AWS Security Blog
How to use service control policies to set permission guardrails across accounts in your AWS Organization
AWS Organizations provides central governance and management for multiple accounts. Central security administrators use service control policies (SCPs) with AWS Organizations to establish controls that all IAM principals (users and roles) adhere to. Now, you can use SCPs to set permission guardrails with the fine-grained control supported in the AWS Identity and Access Management (IAM) policy language. This makes it easier for you to fine-tune policies to meet the precise requirements of your organization’s governance rules.
Now, using SCPs, you can specify Conditions, Resources, and NotAction to deny access across accounts in your organization or organizational unit. For example, you can use SCPs to restrict access to specific AWS Regions, or prevent your IAM principals from deleting common resources, such as an IAM role used for your central administrators. You can also define exceptions to your governance controls, restricting service actions for all IAM entities (users, roles, and root) in the account except a specific administrator role.
To implement permission guardrails using SCPs, you can use the new policy editor in the AWS Organizations console. This editor makes it easier to author SCPs by guiding you to add actions, resources, and conditions. In this post, I review SCPs, walk through the new capabilities, and show how to construct an example SCP you can use in your organization today.
Overview of Service Control Policy concepts
Before I walk through some examples, I’ll review a few features of SCPs and AWS Organizations.
SCPs offer central access controls for all IAM entities in your accounts. You can use them to enforce the permissions you want everyone in your business to follow. Using SCPs, you can give your developers more freedom to manage their own permissions because you know they can only operate within the boundaries you define.
You create and apply SCPs through AWS Organizations. When you create an organization, AWS Organizations automatically creates a root, which forms the parent container for all the accounts in your organization. Inside the root, you can group accounts in your organization into organizational units (OUs) to simplify management of these accounts. You can create multiple OUs within a single organization, and you can create OUs within other OUs to form a hierarchical structure. You can attach SCPs to the organization root, OUs, and individual accounts. SCPs attached to the root and OUs apply to all OUs and accounts inside of them.
SCPs use the AWS Identity and Access Management (IAM) policy language; however, they do not grant permissions. SCPs enable you set permission guardrails by defining the maximum available permissions for IAM entities in an account. If a SCP denies an action for an account, none of the entities in the account can take that action, even if their IAM permissions allow them to do so. The guardrails set in SCPs apply to all IAM entities in the account, which include all users, roles, and the account root user.
Policy Elements Available in SCPs
The table below summarizes the IAM policy language elements available in SCPs. You can read more about the different IAM policy elements in the IAM JSON Policy Reference.
The Supported Statement Effect column describes the effect type you can use with each policy element in SCPs.
|Policy Element||Definition||Supported Statement Effect|
|Statement||Main element for a policy. Each policy can have multiple statements.||Allow, Deny|
|Sid||(Optional) Friendly name for the statement.||Allow, Deny|
|Effect||Define whether a SCP statement allows or denies actions in an account.||Allow, Deny|
|Action||List the AWS actions the SCP applies to.||Allow, Deny|
|NotAction (New)||(Optional) List the AWS actions exempt from the SCP. Used in place of the Action element.||Deny|
|Resource (New)||List the AWS resources the SCP applies to.||Deny|
|Condition (New)||(Optional) Specify conditions for when the statement is in effect.||Deny|
Note: Some policy elements are only available in SCPs that deny actions.
You can use the new policy elements in new or existing SCPs in your organization. In the next section, I use the new elements to create a SCP using the AWS Organizations console.
Create an SCP in the AWS Organizations console
In this section, you’ll create an SCP that restricts IAM principals in accounts from making changes to a common administrative IAM role created in all accounts in your organization. Imagine your central security team uses these roles to audit and make changes to AWS settings. For the purposes of this example, you have a role in all your accounts named AdminRole that has the AdministratorAccess managed policy attached to it. Using an SCP, you can restrict all IAM entities in the account from modifying AdminRole or its associated permissions. This helps you ensure this role is always available to your central security team. Here are the steps to create and attach this SCP.
- Ensure you’ve enabled all features in AWS Organizations and SCPs through the AWS Organizations console.
