IAM policy should avoid use of wildcards and instead apply the principle of least privilege

Explanation

You should use the principle of least privilege when defining your IAM policies. This means you should specify each exact permission required without using wildcards, as this could cause the granting of access to certain undesired actions, resources and principals.

Possible Impact

Overly permissive policies may grant access to sensitive resources

Suggested Resolution

Specify the exact permissions required, and to which resources they should apply instead of using wildcards.

Insecure Example

The following example will fail the aws-iam-no-policy-wildcards check.

resource "aws_iam_role_policy" "test_policy" {
	name = "test_policy"
	role = aws_iam_role.test_role.id

	policy = data.aws_iam_policy_document.s3_policy.json
}

resource "aws_iam_role" "test_role" {
	name = "test_role"
	assume_role_policy = jsonencode({
		Version = "2012-10-17"
		Statement = [
		{
			Action = "sts:AssumeRole"
			Effect = "Allow"
			Sid    = ""
			Principal = {
			Service = "s3.amazonaws.com"
			}
		},
		]
	})
}

data "aws_iam_policy_document" "s3_policy" {
  statement {
    principals {
      type        = "AWS"
      identifiers = ["arn:aws:iam::${data.aws_caller_identity.current.account_id}:root"]
    }
    actions   = ["s3:*"]
    resources = ["*"]
  }
}

Secure Example

The following example will pass the aws-iam-no-policy-wildcards check.

resource "aws_iam_role_policy" "test_policy" {
	name = "test_policy"
	role = aws_iam_role.test_role.id

	policy = data.aws_iam_policy_document.s3_policy.json
}

resource "aws_iam_role" "test_role" {
	name = "test_role"
	assume_role_policy = jsonencode({
		Version = "2012-10-17"
		Statement = [
		{
			Action = "sts:AssumeRole"
			Effect = "Allow"
			Sid    = ""
			Principal = {
			Service = "s3.amazonaws.com"
			}
		},
		]
	})
}

data "aws_iam_policy_document" "s3_policy" {
  statement {
    principals {
      type        = "AWS"
      identifiers = ["arn:aws:iam::${data.aws_caller_identity.current.account_id}:root"]
    }
    actions   = ["s3:GetObject"]
    resources = [aws_s3_bucket.example.arn]
  }
}

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