AWS Security Blog
Establishing a data perimeter on AWS: Allow only trusted identities to access company data
As described in an earlier blog post, Establishing a data perimeter on AWS, Amazon Web Services (AWS) offers a set of capabilities you can use to implement a data perimeter to help prevent unintended access. One type of unintended access that companies want to prevent is access to corporate data by users who do not belong to the company. A combination of AWS Identity and Access Management (AWS IAM) features and capabilities that can help you achieve this goal in AWS while fostering innovation and agility form the identity perimeter. In this blog post, I will provide an overview of some of the security risks the identity perimeter is designed to address, policy examples, and implementation guidance for establishing the perimeter.
The identity perimeter is a set of coarse-grained preventative controls that help achieve the following objectives:
- Only trusted identities can access my resources
- Only trusted identities are allowed from my network
Trusted identities encompass IAM principals that belong to your company, which is typically represented by an AWS Organizations organization. In AWS, an IAM principal is a person or application that can make a request for an action or operation on an AWS resource. There are also scenarios when AWS services perform actions on your behalf using identities that do not belong to your organization. You should consider both types of data access patterns when you create a definition of trusted identities that is specific to your company and your use of AWS services. All other identities are considered untrusted and should have no access except by explicit exception.
Security risks addressed by the identity perimeter
The identity perimeter helps address several security risks, including the following.
Unintended data disclosure due to misconfiguration. Some AWS services support resource-based IAM policies that you can use to grant principals (including principals outside of your organization) permissions to perform actions on the resources they are attached to. While this allows developers to configure resource-based policies based on their application requirements, you should ensure that access to untrusted identities is prohibited even if the developers grant broad access to your resources, such as Amazon Simple Storage Service (Amazon S3) buckets. Figure 1 illustrates examples of access patterns you would want to prevent—specifically, principals outside of your organization accessing your S3 bucket from a non-corporate AWS account, your on-premises network, or the internet.
Figure 1: Unintended access to your S3 bucket by identities outside of your organization
Unintended data disclosure through non-corporate credentials. Some AWS services, such as Amazon Elastic Compute Cloud (Amazon EC2) and AWS Lambda, let you run code using the IAM credentials of your choosing. Similar to on-premises environments where developers might have access to physical and virtual servers, there is a risk that the developers can bring personal IAM credentials to a corporate network and attempt to move company data to personal AWS resources. For example, Figure 2 illustrates unintended access patterns where identities outside of your AWS Organizations organization are used to transfer data from your on-premises networks or VPC to an S3 bucket in a non-corporate AWS account.
Figure 2: Unintended access from your networks by identities outside of your organization
Implementing the identity perimeter
Before you can implement the identity perimeter by using preventative controls, you need to have a way to evaluate whether a principal is trusted and do this evaluation effectively in a multi-account AWS environment. IAM policies allow you to control access based on whether the IAM principal belongs to a particular account or an organization, with the following IAM condition keys:
- The aws:PrincipalOrgID condition key gives you a succinct way to refer to all IAM principals that belong to a particular organization. There are similar condition keys, such as aws:PrincipalOrgPaths and aws:PrincipalAccount, that allow you to define different granularities of trust.
- The aws:PrincipalIsAWSService condition key gives you a way to refer to AWS service principals when those are used to access resources on your behalf. For example, when you create a flow log with an S3 bucket as the destination, VPC Flow Logs uses a service principal, delivery.logs.amazonaws.com, which does not belong to your organization, to publish logs to Amazon S3.
In the context of the identity perimeter, there are two types of IAM policies that can help you ensure that the call to an AWS resource is made by a trusted identity:
- Resource-based policies – Policies that are attached to resources. For a list of services that support resource-based policies, see AWS services that work with IAM.
- VPC endpoint policies – Policies that are attached to VPC endpoints. For a list of services that support VPC endpoints and VPC endpoint policies, see AWS services that integrate with AWS PrivateLink.
Using the IAM condition keys and the policy types just listed, you can now implement the identity perimeter. The following table illustrates the relationship between identity perimeter objectives and the AWS capabilities that you can use to achieve them.
| Data perimeter | Control objective | Implemented by using | Primary IAM capability |
| Identity | Only trusted identities can access my resources. | Resource-based policies | aws:PrincipalOrgID aws:PrincipalIsAWSService |
| Only trusted identities are allowed from my network. | VPC endpoint policies |
Let’s see how you can use these capabilities to mitigate the risk of unintended access to your data.
Only trusted identities can access my resources
Resource-based policies allow you to specify who has access to the resource and what actions they can perform. Resource-based policies also allow you to apply identity perimeter controls to mitigate the risk of unintended data disclosure due to misconfiguration. The following is an example of a resource-based policy for an S3 bucket that limits access to only trusted identities. Make sure to replace <DOC-EXAMPLE-MY-BUCKET> and <MY-ORG-ID> with your information.