- In the AWS Organizations console, select the Policies tab, and then select Create policy.
- Give your policy a name and description that will help you quickly identify it. For this example, I use the following name and description.
- Name: DenyChangesToAdminRole
- Description: Prevents all IAM principals from making changes to AdminRole.
- The policy editor provides you with an empty statement in the text editor to get started. Position your cursor inside the policy statement. The editor detects the content of the policy statement you selected, and allows you to add relevant Actions, Resources, and Conditions to it using the left panel.
- Change the Statement ID to describe what the statement does. For this example, I reused the name of the policy, DenyChangesToAdminRole, because this policy has only one statement.
- Next, add the actions you want to restrict. Using the left panel, select the IAM service. You’ll see a list of actions. To learn about the details of each action, you can hover over the action with your mouse. For this example, we want to allow principals in the account to view the role, but restrict any actions that could modify or delete it. We use the new NotAction policy element to deny all actions except the view actions for the role. Select the following view actions from the list:
- Now position your cursor at the Action element and change it to NotAction. After you perform the steps above, your policy should look like the one below.
- Next, apply these controls to only the AdminRole role in your accounts. To do this, use the Resource policy element, which now allows you to provide specific resources.
- On the left, near the bottom, select the Add Resources button.
- In the prompt, select the IAM service from the dropdown menu.
- Select the role as the resource type, and then type “arn:aws:iam::*:role/AdminRole” in the resource ARN prompt.
- Select Add resource.
Note: The AdminRole has a common name in all accounts, but the account IDs will be different for each individual role. To simplify the policy statement, use the * wildcard in place of the account ID to account for all roles with this name regardless of the account.
- Your policy should look like this:
- Select the Save changes button to create your policy. You can see the new policy in the Policies tab.
- Finally, attach the policy to the AWS account where you want to apply the permissions.
When you attach the SCP, it prevents changes to the role’s configuration. The central security team that uses the role might want to make changes later on, so you may want to allow the role itself to modify the role’s configuration. I’ll demonstrate how to do this in the next section.
Grant an exception to your SCP for an administrator role
In the previous section, you created a SCP that prevented all principals from modifying or deleting the AdminRole IAM role. Administrators from your central security team may need to make changes to this role in your organization, without lifting the protection of the SCP. In this next example, I build on the previous policy to show how to exclude the AdminRole from the SCP guardrail.
- In the AWS Organizations console, select the Policies tab, select the DenyChangesToAdminRole policy, and then select Policy editor.
- Select Add Condition. You’ll use the new Condition element of the policy, using the aws:PrincipalARN global condition key, to specify the role you want to exclude from the policy restrictions.
- The aws:PrincipalARN condition key returns the ARN of the principal making the request. You want to ignore the policy statement if the requesting principal is the AdminRole. Using the StringNotLike operator, assert that this SCP is in effect if the principal ARN is not the AdminRole. To do this, fill in the following values for your condition.
- Condition key: aws:PrincipalARN
- Qualifier: Default
- Operator: StringNotLike
- Value: arn:aws:iam::*:role/AdminRole
- Select Add condition. The following policy will appear in the edit window.
- After you validate the policy, select Save. If you already attached the policy in your organization, the changes will immediately take effect.
Now, the SCP denies all principals in the account from updating or deleting the AdminRole, except the AdminRole itself.
You can now use SCPs to restrict access to specific resources, or define conditions for when SCPs are in effect. You can use the new functionality in your existing SCPs today, or create new permission guardrails for your organization. I walked through one example in this blog post, and there are additional use cases for SCPs that you can explore in the documentation. Below are a few that we have heard from customers that you may want to look at.
- Account may only operate in certain AWS regions (example)
- Account may only deploy certain EC2 instance types (example)
- Account requires MFA is enabled before taking an action (example)
You can start applying SCPs using the AWS Organizations console, CLI, or API. See the Service Control Policies Documentation or the AWS Organizations Forums for more information about SCPs, how to use them in your organization, and additional examples.
Want more AWS Security how-to content, news, and feature announcements? Follow us on Twitter.