Post Syndicated from Rishi Mehrotra original https://aws.amazon.com/blogs/security/how-to-control-access-to-aws-resources-based-on-aws-account-ou-or-organization/

AWS Identity and Access Management (IAM) recently launched new condition keys to make it simpler to control access to your resources along your Amazon Web Services (AWS) organizational boundaries. AWS recommends that you set up multiple accounts as your workloads grow, and you can use multiple AWS accounts to isolate workloads or applications that have specific security requirements. By using the new conditions, aws:ResourceOrgID, aws:ResourceOrgPaths, and aws:ResourceAccount, you can define access controls based on an AWS resource’s organization, organizational unit (OU), or account. These conditions make it simpler to require that your principals (users and roles) can only access resources inside a specific boundary within your organization. You can combine the new conditions with other IAM capabilities to restrict access to and from AWS accounts that are not part of your organization.

This post will help you get started using the new condition keys. We’ll show the details of the new condition keys and walk through a detailed example based on the following scenario. We’ll also provide references and links to help you learn more about how to establish access control perimeters around your AWS accounts.

Consider a common scenario where you would like to prevent principals in your AWS organization from adding objects to Amazon Simple Storage Service (Amazon S3) buckets that don’t belong to your organization. To accomplish this, you can configure an IAM policy to deny access to S3 actions unless aws:ResourceOrgID matches your unique AWS organization ID. Because the policy references your entire organization, rather than individual S3 resources, you have a convenient way to maintain this security posture across any number of resources you control. The new conditions give you the tools to create a security baseline for your IAM principals and help you prevent unintended access to resources in accounts that you don’t control. You can attach this policy to an IAM principal to apply this rule to a single user or role, or use service control policies (SCPs) in AWS Organizations to apply the rule broadly across your AWS accounts. IAM principals that are subject to this policy will only be able to perform S3 actions on buckets and objects within your organization, regardless of their other permissions granted through IAM policies or S3 bucket policies.

New condition key details

You can use the aws:ResourceOrgID, aws:ResourceOrgPaths, and aws:ResourceAccount condition keys in IAM policies to place controls on the resources that your principals can access. The following table explains the new condition keys and what values these keys can take.

Note: Of the three keys, only aws:ResourceOrgPaths is a multi-value condition key, while aws:ResourceAccount and aws:ResourceOrgID are single-value keys. For information on how to use multi-value keys, see Creating a condition with multiple keys or values in the IAM documentation.

Resource owner keys compared to principal owner keys

The new IAM condition keys complement the existing principal condition keys aws:PrincipalAccount, aws:PrincipalOrgPaths, and aws:PrincipalOrgID. The principal condition keys help you define which AWS accounts, organizational units (OUs), and organizations are allowed to access your resources. For more information on the principal conditions, see Use IAM to share your AWS resources with groups of AWS accounts in AWS Organizations on the AWS Security Blog.

Using the principal and resource keys together helps you establish permission guardrails around your AWS principals and resources, and makes it simpler to keep your data inside the organization boundaries you define as you continue to scale. For example, you can define identity-based policies that prevent your IAM principals from accessing resources outside your organization (by using the aws:ResourceOrgID condition). Next, you can define resource-based policies that prevent IAM principals outside your organization from accessing resources that are inside your organization boundary (by using the aws:PrincipalOrgID condition). The combination of both policies prevents any access to and from AWS accounts that are not part of your organization. In the next sections, we’ll walk through an example of how to configure the identity-based policy in your organization. For the resource-based policy, you can follow along with the example in An easier way to control access to AWS resources by using the AWS organization of IAM principals on the AWS Security blog.

Setup for the examples

In the following sections, we’ll show an example IAM policy for each of the new conditions. To follow along with Example 1, which uses aws:ResourceAccount, you’ll just need an AWS account.

