How AWS Control Tower works with roles to create and manage accounts - AWS Control Tower
Roles and account creationHow AWS Control Tower aggregates AWS Config rules in unmanaged OUs and accountsProgrammatic roles and trust relationships for the AWS Control Tower audit accountAutomated Account Provisioning With IAM Roles

How AWS Control Tower works with roles to create and manage accounts

In general, roles are a part of identity and access management (IAM) in AWS. For general information about IAM and roles in AWS, see the IAM roles topic in the AWS IAM User Guide.

Roles and account creation

AWS Control Tower creates a customer's account by calling the CreateAccount API of AWS Organizations. When AWS Organizations creates this account, it creates a role within that account, which AWS Control Tower names by passing in a parameter to the API. The name of the role is AWSControlTowerExecution.

AWS Control Tower takes over the AWSControlTowerExecution role for all accounts created by Account Factory. Using this role, AWS Control Tower baselines the account and applies mandatory (and any other enabled) controls, which results in creation of other roles. These roles in turn are used by other services, such as AWS Config.

Note

To baseline an account is to set up its resources, which include Account Factory templates, sometimes referred to as blueprints, and controls. The baselining process also sets up the centralized logging and security audit roles on the account, as part of deploying the templates. AWS Control Tower baselines are contained in the roles that you apply to every enrolled account.

For more information about accounts and resources, see About AWS accounts in AWS Control Tower.

How AWS Control Tower aggregates AWS Config rules in unmanaged OUs and accounts

The AWS Control Tower management account creates an organization-level aggregator, which assists in detecting external AWS Config rules, so that AWS Control Tower does not need to gain access to unmanaged accounts. The AWS Control Tower console shows you how many externally created AWS Config rules you have for a given account. You can view details about those external rules in the External Config Rule Compliance tab of the Account details page.

To create the aggregator, AWS Control Tower adds a role with the permissions required to describe an organization and list the accounts under it. The AWSControlTowerConfigAggregatorRoleForOrganizations role requires the AWSConfigRoleForOrganizations managed policy and a trust relationship with config.amazonaws.com.

Here is the IAM policy (JSON artifact) attached to the role:

{
    "Version": "2012-10-17",
      "Statement": [
       {
        "Effect": "Allow",
        "Action": [
          "organizations:ListAccounts",
          "organizations:DescribeOrganization",
          "organizations:ListAWSServiceAccessForOrganization"
         ],
       "Resource": "*"
      }
    ]
  }

Here is the AWSControlTowerConfigAggregatorRoleForOrganizations trust relationship:

{
    "Version": "2012-10-17",
      "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
        "Service": "config.amazonaws.com"
        },
        "Action": "sts:AssumeRole"
      }
    ]
  }
}

To deploy this functionality in the management account, the following permissions are added in the managed policy AWSControlTowerServiceRolePolicy, which is used by the AWSControlTowerAdmin role when it creates the AWS Config aggregator:

{
  "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Action": [
          "config:PutConfigurationAggregator",
          "config:DeleteConfigurationAggregator",
          "iam:PassRole"
          ],
        "Resource": [
          "arn:aws:iam:::role/service-role/AWSControlTowerConfigAggregatorRoleForOrganizations",
          "arn:aws:config:::config-aggregator/"
          ]
        },
      {
        "Effect": "Allow",
        "Action": "organizations:EnableAWSServiceAccess",
        "Resource": "*"
      }
    ]
}

New resources created: AWSControlTowerConfigAggregatorRoleForOrganizations and aws-controltower-ConfigAggregatorForOrganizations

When you are ready, you can enroll accounts individually, or enroll them as a group by registering an OU. When you've enrolled an account, if you create a rule in AWS Config, AWS Control Tower detects the new rule. The aggregator shows the number of external rules and provides a link to the AWS Config console where you can view the details of each external rule for your account. Use the information in the AWS Config console and the AWS Control Tower console to determine whether you have the appropriate controls enabled for the account.

Programmatic roles and trust relationships for the AWS Control Tower audit account

You can sign into the audit account and assume a role to review other accounts programmatically. The audit account does not allow you to log in to other accounts manually.

The audit account gives you programmatic access to other accounts, by means of some roles that are granted to AWS Lambda functions only. For security purposes, these roles have trust relationships with other roles, which means that the conditions under which the roles can be utilized are strictly defined.

