Perimeter implementation

This section describes the complete perimeter solution by evaluating each perimeter authorization condition and how the different policy types are used to achieve it. Each section will describe the overall solution for that objective, provide links to detailed policy examples, explain how exceptions can be implemented, and demonstrate how the controls prevent the unintended access pattern.

Only trusted identities

The objectives for this condition ensure that only my principals can access my resources and only my principals are allowed from my networks. You'll use resource-based policies and VPC endpoint policies to constrain which principals are allowed access.

The primary way to ensure IAM principals belong to my AWS is by specifying the aws:PrincipalOrgId IAM policy condition in those policies. This requires that the principal being considered during the authorization of access to a resource you own or originating from a network you own (regardless of the resource owner) belongs to your AWS organization.

You can implement a more granular restriction with the aws:PrincipalAccount or aws:PrincipalOrgPaths IAM policy conditions as well. To ensure that your resource policies only allow the intended access, you can use IAM Access Analyzer for supported resources to identify resource-based policies that are too permissive.

The following diagram demonstrates how these controls prevent unintended principals from accessing your resources or using your networks.

Preventing unintended principals in a resource-based policy and a VPC endpoint policy

In certain cases, AWS services might use an IAM service principal instead of an IAM role to interact with your resources. A service principal is not part of your AWS organization like IAM roles are. These are intended actions, but they need to be explicitly allowed in your resource-based policies and, in some cases, VPC endpoint policies. For example, AWS CloudTrail uses the IAM service principal cloudtrail.amazonaws.com to deliver logs to your Amazon S3 bucket, which requires an exception in your Amazon S3 bucket policy. Similarly, when you use a pre-signed URL for wait condition signaling from a VPC with AWS CloudFormation, you need to create an exception in your policy to allow it. In Allow statements, the service principal can be listed explicitly. However, you might also have Deny statements that the service principal needs to be exempt from. You can’t use a NotPrincipal statement with an AWS service principal; instead, use the aws:PrincipalIsAWSService condition to exempt service principals from Deny statements.

Refer to the AWS data perimeter policy examples repo for templates and instructions on how to create the required resource-based policies and VPC endpoint policies to achieve this objective as well as create any required exceptions. Specifically, look at the example resource-based policies with "Sid": "EnforceIdentityPerimeter" and the example default VPC endpoint policy.

Only trusted resources

The objectives for this condition ensure that my principals can only access my resources and that access from my networks only targets my resources (regardless of the principal involved). You’ll use SCPs and VPC endpoint policies to constrain which resources are allowed to be accessed.

The primary way to ensure targeted resources belong to my AWS is by specifying the aws:ResourceOrgId IAM policy condition in SCPs and VPC endpoint policies. This ensures that the resource being considered during authorization, either being accessed directly or through a VPC endpoint, belongs to your AWS organization. The following diagram demonstrates how these policies prevent access to an unintended resource.

Preventing access to unintended resources with SCPs and VPC endpoint policies

Using VPC endpoint policies in this way can be considered a defense in depth approach. This is because implementing the controls for the Only trusted identities objectives apply a policy on each endpoint that ensures only my principals can access resources from my networks. Then, the SCP used for this objective always applies to these principals to constrain what resources they can access. This indirectly accomplishes the same outcome as applying an aws:ResourceOrgId condition to your VPC endpoint policies.

In some cases, you might need to directly access resources outside of my AWS. These could be Amazon S3 buckets that AWS provides for things like Amazon Linux packages, CloudWatch agent installation, or public data repositories. They could also be resources like public SSM parameters. When you use cfn-hup, it supports the on.command hook, which allows you to use Amazon SQS messages to invoke the cfn-hup actions. This is used by AWS Elastic Beanstalk environments where the cfn-hup daemon retrieves a AWS CloudFormation specific credential to query an Amazon SQS queue owned by AWS. Remember that SCPs don’t apply to SLRs or AWS service principals, so when you need to create exceptions to allow them to access resources from your VPCs, you’ll need to use VPC endpoint policies. For example, in order to use AWS CloudFormation wait condition signaling, which uses an Amazon S3 pre-signed URL to a bucket owned by AWS, you have to create an exception to allow this access in a VPC endpoint policy.

You will need to use SCPs and VPC endpoint policies together to create the necessary exceptions. In VPC endpoint policies, you can list the trusted resources explicitly in the resource element of an Allow statement. However, in an SCP, you can only list specific resources in a Deny statement. In this case, we don’t want to deny the resources; we want to allow them. Instead, you can amend the trusted resource guardrail SCP that requires aws:ResourceOrgId. This statement can use a NotAction element to exempt specific actions from meeting the aws:ResourceOrgId condition. This combined approach allows you to exempt specific actions in your SCPs from the aws:ResourceOrgId guardrail and ensures that only specific resources can be accessed with those permissions by explicitly allowing them in a VPC endpoint policy.

