How to restrict IAM roles to access AWS resources from specific geolocations using AWS Client VPN

You can improve your organization’s security posture by enforcing access to Amazon Web Services (AWS) resources based on IP address and geolocation. For example, users in your organization might bring their own devices, which might require additional security authorization checks and posture assessment in order to comply with corporate security requirements. Enforcing access to AWS resources based on geolocation can help you to automate compliance with corporate security requirements by auditing the connection establishment requests. In this blog post, we walk you through the steps to allow AWS Identity and Access Management (IAM) roles to access AWS resources only from specific geographic locations.

Solution overview

AWS Client VPN is a managed client-based VPN service that enables you to securely access your AWS resources and your on-premises network resources. With Client VPN, you can access your resources from any location using an OpenVPN-based VPN client. A client VPN session terminates at the Client VPN endpoint, which is provisioned in your Amazon Virtual Private Cloud (Amazon VPC) and therefore enables a secure connection to resources running inside your VPC network.

This solution uses Client VPN to implement geolocation authentication rules. When a client VPN connection is established, authentication is implemented at the first point of entry into the AWS Cloud. It’s used to determine if clients are allowed to connect to the Client VPN endpoint. You configure an AWS Lambda function as the client connect handler for your Client VPN endpoint. You can use the handler to run custom logic that authorizes a new connection. When a user initiates a new client VPN connection, the custom logic is the point at which you can determine the geolocation of this user. In order to enforce geolocation authorization rules, you need:

AWS WAF to determine the user’s geolocation based on their IP address.
A Network Address Translation (NAT) gateway to be used as the public origin IP address for all requests to your AWS resources.
An IAM policy that is attached to the IAM role and validated by AWS when the request origin IP address matches the IP address of the NAT gateway.

One of the key features of AWS WAF is the ability to allow or block web requests based on country of origin. When the client connection handler Lambda function is invoked by your Client VPN endpoint, the Client VPN service invokes the Lambda function on your behalf. The Lambda function receives the device, user, and connection attributes. The user’s public IP address is one of the device attributes that are used to identify the user’s geolocation by using the AWS WAF geolocation feature. Only connections that are authorized by the Lambda function are allowed to connect to the Client VPN endpoint.

Note: The accuracy of the IP address to country lookup database varies by region. Based on recent tests, the overall accuracy for the IP address to country mapping is 99.8 percent. We recommend that you work with regulatory compliance experts to decide if your solution meets your compliance needs.

A NAT gateway allows resources in a private subnet to connect to the internet or other AWS services, but prevents a host on the internet from connecting to those resources. You must also specify an Elastic IP address to associate with the NAT gateway when you create it. Since an Elastic IP address is static, any request originating from a private subnet will be seen with a public IP address that you can trust because it will be the elastic IP address of your NAT gateway.

IAM is a web service for securely controlling access to AWS services. You manage access in AWS by creating policies and attaching them to IAM identities (users, groups of users, or roles) or AWS resources. A policy is an object in AWS that, when associated with an identity or resource, defines their permissions. In an IAM policy, you can define the global condition key aws:SourceIp to restrict API calls to your AWS resources from specific IP addresses.

Note: Throughout this post, the user is authenticating with a SAML identity provider (IdP) and assumes an IAM role.

Figure 1 illustrates the authentication process when a user tries to establish a new Client VPN connection session.

Figure 1: Enforce connection to Client VPN from specific geolocations

Let’s look at how the process illustrated in Figure 1 works.

The user device initiates a new client VPN connection session.
The Client VPN service redirects the user to authenticate against an IdP.
After user authentication succeeds, the client connects to the Client VPN endpoint.
The Client VPN endpoint invokes the Lambda function synchronously. The function is invoked after device and user authentication, and before the authorization rules are evaluated.
The Lambda function extracts the public-ip device attribute from the input and makes an HTTPS request to the Amazon API Gateway endpoint, passing the user’s public IP address in the X-Forwarded-For header. Because you’re using AWS WAF to protect API Gateway, and have geographic match conditions configured, a response with the status code 200 is returned only if the user’s public IP address originates from an allowed country of origin. Additionally, AWS WAF has another rule configured that blocks all requests to API Gateway if the request doesn’t originate from one of the NAT gateway IP addresses. Because Lambda is deployed in a VPC, it has a NAT gateway IP address, and therefore the request isn’t blocked by AWS WAF. To learn more about running a Lambda function in a VPC, see Configuring a Lambda function to access resources in a VPC. The following code example showcases Lambda code that performs the described step.