The AWS Control Tower stack StackSet-AWSControlTowerBP-BASELINE-ROLES creates these programmatic-only, cross-account IAM roles in the audit account:

  • aws-controltower-AdministratorExecutionRole

  • aws-controltower-ReadOnlyExecutionRole

The AWS Control Tower stack StackSet-AWSControlTowerSecurityResources creates these programmatic-only, cross-account IAM roles in the audit account:

  • aws-controltower-AuditAdministratorRole

  • aws-controltower-AuditReadOnlyRole

ReadOnlyExecutionRole: Note that this role allows the audit account to read objects in Amazon S3 buckets across the entire organization (in contrast to the SecurityAudit policy, which allows for metadata access only).

aws-controltower-AdministratorExecutionRole:

  • Has administrator permissions

  • Cannot be assumed from the console

  • Can be assumed only by a role in the audit account – the aws-controltower-AuditAdministratorRole

The following artifact shows the trust relationship for aws-controltower-AdministratorExecutionRole. The placeholder number 012345678901 will be replaced by the Audit_acct_ID number for your audit account.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::012345678901:role/aws-controltower-AuditAdministratorRole"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}
aws-controltower-AuditAdministratorRole:

  • Can be assumed by the AWS Lambda service only

  • Has permission to perform read (Get) and write (Put) operations on Amazon S3 objects with names that start with the string log

Attached policies:

1. AWSLambdaExecute – AWS managed policy

2. AssumeRole-aws-controltower-AuditAdministratorRole – inline policy – Created by AWS Control Tower, artifact follows.

{
  "Version": "2012-10-17",
  "Statement": [
	{
	"Action": [
		 "sts:AssumeRole"
		 ],
	"Resource": [
		 "arn:aws:iam::*:role/aws-controltower-AdministratorExecutionRole"
		 ],
	"Effect": "Allow"
	}
   ]
}

The following artifact shows the trust relationship for aws-controltower-AuditAdministratorRole:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}
aws-controltower-ReadOnlyExecutionRole:

  • Cannot be assumed from the console

  • Can be assumed only by another role in the audit account – the AuditReadOnlyRole

The following artifact shows the trust relationship for aws-controltower-ReadOnlyExecutionRole. The placeholder number 012345678901 will be replaced by the Audit_acct_ID number for your audit account.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::012345678901:role/aws-controltower-AuditReadOnlyRole "
      },
      "Action": "sts:AssumeRole"
    }
  ]
}
aws-controltower-AuditReadOnlyRole:

  • Can be assumed by the AWS Lambda service only

  • Has permission to perform read (Get) and write (Put) operations on Amazon S3 objects with names that start with the string log

Attached policies:

1. AWSLambdaExecute – AWS managed policy

2. AssumeRole-aws-controltower-AuditReadOnlyRole – inline policy – Created by AWS Control Tower, artifact follows.

{
  "Version": "2012-10-17",
  "Statement": [
	{
	"Action": [
		"sts:AssumeRole"
	],
	"Resource": [
		"arn:aws:iam::*:role/aws-controltower-ReadOnlyExecutionRole"
	],
	"Effect": "Allow"
   }
  ]
}

The following artifact shows the trust relationship for aws-controltower-AuditAdministratorRole:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Automated Account Provisioning With IAM Roles

To configure Account Factory accounts in a more automated way, you can create Lambda functions in the AWS Control Tower management account, which assumes the AWSControlTowerExecution role in the member account. Then, using the role, the management account performs the desired configuration steps in each member account.

If you're provisioning accounts using Lambda functions, the identity that will perform this work must have the following IAM permissions policy, in addition to AWSServiceCatalogEndUserFullAccess.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AWSControlTowerAccountFactoryAccess",
            "Effect": "Allow",
            "Action": [
                "sso:GetProfile",
                "sso:CreateProfile",
                "sso:UpdateProfile",
                "sso:AssociateProfile",
                "sso:CreateApplicationInstance",
                "sso:GetSSOStatus",
                "sso:GetTrust",
                "sso:CreateTrust",
                "sso:UpdateTrust",
                "sso:GetPeregrineStatus",
                "sso:GetApplicationInstance",
                "sso:ListDirectoryAssociations",
                "sso:ListPermissionSets",
                "sso:GetPermissionSet",
                "sso:ProvisionApplicationInstanceForAWSAccount",
                "sso:ProvisionApplicationProfileForAWSAccountInstance",
                "sso:ProvisionSAMLProvider",
                "sso:ListProfileAssociations",
                "sso-directory:ListMembersInGroup",
                "sso-directory:AddMemberToGroup",
                "sso-directory:SearchGroups",
                "sso-directory:SearchUsers",
                "sso-directory:CreateUser",
                "sso-directory:DescribeGroups",
                "sso-directory:DescribeDirectory",
                "sso-directory:GetUserPoolInfo",
                "controltower:CreateManagedAccount",
                "controltower:DescribeManagedAccount",
                "controltower:DeregisterManagedAccount",
                "s3:GetObject",
                "organizations:describeOrganization",
                "sso:DescribeRegisteredRegions"
            ],
            "Resource": "*"
        }
    ]
}

The permissions sso:GetPeregrineStatus, sso:ProvisionApplicationInstanceForAWSAccount, sso:ProvisionApplicationProfileForAWSAccountInstance, and sso:ProvisionSAMLProvide are required by AWS Control Tower Account Factory to interact with AWS IAM Identity Center.