The Deny statement in the preceding policy has two condition keys where both conditions must resolve to true to invoke the Deny effect. This means that this policy will deny any S3 action unless it is performed by an IAM principal within your organization (StringNotEqualsIfExists with aws:PrincipalOrgID) or a service principal (BoolIfExists with aws:PrincipalIsAWSService). Note that resource-based policies on AWS resources do not allow access outside of the account by default. Therefore, in order for another account or an AWS service to be able to access your resource directly, you need to explicitly grant access permissions with appropriate Allow statements added to the preceding policy.
Some AWS resources allow sharing through the use of AWS Resource Access Manager (AWS RAM). When you create a resource share in AWS RAM, you should choose Allow sharing with principals in your organization only to help prevent access from untrusted identities. In addition to the primary capabilities for the identity perimeter, you should also use the ram:RequestedAllowsExternalPrincipals condition key in the AWS Organizations service control policies (SCPs) to specify that resource shares cannot be created or modified to allow sharing with untrusted identities. For an example SCP, see Example service control policies for AWS Organizations and AWS RAM in the AWS RAM User Guide.
Only trusted identities are allowed from my network
When you access AWS services from on-premises networks or VPCs, you can use public service endpoints or connect to supported AWS services by using VPC endpoints. VPC endpoints allow you to apply identity perimeter controls to mitigate the risk of unintended data disclosure through non-corporate credentials. The following is an example of a VPC endpoint policy that allows access to all actions but limits the access to trusted identities only. Replace <MY-ORG-ID> with your information.
As opposed to the resource-based policy example, the preceding policy uses Allow statements to enforce the identity perimeter. This is because VPC endpoint policies do not grant any permissions but define the maximum access allowed through the endpoint. Your developers will be using identity-based or resource-based policies to grant permissions required by their applications. We use two statements in this example policy to invoke the Allow effect in two scenarios: if an action is performed by an IAM principal that belongs to your organization (StringEquals with aws:PrincipalOrgID in the AllowRequestsByOrgsIdentities statement) or if an action is performed by a service principal (Bool with aws:PrincipalIsAWSService in the AllowRequestsByAWSServicePrincipals statement). We do not use IfExists in the end of the condition operators in this case, because we want the condition elements to evaluate to true only if the specified keys exist in the request.
It is important to note that in order to apply the VPC endpoint policies to requests originating from your on-premises environment, you need to configure private connectivity to AWS through AWS Direct Connect and/or AWS Site-to-Site VPN. Proper routing rules and DNS configurations will help you to ensure that traffic to AWS services is flowing through your VPC interface endpoints and is governed by the applied policies for supported services. You might also need to implement a mechanism to prevent cross-Region API requests from bypassing the identity perimeter controls within your network.
Extending your identity perimeter
There might be circumstances when you want to grant access to your resources to principals outside of your organization. For example, you might be hosting a dataset in an Amazon S3 bucket that is being accessed by your business partners from their own AWS accounts. In order to support this access pattern, you can use the aws:PrincipalAccount condition key to include third-party account identities as trusted identities in a policy. This is shown in the following resource-based policy example. Replace <DOC-EXAMPLE-MY-BUCKET>, <MY-ORG-ID>, <THIRD-PARTY-ACCOUNT-A>, and <THIRD-PARTY-ACCOUNT-B> with your information.
The preceding policy adds the aws:PrincipalAccount condition key to the StringNotEqualsIfExists operator. You now have a Deny statement with three condition keys where all three conditions must resolve to true to invoke the Deny effect. Therefore, this policy denies any S3 action unless it is performed by an IAM principal that belongs to your organization (StringNotEqualsIfExists with aws:PrincipalOrgID), by an IAM principal that belongs to specified third-party accounts (StringNotEqualsIfExists with aws:PrincipalAccount), or a service principal (BoolIfExists with aws:PrincipalIsAWSService).
There might also be circumstances when you want to grant access from your networks to identities external to your organization. For example, your applications could be uploading or downloading objects to or from a third-party S3 bucket by using third-party generated pre-signed Amazon S3 URLs. The principal that generates the pre-signed URL will belong to the third-party AWS account. Similar to the previously discussed S3 bucket policy, you can extend your identity perimeter to include identities that belong to trusted third-party accounts by using the aws:PrincipalAccount condition key in your VPC endpoint policy.
Additionally, some AWS services make unauthenticated requests to AWS owned resources through your VPC endpoint. An example of such a pattern is Kernel Live Patching on Amazon Linux 2, which allows you to apply security vulnerability and critical bug patches to a running Linux kernel. Amazon EC2 makes an unauthenticated call to Amazon S3 to download packages from Amazon Linux repositories hosted on Amazon EC2 service-owned S3 buckets. To include this access pattern into your identity perimeter definition, you can choose to allow unauthenticated API calls to AWS owned resources in the VPC endpoint policies.