In a few other scenarios, you can use services that use your credentials on your behalf to interact with resources that are not part of your organization. For example, AWS Service Catalog and AWS Data Exchange will write data to an Amazon S3 bucket that you don’t own. Amazon Athena can also write data to a bucket you don’t own. In the former cases, AWS owns the buckets those services store the data in, but in the latter case with Athena, the bucket could be owned by anyone. For that reason, you might not want to create a broad exception to accessing external resources with the aws:ViaAWSService condition. Not all of the resources that can be accessed through an AWS service should be considered a trusted resource. Instead, you can use the aws:CalledVia condition to explicitly allow services to behave this way.

Refer to the AWS data perimeter policy examples repo for templates and instructions on how to create the required SCPs and VPC endpoint policies to achieve this objective as well as manage any required exceptions. Specifically look at the resource perimeter policy and the default endpoint policy.

Only expected networks

This final condition’s objectives ensures that only my networks can be the source of requests from my principals or to my resources. You’ll use SCPs and resource-based policies to constrain which networks are allowed for access.

Within an SCP, you can define the expected networks by using an IP address with the aws:SourceIp IAM policy condition or a VPC identifier with the aws:SourceVpc condition. The following diagram shows how these policies prevent access from unexpected network locations.

Preventing access from unexpected networks with SCPs and resource-based policies

Applying a similar constraint with a resource-based policy can also be considered a defense in depth approach. This is because the Only trusted identities objectives apply a constraint on each resource-based policy that ensures only my principals can access those resources. Then, the network boundary SCP always applies to these principals to constrain the networks they can access resources from. This indirectly accomplishes the same outcome as applying an aws:SourceIp condition or aws:SourceVpc condition to your resource-based policies.

There are several scenarios where AWS will act on your behalf with your IAM credentials from networks that AWS owns that will require exceptions to these policies. For example, you can use AWS CloudFormation to define a template of resources for which AWS orchestrates the creation, update, and deletion. The initial request to create a AWS CloudFormation stack will originate from an expected network, but the subsequent requests for each resource in the template are made by the AWS CloudFormation service in an AWS network using your credentials or a service role for AWS CloudFormation that you’ve specified. This situation also occurs when you use Athena to run queries on CloudTrail logs. The aws:ViaAWSService IAM policy condition provides a way to implement an exception for some of these common scenarios where your IAM credentials are used in requests made by AWS on your behalf.

When AWS uses a service principal or SLR to interact with your resources, the source of that interaction is typically a network owned by AWS. In order to allow this access, you need to create an exception to the trusted network control in a resource-based policy since SCPs don’t apply to service principals or SLRs. A typical example of this pattern is CloudTrail log delivery to your Amazon S3 bucket.

The last consideration in implementing network controls is AWS services that operate in compute environments that are not part of your network. For example, Lambda functions and SageMaker Studio Notebooks both provide an option to run in AWS-owned networks.

Some of these services provide a configuration option for running the service in your VPC as well. If you want to use the same VPC network boundary for these services, you should monitor and, where possible, enforce it using the VPC configuration.

For example, customers can enforce AWS Lambda function deployments and updates to use Amazon Virtual Private Cloud (Amazon VPC) settings with IAM condition keys, use AWS Config Rules to audit this configuration, and then implement remediation with AWS Config Remediation Actions and AWS Systems Manager Automation documents.

It is important to note that not all AWS services are hosted as an AWS-owned endpoint authorized with IAM; for example, Amazon Relational Database Service (Amazon RDS) databases. Instead, these services expose their data plane inside a customer VPC.

The data plane is the part of the service that provides the day-to-day functionality of that thing. For MySQL RDS, it would be the IP address of the RDS instance on port 3306. Network controls such as firewalls or security groups should be used as part of your network boundary to prevent access to AWS services that are hosted in customer VPCs, but are not authorized with IAM credentials. Additionally, customers should leverage alternative authentication and authorization systems to access those services, such as AWS Secrets Manager for Amazon RDS access, when possible.

Refer to the AWS data perimeter policy examples repo for templates and instructions on how to create the required SCPs and resource-based policies to achieve this objective as well as create any required exceptions. Specifically, look at the network perimeter policy, the Amazon S3 bucket policy statement with “Sid” : “EnforceNetworkPerimeter”, and the data perimeter governance policy (for preventing non-VPC deployments of Lambda and SageMaker) for details on how to write policies for this objective as well as manage exceptions.