To follow along with Examples 2 and 3 that use aws:ResourceOrgPaths and aws:ResourceOrgID respectively, you’ll need to have an organization in AWS Organizations and at least one OU created. This blog post assumes that you have some familiarity with the basic concepts in IAM and AWS Organizations. If you need help creating an organization or want to learn more about AWS Organizations, visit Getting Started with AWS Organizations in the AWS documentation.

Which IAM policy type should I use?

You can implement the following examples as identity-based policies, or in SCPs that are managed in AWS Organizations. If you want to establish a boundary for some of your IAM principals, we recommend that you use identity-based policies. If you want to establish a boundary for an entire AWS account or for your organization, we recommend that you use SCPs. Because SCPs apply to an entire AWS account, you should take care when you apply the following policies to your organization, and account for any exceptions to these rules that might be necessary for some AWS services to function properly.

Example 1: Restrict access to AWS resources within a specific AWS account

Let’s look at an example IAM policy that restricts access along the boundary of a single AWS account. For this example, say that you have an IAM principal in account 222222222222, and you want to prevent the principal from accessing S3 objects outside of this account. To create this effect, you could attach the following IAM policy.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": " DenyS3AccessOutsideMyBoundary",
      "Effect": "Deny",
      "Action": [
        "s3:*"
      ],
      "Resource": "*",
      "Condition": {
        "StringNotEquals": {
          "aws:ResourceAccount": [
            "222222222222"
          ]
        }
      }
    }
  ]
}

Note: This policy is not meant to replace your existing IAM access controls, because it does not grant any access. Instead, this policy can act as an additional guardrail for your other IAM permissions. You can use a policy like this to prevent your principals from access to any AWS accounts that you don’t know or control, regardless of the permissions granted through other IAM policies.

This policy uses a Deny effect to block access to S3 actions unless the S3 resource being accessed is in account 222222222222. This policy prevents S3 access to accounts outside of the boundary of a single AWS account. You can use a policy like this one to limit your IAM principals to access only the resources that are inside your trusted AWS accounts. To implement a policy like this example yourself, replace account ID 222222222222 in the policy with your own AWS account ID. For a policy you can apply to multiple accounts while still maintaining this restriction, you could alternatively replace the account ID with the aws:PrincipalAccount condition key, to require that the principal and resource must be in the same account (see example #3 in this post for more details how to accomplish this).

Organization setup: Welcome to AnyCompany

For the next two examples, we’ll use an example organization called AnyCompany that we created in AWS Organizations. You can create a similar organization to follow along directly with these examples, or adapt the sample policies to fit your own organization. Figure 1 shows the organization structure for AnyCompany.

Figure 1: Organization structure for AnyCompany

Like all organizations, AnyCompany has an organization root. Under the root are three OUs: Media, Sports, and Governance. Under the Sports OU, there are three more OUs: Baseball, Basketball, and Football. AWS accounts in this organization are spread across all the OUs based on their business purpose. In total, there are six OUs in this organization.

Example 2: Restrict access to AWS resources within my organizational unit

Now that you’ve seen what the AnyCompany organization looks like, let’s walk through another example IAM policy that you can use to restrict access to a specific part of your organization. For this example, let’s say you want to restrict S3 object access within the following OUs in the AnyCompany organization:

• Media
• Sports
• Baseball
• Basketball
• Football

To define a boundary around these OUs, you don’t need to list all of them in your IAM policy. Instead, you can use the organization structure to your advantage. The Baseball, Basketball, and Football OUs share a parent, the Sports OU. You can use the new aws:ResourceOrgPaths key to prevent access outside of the Media OU, the Sports OU, and any OUs under it. Here’s the IAM policy that achieves this effect.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": " DenyS3AccessOutsideMyBoundary",
      "Effect": "Deny",
      "Action": [
        "s3:*"
      ],
      "Resource": "*",
      "Condition": {
        "ForAllValues:StringNotLike": {
          "aws:ResourceOrgPaths": [
            "o-acorg/r-acroot/ou-acroot-mediaou/",
            "o-acorg/r-acroot/ou-acroot-sportsou/*"
          ] 
        }
      }
    }
  ]
}

Note: Like the earlier example, this policy does not grant any access. Instead, this policy provides a backstop for your other IAM permissions, preventing your principals from accessing S3 objects outside an OU-defined boundary. If you want to require that your IAM principals consistently follow this rule, we recommend that you apply this policy as an SCP. In this example, we attached this policy to the root of our organization, applying it to all principals across all accounts in the AnyCompany organization.

