- A
Create an IAM role in Account A with a trust policy allowing the user from Account B to assume it.
Why wrong: This is a valid approach but requires the user to assume a role; the question asks for the most secure, and a bucket policy is simpler.
- B
Share the access keys of an IAM user in Account A with the user in Account B.
Why wrong: Sharing access keys is insecure.
- C
Add a bucket policy in Account A that grants access to the user in Account B, and attach an IAM policy to the user in Account B allowing the S3 actions.
This uses a resource-based policy (bucket policy) and IAM policy, following least privilege.
- D
Attach an IAM policy to the user in Account B that grants access to the S3 bucket.
Why wrong: Without a bucket policy, the user in Account B cannot access the bucket.
Quick Answer
The most secure approach is to combine a resource-based bucket policy in Account A that grants access to the specific user in Account B with an identity-based IAM policy attached to that user in Account B. This dual-policy method enforces the principle of least privilege because the user can only perform S3 actions when both policies explicitly allow the operation, effectively creating a logical AND condition between the two accounts. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of cross-account S3 access and the difference between resource-based and identity-based policies—a common trap is assuming a bucket policy alone is sufficient, but without the IAM policy in the target account, the user has no permission to even attempt the action. Remember the memory tip: “Both sides must say yes” — the bucket policy says “you can come in,” and the IAM policy says “you are allowed to enter.”
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 cross-account access to an S3 bucket owned by Account A to a user in Account B. Which approach is the most secure?
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
Add a bucket policy in Account A that grants access to the user in Account B, and attach an IAM policy to the user in Account B allowing the S3 actions.
Option C is the most secure because it combines a resource-based bucket policy in Account A that explicitly grants access to the user in Account B with an identity-based IAM policy attached to that user in Account B. This dual-policy approach ensures that the user can only access the bucket when both policies allow the action, following the principle of least privilege and avoiding the need to share long-term credentials.
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.
- ✗
Create an IAM role in Account A with a trust policy allowing the user from Account B to assume it.
Why it's wrong here
This is a valid approach but requires the user to assume a role; the question asks for the most secure, and a bucket policy is simpler.
- ✗
Share the access keys of an IAM user in Account A with the user in Account B.
Why it's wrong here
Sharing access keys is insecure.
- ✓
Add a bucket policy in Account A that grants access to the user in Account B, and attach an IAM policy to the user in Account B allowing the S3 actions.
Why this is correct
This uses a resource-based policy (bucket policy) and IAM policy, following least privilege.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Attach an IAM policy to the user in Account B that grants access to the S3 bucket.
Why it's wrong here
Without a bucket policy, the user in Account B cannot access the bucket.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume an IAM policy in the target account alone is sufficient for cross-account S3 access, forgetting that the owning account must explicitly allow the access via a resource-based policy like a bucket policy.
Detailed technical explanation
How to think about this question
Under the hood, S3 bucket policies are resource-based policies evaluated at the bucket level, while IAM policies are identity-based evaluated at the user/role level. For cross-account access, the bucket policy must include a Principal element specifying the AWS account ID or user ARN of the external user, and the external user must have an IAM policy allowing the same S3 actions. This dual evaluation ensures that neither account can unilaterally grant access, providing defense in depth. A common real-world scenario is granting a partner account read-only access to a data lake while preventing accidental writes.
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.
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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: Add a bucket policy in Account A that grants access to the user in Account B, and attach an IAM policy to the user in Account B allowing the S3 actions. — Option C is the most secure because it combines a resource-based bucket policy in Account A that explicitly grants access to the user in Account B with an identity-based IAM policy attached to that user in Account B. This dual-policy approach ensures that the user can only access the bucket when both policies allow the action, following the principle of least privilege and avoiding the need to share long-term credentials.
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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Same concept, more angles
2 more ways this is tested on DVA-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A developer needs to grant cross-account access to an S3 bucket for an IAM user from another AWS account. The developer has added a bucket policy that allows the user's ARN. However, the user still cannot access the bucket. What additional step is required?
easy- ✓ A.The user must have an IAM policy allowing the required S3 actions on that bucket
- B.The bucket must be made public
- C.The user must use a different AWS CLI profile
- D.The resource-based policy must explicitly allow the user's ARN
Why A: A is correct because cross-account access to an S3 bucket requires both a resource-based policy (the bucket policy) that grants access to the user's ARN and an identity-based policy (an IAM policy attached to the user) that explicitly allows the required S3 actions on that bucket. Without the IAM policy, the user's account denies the request by default, even if the bucket policy permits it. This is the principle of 'permission delegation' in AWS: the resource owner can grant access, but the user's own account must also authorize the action.
Variation 2. A developer wants to grant a user in a different AWS account access to an S3 bucket. The developer has written a bucket policy that allows the user's IAM user ARN. However, the access is still denied. What is the most likely reason?
easy- ✓ A.The user's IAM user policy does not explicitly allow the required S3 action
- B.The bucket policy does not have a principal of '*' to allow external accounts
- C.The bucket is in a different region than the user's account
- D.The user is using the wrong S3 endpoint (e.g., path-style vs virtual-hosted)
Why A: When granting cross-account access to an S3 bucket, both the bucket policy (resource-based policy) and the user's IAM policy (identity-based policy) must explicitly allow the action. The bucket policy alone is insufficient if the user's IAM policy does not include an explicit Allow for the S3 action, because IAM denies by default. Even though the bucket policy grants access, the user's own IAM policy must also permit the operation for the request to succeed.
Last reviewed: Jun 24, 2026
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