Mobile devices

In on-premises networks, there are some resources that are physically static, such as servers. Other resources such as laptops, however, are inherently mobile and can connect to networks outside of your control.

For example, a laptop could be connected to a corporate network when accessing data, which is temporarily stored locally, but then joins a public Wi-Fi network and sends the data to a personal Amazon S3 bucket. This network access pattern could allow access to unintended resources by bypassing corporate network controls and is a use case that you will need to consider with care.

Customers have generally tried to solve this problem with preventative controls such as always-on VPNs to keep devices connected to a corporate network. They also use detective controls (including agents) to monitor traffic and identify when preventative controls are disabled.

However, these controls aren’t fool-proof. There is still some risk that the device could join non-corporate networks. Virtual Desktop Infrastructure (VDI) is typically implemented when the risk of being able to operate a device outside of a controlled network is unacceptable and requires forcing access to AWS resources from non-mobile assets.

Amazon WorkSpaces offers a VDI solution that can be used to require users, developers, and data scientists to use a static asset to interact with AWS resources that is subject to the same network controls as other resources in AWS VPCs. VDI solutions can also be operated by customers natively using Amazon Elastic Compute Cloud (Amazon EC2) instances in a VPC.

Additional considerations

There are a few additional considerations for specific scenarios when building a data perimeter on AWS.

Amazon S3 resource considerations

Amazon S3 is widely used to store and present publicly available website content and public data sets. Access to this content is typically performed anonymously, meaning that the HTTP requests do not have an authorization header or query string parameter generated from AWS credentials.

You might need this anonymous access for users to browse internet websites from VPC networks or on-premises networks that are routed through VPC endpoints. It is also used for workloads that might need to access public data (such as package repositories hosted on Amazon S3 or agent downloads). In order to allow this type of access, you can allow anonymous GetObject API calls in your VPC endpoint policies. This is true whether the Amazon S3 content is being accessed using the virtual or path style endpoints or is being accessed via an Amazon S3 website endpoint.

Access to all other Amazon S3 APIs should be authenticated. Refer to the S3 endpoint policy example, which includes details of how to allow anonymous GetObject API calls while enforcing authentication and guardrails for intended resources for the remainder of Amazon S3 actions. The example lists specific AWS resources, but you might also need unauthenticated access to resources where you do not know the bucket or object names (for example, downloading static content on a website in your browser). For those scenarios, you might want to use a dedicated Amazon S3 VPC endpoint that allows unauthenticated GetObject API calls for all Amazon S3 resources that is just used for networks where you expect this scenario to be required.

Cross-Region requests

VPC endpoints only support routing AWS API calls to service endpoints that are in the same Region as the VPC endpoint itself. For example, an Amazon S3 VPC endpoint in a VPC in us-east-1 only supports routing traffic for requests made to Amazon S3 buckets in us-east-1. A call to PutObject for a bucket in us-west-2 would not traverse the VPC endpoint and would not have the endpoint policy applied to the request. To ensure the intended security controls are applied consistently, you can handle cross-Region requests in three ways:

• Prevent cross-Region API calls using a proxy. This does not require inspecting Transport Layer Security (TLS) and can be done by looking at the hostname in the CONNECT request. You can also use Server Name Indication (SNI) because the hostname presented in the ClientHello includes the AWS Region in the domain name of the URL (with the exception of global services).

• Use a centralized VPC endpoint approach for each Region and share Route 53 private hosted zones (PHZ) for the endpoints. This allows instances to resolve the out-of-Region endpoint domain name locally and send their request directly to the VPC endpoint. This pattern is described in more detail in Centralized access to VPC private endpoints.

• If you use HTTP/S proxies in your environment, you can use them to forward out-of-Region requests. There are two variations for this option:

  • Use proxy-chaining - The proxy in the local Region forwards traffic to a peer proxy running in a VPC in the destination Region. The out-of-Region proxy delivers the traffic to the appropriate VPC endpoint in its Region. See Appendix A – Proxy configuration example for an example proxy configuration that implements this proxy-chaining solution. The following diagram demonstrates a high-level reference architecture.

    

    Using proxy-chaining to send out-of-Region requests through VPC endpoints

  • Implement a centralized VPC endpoint approach using shared Route 53 private hosted zones - The local proxy uses AWS-provided VPC DNS and sends the request directly to the out-of-Region VPC endpoint. This eliminates the need to configure proxy-chaining, but does require the creation of a PHZ for each endpoint that will need to be shared with every VPC hosting a proxy. Refer to Cross Region endpoint access for more details.