Note: Optionally, you can implement additional controls by creating specific authorization rules. Authorization rules act as ﬁrewall rules that grant access to networks. You should have an authorization rule for each network for which you want to grant access. To learn more, see Authorization rules.
The Lambda function returns the authorization request response to Client VPN.
When the Lambda function—shown following—returns an allow response, Client VPN establishes the VPN session.

import os
import http.client


cloud_front_url = os.getenv("ENDPOINT_DNS")
endpoint = os.getenv("ENDPOINT")
success_status_codes = [200]


def build_response(allow, status):
    return {
        "allow": allow,
        "error-msg-on-failed-posture-compliance": "Error establishing connection. Please contact your administrator.",
        "posture-compliance-statuses": [status],
        "schema-version": "v1"
    }


def handler(event, context):
    ip = event['public-ip']

    conn = http.client.HTTPSConnection(cloud_front_url)
    conn.request("GET", f'/{endpoint}', headers={'X-Forwarded-For': ip})
    r1 = conn.getresponse()
    conn.close()

    status_code = r1.status

    if status_code in success_status_codes:
        print("User's IP is based from an allowed country. Allowing the connection to VPN.")
        return build_response(True, 'compliant')

    print("User's IP is NOT based from an allowed country. Blocking the connection to VPN.")
    return build_response(False, 'quarantined')

After the client VPN session is established successfully, the request from the user device flows through the NAT gateway. The originating source IP address is recognized, because it is the Elastic IP address associated with the NAT gateway. An IAM policy is defined that denies any request to your AWS resources that doesn’t originate from the NAT gateway Elastic IP address. By attaching this IAM policy to users, you can control which AWS resources they can access.

Figure 2 illustrates the process of a user trying to access an Amazon Simple Storage Service (Amazon S3) bucket.

Figure 2: Enforce access to AWS resources from specific IPs

Let’s look at how the process illustrated in Figure 2 works.

A user signs in to the AWS Management Console by authenticating against the IdP and assumes an IAM role.
Using the IAM role, the user makes a request to list Amazon S3 buckets. The IAM policy of the user is evaluated to form an allow or deny decision.
If the request is allowed, an API request is made to Amazon S3.

The aws:SourceIp condition key is used in a policy to deny requests from principals if the origin IP address isn’t the NAT gateway IP address. However, this policy also denies access if an AWS service makes calls on a principal’s behalf. For example, when you use AWS CloudFormation to provision a stack, it provisions resources by using its own IP address, not the IP address of the originating request. In this case, you use aws:SourceIp with the aws:ViaAWSService key to ensure that the source IP address restriction applies only to requests made directly by a principal.

IAM deny policy

The IAM policy doesn’t allow any actions. What the policy does is deny any action on any resource if the source IP address doesn’t match any of the IP addresses in the condition. Use this policy in combination with other policies that allow specific actions.

Prerequisites

Make sure that you have the following in place before you deploy the solution:

Client VPN to connect to a Client VPN endpoint. You can follow the AWS Client VPN download instructions to download the desktop client.
You must generate and import a private certificate into AWS Certificate Manager (ACM). The certificate is used to establish mutual authentication. Client VPN uses certificates to perform authentication between the client and the server.
An IdP, such as AWS Directory Service for Microsoft Active Directory, or an external IdP for user-based client VPN authentication. Then you use the same user to federate the sign-in to the console. To learn more and to configure your IdP client or AWS Managed Microsoft AD for authorization, see client authorization.

Implementation and deployment details

In this section, you create a CloudFormation stack that creates AWS resources for this solution. To start the deployment process, select the following Launch Stack button.

You also can download the CloudFormation template if you want to modify the code before the deployment.

The template in Figure 3 takes several parameters. Let’s go over the key parameters.

Figure 3: CloudFormation stack parameters

The key parameters are:

AuthenticationOption: Information about the authentication method to be used to authenticate clients. You can choose either AWS Managed Microsoft AD or IAM SAML identity provider for authentication.
AuthenticationOptionResourceIdentifier: The ID of the AWS Managed Microsoft AD directory to use for Active Directory authentication, or the Amazon Resource Number (ARN) of the SAML provider for federated authentication.
ServerCertificateArn: The ARN of the server certificate. The server certificate must be provisioned in ACM.
CountryCodes: A string of comma-separated country codes. For example: US,GB,DE. The country codes must be alpha-2 country ISO codes of the ISO 3166 international standard.
LambdaProvisionedConcurrency: Provisioned concurrency for the client connection handler. We recommend that you configure provisioned concurrency for the Lambda function to enable it to scale without fluctuations in latency.

All other input fields have default values that you can either accept or override. Once you provide the parameter input values and reach the final screen, choose Create stack to deploy the CloudFormation stack.

This template creates several resources in your AWS account, as follows:

A VPC and associated resources, such as InternetGateway, Subnets, ElasticIP, NatGateway, RouteTables, and SecurityGroup.
A Client VPN endpoint, which provides connectivity to your VPC.
A Lambda function, which is invoked by the Client VPN endpoint to determine the country origin of the user’s IP address.
An API Gateway for the Lambda function to make an HTTPS request.
AWS WAF in front of API Gateway, which only allows requests to go through to API Gateway if the user’s IP address is based in one of the allowed countries.
A deny policy with a NAT gateway IP addresses condition. Attaching this policy to a role or user enforces that the user can’t access your AWS resources unless they are connected to your client VPN.

Note: CloudFormation stack deployment can take up to 20 minutes to provision all AWS resources.