The policy denies access to S3 actions unless the S3 resource being accessed is in a specific set of OUs in the AnyCompany organization. This policy is identical to Example 1, except for the condition block: The condition requires that aws:ResourceOrgPaths contains any of the listed OU paths. Because aws:ResourceOrgPaths is a multi-value condition, the policy uses the ForAllValues:StringNotLike operator to compare the values of aws:ResourceOrgPaths to the list of OUs in the policy.

The first OU path in the list is for the Media OU. The second OU path is the Sports OU, but it also adds the wildcard character * to the end of the path. The wildcard * matches any combination of characters, and so this condition matches both the Sports OU and any other OU further down its path. Using wildcards in the OU path allows you to implicitly reference other OUs inside the Sports OU, without having to list them explicitly in the policy. For more information about wildcards, refer to Using wildcards in resource ARNs in the IAM documentation.

Example 3: Restrict access to AWS resources within my organization

Finally, we’ll look at a very simple example of a boundary that is defined at the level of an entire organization. This is the same use case as the preceding two examples (restrict access to S3 object access), but scoped to an organization instead of an account or collection of OUs.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyS3AccessOutsideMyBoundary",
      "Effect": "Deny",
      "Action": [
        "s3:*"
      ],
      "Resource": "arn:aws:s3:::*/*",
      "Condition": {
        "StringNotEquals": {
          "aws:ResourceOrgID": "${aws:PrincipalOrgID}"
        }
      }
    }
  ]
}

Note: Like the earlier examples, this policy does not grant any access. Instead, this policy provides a backstop for your other IAM permissions, preventing your principals from accessing S3 objects outside your organization regardless of their other access permissions. If you want to require that your IAM principals consistently follow this rule, we recommend that you apply this policy as an SCP. As in the previous example, we attached this policy to the root of our organization, applying it to all accounts in the AnyCompany organization.

The policy denies access to S3 actions unless the S3 resource being accessed is in the same organization as the IAM principal that is accessing it. This policy is identical to Example 1, except for the condition block: The condition requires that aws:ResourceOrgID and aws:PrincipalOrgID must be equal to each other. With this requirement, the principal making the request and the resource being accessed must be in the same organization. This policy also applies to S3 resources that are created after the policy is put into effect, so it is simple to maintain the same security posture across all your resources.

For more information about aws:PrincipalOrgID, refer to AWS global condition context keys in the IAM documentation.

Learn more

In this post, we explored the new conditions, and walked through a few examples to show you how to restrict access to S3 objects across the boundary of an account, OU, or organization. These tools work for more than just S3, though: You can use the new conditions to help you protect a wide variety of AWS services and actions. Here are a few links that you may want to look at:

• IAM global condition context keys in the IAM documentation provides reference material for the new conditions.
• AWS: Deny access to SNS resources outside your account except via CloudFormation in the IAM documentation shows you how to create an IAM policy that denies access to all resources in AWS that don’t belong to your account, except for resources that AWS CloudFormation requires for normal operations.
• Example: Deny access to Amazon S3 resources outside your account except AWS Data Exchange in the IAM documentation shows you how to allow AWS Data Exchange to access common S3 data through your access control perimeter.
• Video: How to ensure that my identities only access my resources from a trusted network shows you how to use the new conditions in your network-based access controls.
• Whitepaper: Building an AWS Perimeter provides an in-depth overview of how to use these keys as part of a broader strategy to secure your AWS resources.

If you have any questions, comments, or concerns, contact AWS Support or start a new thread on the AWS Identity and Access Management forum. Thanks for reading about this new feature. If you have feedback about this post, submit comments in the Comments section below.

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