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 DVA-C02 exam.