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Question 61 of 1,616

Security →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that you need three policies working together: the S3 bucket policy in Account A granting s3:GetObject to the IAM role in Account B, the KMS key policy granting kms:Decrypt to that same role, and the role’s own IAM policy in Account B allowing both s3:GetObject and kms:Decrypt. This is required because cross-account access to an SSE-KMS encrypted S3 bucket involves two separate authorization layers—S3 permissions control object retrieval, while KMS permissions control decryption of the data key—and the request fails if either layer is missing. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of how resource-based policies (bucket and key policies) interact with identity-based policies across accounts, and a common trap is assuming the bucket policy alone suffices. Remember the “three-link chain” memory tip: Bucket Policy + Key Policy + Role Policy, and if any link is missing, the chain breaks.

DVA-C02 Security Practice Question

This DVA-C02 practice question tests your understanding of security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 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?

Question 1hardmultiple choice

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A
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
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.
B
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 wrong: The question specifies a customer managed key, which requires explicit key policy grants. AWS managed keys cannot be used in cross-account scenarios.
C
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 wrong: IAM roles require IAM policies to authorize actions, even if resource-based policies grant access. The role must have an IAM policy allowing s3:GetObject and kms:Decrypt.
D
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 wrong: SSE-KMS with a customer managed key requires explicit KMS permissions. The default key (AWS managed key) cannot be used for cross-account access.

Full breakdown with real-world context →

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

Option A is correct because 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.

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 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.
Related concept
Read the scenario before looking for a memorised answer.
✗
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
The question specifies a customer managed key, which requires explicit key policy grants. AWS managed keys cannot be used in cross-account scenarios.
✗
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
IAM roles require IAM policies to authorize actions, even if resource-based policies grant access. The role must have an IAM policy allowing s3:GetObject and kms:Decrypt.
✗
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
SSE-KMS with a customer managed key requires explicit KMS permissions. The default key (AWS managed key) cannot be used for cross-account access.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates 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.

Trap categories for this question

Scenario analysis trap
The question specifies a customer managed key, which requires explicit key policy grants. AWS managed keys cannot be used in cross-account scenarios.

Detailed technical explanation

How to think about this question

Under the hood, when an IAM role in Account B makes a GetObject request to an SSE-KMS encrypted bucket in Account A, S3 first checks the bucket policy for s3:GetObject permission. If allowed, S3 then calls KMS to decrypt the object's data key, which requires the key policy to grant kms:Decrypt to the role's principal ARN. Finally, the role's own IAM policy must explicitly allow both actions because AWS evaluates all policy boundaries—without the IAM policy, the role lacks authorization to invoke the KMS Decrypt API. This three-way authorization chain is a common source of cross-account access failures.

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.

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FAQ

Questions learners often ask

What does this DVA-C02 question test?

Security — This question tests Security — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: 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 — Option A is correct because 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.

What should I do if I get this DVA-C02 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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