In a greenfield environment you generally have the luxury to right any wrongs of any past tech debt. It can be more difficult to apply this to an existing environment? For example, my setup is configured to just work with the AWS CLI and various litmus tests to validate that. Generally instructions would include, valid your AWS access.  This can be as simple as: 

$ aws ec2 describe-regions
$ aws ec2 describe-availability-zones --profile oh

As part of documenting some upcoming Athena/Hadoop/Pig/RDBMS posts I decided it was important to separate out the AWS IAM privileges with a new user and permission policies.This introduced a number of steps that simply do not work.  Creating a new AWS IAM user is not complex. Validating console and API access of that user required some revised setup.

$ aws ec2 describe-regions

An error occurred (AuthFailure) when calling the DescribeRegions operation: AWS was not able to validate the provided access credentials

In order to be able to use the CLI you require your aws_access_key_id and aws_secret_access_key information as well as aws_session_token if used. In order for a new individual user to gain this information, you also need a number of policy rules including the ability to ListAccessKeys, CreateAccessKey and potentially DeleteAccessKey.

 
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "iam:DeleteAccessKey",
                "iam:CreateAccessKey",
                "iam:ListAccessKeys"
            ],
            "Resource": "arn:aws:iam::[account]:user/[username]"
        }
    ]
}

In this example, we also restrict the least privileged model with a specific user resource ARN. For a single user account that is workable, for a large organization it would not.
This gives the ability to configure your AWS CLI via typical ~/.aws/credentials and/or ~/aws/config settings. Performing  the litmus test now gives:

$ aws ec2 describe-regions

An error occurred (UnauthorizedOperation) when calling the DescribeRegions operation: You are not authorized to perform this operation.

This requires a policy of:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "ec2:DescribeAvailabilityZones",
                "ec2:DescribeRegions"
            ],
            "Resource": "*"
        }
    ]
}

$ aws ec2 describe-regions | jq '.Regions[0]'
{
  "Endpoint": "ec2.eu-north-1.amazonaws.com",
  "RegionName": "eu-north-1",
  "OptInStatus": "opt-in-not-required"
}


$ aws ec2 describe-availability-zones --filter "Name=region-name,Values=us-east-1" | jq -r '.AvailabilityZones[].ZoneName'

us-east-1a
us-east-1b
us-east-1c
us-east-1d
us-east-1e
us-east-1f

However, this may be too restrictive for a larger organization.  The EC2 Access level for ‘list’ includes currently over 120 individual permissions. A more open policy could be:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "ec2:Describe*"
            ],
            "Resource": "*"
        }
    ]
}

However this does not provide all of the EC2 ‘list’ actions, e.g. ExportClientVpnClientConfiguration, and it includes several ‘read’ actions, e.g. DescribeVolumesModifications.
Selecting the ‘list’ tickbox via the GUI will provide all actions by name individually in the policy action list, currently 117, however this is not forward compatible for any future list defined access level actions.

This is before the exercise to starting granting access to a new AWS service – Athena, and its data source S3.