After creating the stack, there are two outputs in the Outputs section, as shown in Figure 4.

Figure 4: CloudFormation stack outputs

ClientVPNConsoleURL: The URL where you can download the client VPN configuration file.
IAMRoleClientVpnDenyIfNotNatIP: The IAM policy to be attached to an IAM role or IAM user to enforce access control.

Attach the IAMRoleClientVpnDenyIfNotNatIP policy to a role

This policy is used to enforce access to your AWS resources based on geolocation. Attach this policy to the role that you are using for testing the solution. You can use the steps in Adding IAM identity permissions to do so.

Configure the AWS client VPN desktop application

When you open the URL that you see in ClientVPNConsoleURL, you see the newly provisioned Client VPN endpoint. Select Download Client Configuration to download the configuration file.

Figure 5: Client VPN endpoint

Confirm the download request by selecting Download.

Figure 6: Client VPN Endpoint – Download Client Configuration

To connect to the Client VPN endpoint, follow the steps in Connect to the VPN. After a successful connection is established, you should see the message Connected. in your AWS Client VPN desktop application.

Figure 7: AWS Client VPN desktop application – established VPN connection

Troubleshooting

If you can’t establish a Client VPN connection, here are some things to try:

Confirm that the Client VPN connection has successfully established. It should be in the Connected state. To troubleshoot connection issues, you can follow this guide.
If the connection isn’t establishing, make sure that your machine has TCP port 35001 available. This is the port used for receiving the SAML assertion.
Validate that the user you’re using for testing is a member of the correct SAML group on your IdP.
Confirm that the IdP is sending the right details in the SAML assertion. You can use browser plugins, such as SAML-tracer, to inspect the information received in the SAML assertion.

Test the solution

Now that you’re connected to Client VPN, open the console, sign in to your AWS account, and navigate to the Amazon S3 page. Since you’re connected to the VPN, your origin IP address is one of the NAT gateway IPs, and the request is allowed. You can see your S3 bucket, if any exist.

Figure 8: Amazon S3 console view – the user is connected to AWS Client VPN

Now that you’ve verified that you can access your AWS resources, go back to the Client VPN desktop application and disconnect your VPN connection. Once the VPN connection is disconnected, go back to the Amazon S3 page and reload it. This time you should see an error message that you don’t have permission to list buckets, as shown in Figure 9.

Figure 9: Amazon S3 service console view – the user is disconnected from AWS Client VPN

Access has been denied because your origin public IP address is no longer one of the NAT gateway IP addresses. As mentioned earlier, since the policy denies any action on any resource without an established VPN connection to the Client VPN endpoint, access to all your AWS resources is denied.

Scale the solution in AWS Organizations

With AWS Organizations, you can centrally manage and govern your environment as you grow and scale your AWS resources. You can use Organizations to apply policies that give your teams the freedom to build with the resources they need, while staying within the boundaries you set. By organizing accounts into organizational units (OUs), which are groups of accounts that serve an application or service, you can apply service control policies (SCPs) to create targeted governance boundaries for your OUs. To learn more about Organizations, see AWS Organizations terminology and concepts.

SCPs help you to ensure that your accounts stay within your organization’s access control guidelines across all your accounts within OUs. In particular, these are the key benefits of using SCPs in your AWS Organizations:

You don’t have to create an IAM policy with each new account, but instead create one SCP and apply it to one or more OUs as needed.
You don’t have to apply the IAM policy to every IAM user or role, existing or new.
This solution can be deployed in a separate account, such as a shared infrastructure account. This helps to decouple infrastructure tooling from business application accounts.

The following figure, Figure 10, illustrates the solution in an Organizations environment.

Figure 10: Use SCPs to enforce policy across many AWS accounts

The Client VPN account is the account the solution is deployed into. This account can also be used for other networking related services. The SCP is created in the Organizations root account and attached to one or more OUs. This allows you to centrally control access to your AWS resources.

Let’s review the new condition that’s added to the IAM policy:

"ArnNotLikeIfExists": {
    "aws:PrincipalARN": [
    "arn:aws:iam::*:role/service-role/*"
    ]
}

The aws:PrincipalARN condition key allows your AWS services to communicate to other AWS services even though those won’t have a NAT IP address as the source IP address. For instance, when a Lambda function needs to read a file from your S3 bucket.

Note: Appending policies to existing resources might cause an unintended disruption to your application. Consider testing your policies in a test environment or to non-critical resources before applying them to production resources. You can do that by attaching the SCP to a specific OU or to an individual AWS account.

Cleanup

After you’ve tested the solution, you can clean up all the created AWS resources by deleting the CloudFormation stack.

Conclusion

In this post, we showed you how you can restrict IAM users to access AWS resources from specific geographic locations. You used Client VPN to allow users to establish a client VPN connection from a desktop. You used an AWS client connection handler (as a Lambda function), and API Gateway with AWS WAF to identify the user’s geolocation. NAT gateway IPs served as trusted source IPs, and an IAM policy protects access to your AWS resources. Lastly, you learned how to scale this solution to many AWS accounts with Organizations.

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