Cross account role granting S3 bucket access - Permission Denied - amazon-web-services

Question

I have two accounts, account ACCOUNTAAAA, and ACCOUNTBBBB. A bucket (BUCKETAAAA) exists in account ACCOUNTAAAA and an instance with a role (ROLEBBBB) in ACCOUNTBBBB needs to be able to read from it.
I have adding the following permissions to the bucket:
{
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::ACCOUNTBBBB:role/ROLEBBBB"
},
"Action": [
"s3:*"
],
"Resource": "arn:aws:s3:::BUCKETAAAA/*"
}
I have the following permissions on the role in ACOUNTBBBB:
"Sid": "",
"Effect": "Allow",
"Action": [
"s3:*"
],
"Resource": [
"arn:aws:s3:::BUCKETAAAA/",
"arn:aws:s3:::BUCKETAAAA/*"
]
}
My assumption is I should be able to run aws s3 ls s3://BUCKETAAAA on the EC2 instance with the IAM role attached, and see the contants of BUCKETAAAA. When I try this, I get An error occurred (AccessDenied) when calling the ListObjects operation: Access Denied.
What am I missing here?

Answer 1

To reproduce your situation, I did the following:
Created Role-B in Account-B with EC2 as the Trusted Entity ("Allows EC2 instances to call AWS services on your behalf") and a policy granting access to Bucket-A
Created Bucket-A in Account-A
Added a Bucket Policy to Bucket-A, which grants access to Role-B
Assigned Role-B to an Amazon EC2 instance
The Bucket Policy on Bucket-A was:
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AddPerm",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::<account>:role/role-b"
},
"Action": [
"s3:*"
],
"Resource": [
"arn:aws:s3:::bucket-a",
"arn:aws:s3:::bucket-a/*"
]
}
]
}
The permissions on Role-B were:
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "BucketA",
"Effect": "Allow",
"Action": "s3:*",
"Resource": [
"arn:aws:s3:::bucket-a",
"arn:aws:s3:::bucket-a/*"
]
}
]
}
I was able to successfully use aws s3 ls s3://bucket-a and was able to copy files to Bucket-A.
The main difference I see with your attempt was that your Bucket Policy only granted permissions for:
"Resource": "arn:aws:s3:::BUCKETAAAA/*"
This means "anything within Bucket-A" but does not include Bucket-A itself. The command aws s3 ls s3://BUCKETAAAA operates on the bucket, for which the role has no permissions.
Thus, you should also add permissions for the bucket itself:
"Resource": [
"arn:aws:s3:::BUCKETAAAA",
"arn:aws:s3:::BUCKETAAAA/*"