The following example VPC endpoint policy demonstrates how to extend your identity perimeter to include access to Amazon Linux repositories and to Amazon S3 buckets owned by a third-party. Replace <MY-ORG-ID>, <REGION>, <ACTION>, <THIRD-PARTY-ACCOUNT-A>, and <THIRD-PARTY-BUCKET-ARN> with your information.
The preceding example adds two new statements to the VPC endpoint policy. The AllowUnauthenticatedRequestsToAWSResources statement allows the s3:GetObject action on buckets that host Amazon Linux repositories. The AllowRequestsByThirdPartyIdentitiesToThirdPartyResources statement allows actions on resources owned by a third-party entity by principals that belong to the third-party account (StringEquals with aws:PrincipalAccount).
Note that identity perimeter controls do not eliminate the need for additional network protections, such as making sure that your private EC2 instances or databases are not inadvertently exposed to the internet due to overly permissive security groups.
Apart from preventative controls established by the identity perimeter, we also recommend that you configure AWS Identity and Access Management Access Analyzer. IAM Access Analyzer helps you identify unintended access to your resources and data by monitoring policies applied to supported resources. You can review IAM Access Analyzer findings to identify resources that are shared with principals that do not belong to your AWS Organizations organization. You should also consider enabling Amazon GuardDuty to detect misconfigurations or anomalous access to your resources that could lead to unintended disclosure of your data. GuardDuty uses threat intelligence, machine learning, and anomaly detection to analyze data from various sources in your AWS accounts. You can review GuardDuty findings to identify unexpected or potentially malicious activity in your AWS environment, such as an IAM principal with no previous history invoking an S3 API.
IAM policy samples
This AWS git repository contains policy examples that illustrate how to implement identity perimeter controls for a variety of AWS services and actions. The policy samples do not represent a complete list of valid data access patterns and are for reference purposes only. They are intended for you to tailor and extend to suit the needs of your environment. Make sure that you thoroughly test the provided example policies before you implement them in your production environment.
Deploying the identity perimeter at scale
As discussed earlier, you implement the identity perimeter as coarse-grained preventative controls. These controls typically need to be implemented for each VPC by using VPC endpoint policies and on all resources that support resource-based policies. The effectiveness of these controls relies on their ability to scale with the environment and to adapt to its dynamic nature.
The methodology you use to deploy identity perimeter controls will depend on the deployment mechanisms you use to create and manage AWS accounts. For example, you might choose to use AWS Control Tower and the Customizations for AWS Control Tower solution (CfCT) to govern your AWS environment at scale. You can use CfCT or your custom CI/CD pipeline to deploy VPC endpoints and VPC endpoint policies that include your identity perimeter controls.
Because developers will be creating resources such as S3 buckets and AWS KMS keys on a regular basis, you might need to implement automation to enforce identity perimeter controls when those resources are created or their policies are changed. One option is to use custom AWS Config rules. Alternatively, you can choose to enforce resource deployment through AWS Service Catalog or a CI/CD pipeline. With the AWS Service Catalog approach, you can have identity perimeter controls built into the centrally controlled products that are made available to developers to deploy within their accounts. With the CI/CD pipeline approach, the pipeline can have built-in compliance checks that enforce identity perimeter controls during the deployment. If you are deploying resources with your CI/CD pipeline by using AWS CloudFormation, see the blog post Proactively keep resources secure and compliant with AWS CloudFormation Hooks.
Regardless of the deployment tools you select, identity perimeter controls, along with other baseline security controls applicable to your multi-account environment, should be included in your account provisioning process. You should also audit your identity perimeter configurations periodically and upon changes in your organization, which could lead to modifications in your identity perimeter controls (for example, disabling a third-party integration). Keeping your identity perimeter controls up to date will help ensure that they are consistently enforced and help prevent unintended access during the entire account lifecycle.
Conclusion
In this blog post, you learned about the foundational elements that are needed to define and implement the identity perimeter, including sample policies that you can use to start defining guardrails that are applicable to your environment and control objectives.
Following are additional resources that will help you further explore the identity perimeter topic, including a whitepaper and a hands-on-workshop.
- Data perimeters on AWS
- Establishing a data perimeter on AWS (blog)
- Building a Data Perimeter on AWS (whitepaper)
- Data Perimeter (hands-on workshop)
- An easier way to control access to AWS resources by using the AWS organization of IAM principals (blog)
If you have any questions, comments, or concerns, contact AWS Support or browse AWS re:Post. If you have feedback about this post, submit comments in the Comments section below.
Want more AWS Security news? Follow us on Twitter.