Amplify build failing to get object from an S3 bucket in another account

Question

I am trying to get and Amplify build to pull a file from an S3 bucket in another AWS account. Both accounts live under the same organisation. The bucket in account A has the following policy
```
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowGetObject",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::bucket-account-a/*",
            "Condition": {
                "StringEquals": {
                    "aws:PrincipalOrgID": "o-xxxxxxxxxx"
                }
            }
        }
    ]
}
```

Account B has the Amplify application with a service role attached and the following `amplify.yml`:
```
version: 1
frontend:
  phases:
    preBuild:
      commands:
        - aws s3api get-object --bucket bucket-account-a --key file.txt ./file.txt
        - yarn install
    build:
      commands:
        - yarn run build
  artifacts:
    baseDirectory: .next
    files:
      - '**/*'
  cache:
    paths:
      - node_modules/**/*
```

The build fails outputting the following error: `An error occurred (AccessDenied) when calling the GetObject operation: Access Denied`. I have seen that the service role assigned is being assumed and refers to account B, so I am not sure what I am missing or what is stopping this from working. I have tried removing the condition on the above policy and updating the `Principal` on the S3 bucket policy to the assumed role (`arn:aws:sts::bbbbbbbbbbbb:assumed-role/staging-amplify-service-role/BuildSession`), the Amplify service, even just without the account id (`arn:aws:iam::bbbbbbbbbbbb:root`), all producing the same error.

Has anyone tried something similar?

Answer

Hello,
Access Denied generally occurs because of permission related issues. In you scenario I would suggest to check the following:
1. If the S3 bucket is encrypted with AWS KMS. Check if the KMS key policy allowed the other account to access.
2. Does the IAM role created in account B has permissions for the S3 bucket. You have to add the S3 bucket in the IAM role policy.
3. Check if the organisation does not have any restrictive policies.

Please refer the below document
[+] https://repost.aws/knowledge-center/s3-cross-account-access-denied

Amplify build failing to get object from an S3 bucket in another account

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