An error occurred (AccessDenied) when calling the AssumeRole operation

Question

I have a lambda function (lambda-get-details) created using the below IAM role in Account-A

Role name: lambdarole

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": [
                "logs:CreateLogGroup",
                "logs:CreateLogStream",
                "logs:PutLogEvents"
            ],
            "Resource": "arn:aws:logs:*:*:*",
            "Effect": "Allow"
        },
        {
            "Action": [
                "config:PutEvaluations",
                "ec2:DescribeImages",
                "sts:AssumeRole"
            ],
            "Resource": "*",
            "Effect": "Allow"
        }
    ]
}

The same IAM role name (lambdarole) is created in different accounts as well like Account-B

Now the lambda function from Account-A needs to get details from Account-B for example (List of AMI's) and we are getting the below error

"errorMessage": "An error occurred (AccessDenied) when calling the AssumeRole operation: User: arn:aws:sts::Account-A:assumed-role/lambdarole/lambda-get-details is not authorized to perform: sts:AssumeRole on resource: arn:aws:iam::Account-B:role/lambdarole

Can someone help me solving the above issue.

Any help would be appreciated

Thanks

Answer 1

In Account-A, the policy of lambdarole allows role assumption to any role ARN's (including roles in Account-B). This is taken care by this statement

       {
            "Action": [
                "sts:AssumeRole"
            ],
            "Resource": "*",
            "Effect": "Allow"
        }

Similarly in Account-B, the role lambdarole should contain a trust policy which allows role assumption from Account-A.

Add the AccountID or the lambdarole Role ARN of Account-A as the Principal in the Account-B's lambdarole's AssumeRolePolicyDocument.

The AssumeRolePolicyDocument would look like this (if account ID is used as Principal),

"AssumeRolePolicyDocument": {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect" : "Allow",
            "Action": "sts:AssumeRole",
            "Principal": {
                "AWS" : "<Account-ID-of-Account-A>"                                
            }
        }
    ]
}

You can refer here to understand how cross account access is established with IAM roles.