Cross-Account S3 Access: Bucket Policy + IAM Policy
A DevOps engineer needs to grant cross-account access to an S3 bucket. The source account is 111111111111 and the destination account is 222222222222. Which policy should be attached to the S3 bucket?
Quick Answer
The correct answer is the bucket policy that grants s3:GetObject access to the destination account’s root principal, because cross-account S3 access requires a resource-based policy (the bucket policy) to explicitly authorize a principal from another AWS account. An IAM policy alone in the destination account is insufficient—it must be paired with a bucket policy that lists the external account as the Principal. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this tests your understanding of the fundamental distinction between resource-based and identity-based policies for cross-account scenarios. A common trap is thinking an IAM policy in the destination account is enough, but without the bucket policy allowing the cross-account principal, the request is denied. Remember the memory tip: “Bucket policy for the guest, IAM policy for the host”—the bucket owner writes the bucket policy to invite the external account, while the external account uses its own IAM policy to exercise that permission.
⚠ Common exam trap
Test-takers frequently confuse which account's ARN to use in the Principal field, mistakenly using the source account (111111111111) instead of the destination account (222222222222), or they incorrectly assume an IAM policy in the source account can grant cross-account access.
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
✓
{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"AWS":"arn:aws:iam::222222222222:root"},"Action":"s3:GetObject","Resource":"arn:aws:s3:::example-bucket/*"}]}
It uses a bucket policy to grant cross-account access to the S3 bucket. The policy specifies the destination account (222222222222) as the principal, allowing s3:GetObject on the bucket's objects. This is the standard AWS approach for cross-account S3 access, as bucket policies can grant permissions to principals in other AWS accounts.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Attach an IAM policy to the user in account 111111111111 allowing s3:GetObject.
Why it's wrong here
IAM policy only grants access within the same account.
- ✗
Set the bucket ACL to grant full control to account 222222222222.
Why it's wrong here
ACLs are not recommended; bucket policy is preferred.
- ✓
{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"AWS":"arn:aws:iam::222222222222:root"},"Action":"s3:GetObject","Resource":"arn:aws:s3:::example-bucket/*"}]}
Why this is correct
Bucket policy grants cross-account access to objects.
- ✗
{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"AWS":"arn:aws:iam::111111111111:root"},"Action":"s3:GetObject","Resource":"arn:aws:s3:::example-bucket/*"}]}
Why it's wrong here
Principal is source account, not destination.
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
Related to this question
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Same concept, more angles
4 more ways this is tested on DOP-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. An organization wants to grant cross-account access to an S3 bucket in Account A to a user in Account B. Which policy configuration is required?
easy- ✓ A.A bucket policy in Account A and an IAM user policy in Account B
- B.An S3 bucket ACL granting access to the user in Account B
- C.An IAM user policy in Account B allowing access to the bucket
- D.A bucket policy in Account A granting access to the user in Account B
Why A: Cross-account access to an S3 bucket requires both a resource-based policy (bucket policy) on the bucket in Account A granting access to the user in Account B, and an identity-based policy (IAM user policy) in Account B allowing the user to access the bucket. Option A correctly includes both policies. Option B is incorrect because S3 bucket ACLs are legacy and not recommended for cross-account access. Option C is missing the bucket policy in Account A, so it is insufficient. Option D is missing the IAM user policy in Account B, so it is insufficient.
Variation 2. A DevOps engineer needs to grant cross-account access to an S3 bucket in Account A for a user in Account B. Which combination of policies is required?
easy- ✓ A.An S3 bucket policy in Account A and an IAM policy in Account B.
- B.An IAM role in Account A and an IAM policy in Account B.
- C.Only an IAM policy in Account B.
- D.Only an S3 bucket policy in Account A.
Why A: Cross-account access to an S3 bucket requires both a resource-based policy (the S3 bucket policy in Account A) that grants the necessary permissions to the principal from Account B, and an identity-based policy (an IAM policy in Account B) attached to the user that allows the S3 actions. The bucket policy explicitly authorizes the external user (by ARN), while the IAM policy ensures the user has the required permissions to initiate the request. Without both, the request will fail due to the lack of either resource-side authorization or identity-side authorization.
Variation 3. A DevOps engineer needs to grant cross-account access to an S3 bucket in Account A for users in Account B. The users in Account B must be able to list objects and read them. What is the most secure way to configure this access?
medium- A.Create a bucket policy in Account A that grants access to the IAM user ARNs from Account B.
- B.Create a KMS key in Account A and share it with Account B to decrypt objects.
- ✓ C.Create a bucket policy in Account A granting access to the root user of Account B, and create an IAM policy in Account B allowing the users to access the bucket.
- D.Create an IAM role in Account A with trust policy allowing Account B to assume it, and attach a policy granting S3 access.
Why C: It combines a bucket policy in Account A granting access to Account B's root or specific ARN, and an IAM policy in Account B allowing users to access that bucket. Option A is wrong because a bucket policy alone is insufficient; Account B users also need IAM permissions. Option B is wrong because KMS is for encryption, not access control. Option D is wrong because creating an IAM role in Account A with a trust policy allowing Account B to assume it is a valid pattern, but the question specifies users in Account B; they would need to assume the role, which is less direct than Option C for S3 access.
Variation 4. A DevOps engineer needs to grant cross-account access to an S3 bucket. The source account is 111111111111 and the target account is 222222222222. Which combination of a bucket policy and an IAM policy correctly grants the target account access?
easy- A.Bucket policy: { "Effect": "Allow", "Principal": "222222222222", "Action": "s3:GetObject", "Resource": "arn:aws:s3:::example-bucket/*" } and IAM policy: same as A
- ✓ B.Bucket policy: { "Effect": "Allow", "Principal": { "AWS": "arn:aws:iam::222222222222:root" }, "Action": "s3:GetObject", "Resource": "arn:aws:s3:::example-bucket/*" } and IAM policy: { "Effect": "Allow", "Action": "s3:GetObject", "Resource": "arn:aws:s3:::example-bucket/*" }
- C.Only bucket policy as in A
- D.Only IAM policy as in A
Why B: Cross-account S3 access requires a bucket policy in the source account (111111111111) that grants permissions to the target account (222222222222) by specifying the principal as the target account's root ARN (arn:aws:iam::222222222222:root), and an IAM policy in the target account that allows the user to perform the action on the bucket. Option B correctly provides both: the bucket policy uses the correct principal ARN, and the IAM policy allows s3:GetObject on the bucket. Option A is incorrect because 'Principal': '222222222222' is invalid; it must be an ARN. Option C lacks the IAM policy, and option D lacks the bucket policy.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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