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So I want to grant a role that exists in account A the get-object permission on items in a bucket owned by account B.

I read The AWS docs' example policies and created a bucket policy like this:

{
    "Version": "2012-10-17",
    "Id": "foo2",
    "Statement": [
        {
            "Sid": "Stmt199999999999",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::123456789:role/asset_download_role"
            },
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::my-bucket-name/*"
        }
    ]
}

Which to me reads in English: "Give the role asset_download_role from account 123456789 the GetObject permission to any and all objects inside the bucket my-bucket-name"

the test object in the bucket, foo.jpg, has no additional acl or permissions set beyond the standard "Access for object owner"

Now when I assume the role, and try to get object

aws s3api get-object  --profile download_asset --region eu-west-2 --bucket my-bucket-name --key foo.jpg file.jpg

(or using boto3, or signing the url with those credentials and retrieving that url)

I get "An error occurred (AccessDenied) when calling the GetObject operation: Access Denied"

Changing the profile to the bucket owner user credentials, I can download the file.

What am I doing wrong?

[note that while I have access to both Account A and Account B in this test set up, in real life Account B will belong to a third party to whom I want to provide a working policy that they can use to open up what they require]

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The solution was that when granting permission to a role owned by a third party account, then the role needs to have a policy added to it granting access to the bucket as well - the permissions need to be defined and match on both ends

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