    

    Forwarding out-of-Region requests using a shared Route 53 PHZ

Preventing access to temporary credentials

Except for the cases of credential theft or leakage, the only other way for an unintended entity to gain access to temporary credentials derived from IAM roles that are part of my AWS is through misconfigured IAM role trust policies.

IAM role trust policies define the principals that you trust to assume an IAM role. A role trust policy is a required resource-based policy that is attached to a role in IAM. The principals that you can specify in the trust policy include users, roles, accounts, and services.

The trust policy can be configured to help ensure that no one from outside your account or organization can be authorized to assume the role. Audit all IAM role trust policies and ensure either of the following are true:

• If the trusted entity is an IAM principal, such as a role or user, the trust policy uses either the aws:PrincipalOrgId or aws:PrincipalOrgPaths condition. Exceptions can be created with an allow list of known, external, expected accounts and they should use the sts:ExternalId condition.

• If the trusted entity is an AWS service, being either a service principal or IAM service-linked role, as a best practice, the trust policy should not trust more than one AWS service in order to apply least privilege.

Refer to IAM role trust policy example for more details.

Resource sharing and external targets

The final consideration are services that allow resource sharing or targeting external resources. With these services, you cannot use a condition such as aws:ResourceOrgId because the resource being evaluated in the policy belongs to your AWS organization, but its configuration specifies a resource that does not.

Instead, you will need to use different approaches to prevent sharing resources with AWS accounts outside of your organization. AWS has several services that allow you to share a resource with another account or target a resource in another account. Some options include:

• Amazon S3 - Amazon S3 buckets can be made public by using bucket policies or with access control lists (ACLs). You should disable ACLs and deny ACL updates on your Amazon S3 buckets and enforce block public access as a best practice.

• Amazon Machine Images (AMI) - AMIs can be shared with other accounts or made public with the ModifyImageAttribute API. You can deny this action in an SCP and create an exception for a privileged IAM principal if required.

• Amazon EBS Snapshots - Amazon EBS Snapshots can be shared with other accounts or made public with the ModifySnapshotAttribute API. You can deny this action in an SCP and create an exception for a privileged IAM principal if required.

• Amazon RDS Snapshots - Amazon RDS Snapshots can be shared with other accounts or made public with the ModifyDBSnapshotAttribute API. You can deny this action in an SCP and create an exception for a privileged IAM principal if required.

• AWS Resource Access Manager (AWS RAM) - AWS RAM is a service that allows sharing various types of resources with other AWS accounts. Sharing with AWS RAM can be constrained to your AWS organization using the ram:RequestedAllowsExternalPrincipals IAM condition.

• Amazon CloudWatch Logs Subscription Filters - You can send CloudWatch Logs to cross account destinations. You can deny this action in an SCP and create an exception for a privileged IAM principals using a principal tag.

• Amazon EventBridge Targets - You can add targets to an EventBridge rule that are in different accounts than the event bus. Use the events:TargetArn IAM condition to limit which accounts can be used as targets for EventBridge rules or create an exception for privileged IAM principals using a principal tag.

AWS RAM allows for conditioning on sharing outside of your AWS organization in an SCP. For the rest of the services, you can use IAM policies to prevent the actions altogether. However, this isn’t always practical. In these cases, you can either allow just privileged roles to take those actions, or you can build detective controls and remediate the configuration using services such as AWS Config or EventBridge.

For example, an EventBridge rule can look for PutSubscriptionFilter events and invoke a Lambda function to evaluate the destination Amazon Resource Name (ARN) in the subscription. If the ARN is for a resource not in the AWS organization, the function can remove the subscription. Refer to data perimeter governancy policy statements PreventRAMExternalResourceShare, PreventExternalResourceShare, and PreventPublicBucketACL for more detailed examples of policies you can use to prevent resource sharing and create required exceptions.

AWS Management Console

The AWS Management Console can be configured to use a Private Access option. This capability allows you to prevent users from signing in to unintended AWS accounts from within your network. You can use it to limit access to the management console to only a specified set of AWS accounts or organizations. AWS Management Console Private Access helps ensure only trusted identities are allowed from your expected networks. Refer to the documentation for supported AWS Regions, service consoles, and features. Instructions for implementing VPC endpoint policies with the aws:PrincipalOrgID condition to enforce only trusted identities and considerations for enforcing only expected networks with aws:SourceVpc can also be found in the documentation.