A walk through AWS Verified Access policies

AWS Verified Access helps improve your organization’s security posture by using security trust providers to grant access to applications. This service grants access to applications only when the user’s identity and the user’s device meet configured security requirements. In this blog post, we will provide an overview of trust providers and policies, then walk through a Verified Access policy for securing your corporate applications.

Understanding trust data and policies

Verified Access policies enable you to use trust data from trust providers and help protect access to corporate applications that are hosted on Amazon Web Services (AWS). When you create a Verified Access group or a Verified Access endpoint, you create a Verified Access policy, which is applied to the group or both the group and endpoint. Policies are written in Cedar, an AWS policy language. With Verified Access, you can express policies that use the trust data from the trust providers that you configure, such as corporate identity providers and device security state providers.

Verified Access receives trust data or claims from different trust providers. Currently, Verified Access supports two types of trust providers. The first type is an identity trust provider. Identity trust providers manage the identities of digital users, including the user’s email address, groups, and profile information. The second type of trust provider is a device trust provider. Device trust providers manage the device posture for users, including the OS version of the device, risk scores, and other metrics that reflect device posture. When a user makes a request to Verified Access, the request includes claims from the configured trust providers. Verified Access customers permit or forbid access to applications by evaluating the claims in Cedar policies. We will walk through the types of claims that are included from trust providers and the options for custom trust data.

End-to-end Cedar policy use cases

Let’s look at how to use policies with your applications. In general, you use Verified Access to control access to an application for purposes of authentication and initial authorization. This means that you use Verified Access to authenticate the user when they log in and to confirm that the device posture of the end device meets minimum criteria. For authorization logic to control access to actions and resources inside the application, you pass the identity claims to the application. The application uses the information to authorize users within the application after authentication. In other words, not every identity claim needs to be passed or checked in Verified Access to allow traffic to pass to the application. You can and should put additional logic in place to make decisions for users when they gain access to the backend application after initial authentication and authorization by Verified Access. From an identity perspective, this additional criteria might be an email address, a group, and possibly some additional claims. From a device perspective, Verified Access does not at this time pass device trust data to the end application. This means that you should use Verified Access to perform checks involving device posture.

We will explore the evolution of a policy by walking you through four use cases for Cedar policy. You can test the claim data and policies in the Verified Access Cedar Playground. For more information about Verified Access, see Verified Access policies and types of trust providers.

Use case 1: Basic policy

For many applications, you only need a simple policy to provide access to your users. This can include the identity information only. For example, let’s say that you want to write a policy that uses the user’s email address and matches a certain group that the user is part of. Within the Verified Access trust provider configuration, you can include “openid email groups” as the scope, and your OpenID Connect (OIDC) provider will include each claim associated with the scopes that you have configured with the OIDC provider. When the user John in this example uses case logs in to the OIDC provider, he receives the following claims from the OIDC provider. For this provider, the Verified Access Trust Provider is configured for “identity” to be the policy reference name.

{
  "identity": {
    "email": "john@example.com",
    "groups": [
      "finance",
      "employees"
    ]
  }
}

With these claims, you can write a policy that matches the email domain and the group, to allow access to the application, as follows.

permit(principal, action, resource)
when {
    // Returns true if the email ends in "@example.com"
    context.identity.email like "*@example.com" &&
    // Returns true if the user is part of the "finance" group
    context.identity.groups.contains("finance")
};

Use case 2: Custom claims within a policy

Many times, you are also interested in company-specific or custom claims from the identity provider. The claims that exist with the user endpoint are dependent on how you configure the identity provider. For OIDC providers, this is determined by the scopes that you define when you set up the identity provider. Verified Access uses OIDC scopes to authorize access to details of the user. This includes attributes such as the name, email address, email verification, and custom attributes. Each scope that you configure for the identity provider returns a set of user attributes, which we call claims. Depending on which claims you want to match on in your policy, you configure the scopes and claims in the OIDC provider, which the OIDC provider adds to the user endpoint. For a list of standard claims, including profile, email, name, and others, see the Standard Claims OIDC specification.

In this example use case, as your policy evolves from the basic policy, you decide to add additional company-specific claims to Verified Access. This includes both the business unit and the level of each employee. Within the Verified Access trust provider configuration, you can include “openid email groups profile” as the scope, and your OIDC provider will include each claim associated with the scopes that you have configured with the OIDC provider. Now, when the user John logs in to the OIDC provider, he receives the following claims from the OIDC provider, with both the business unit and role as claims from the “profile” scope in OIDC.

{
  "identity": {
    "email": "john@example.com",
    "groups": [
      "finance",
      "employees"
    ],
    "business_unit": "corp",
    "level": 8
  }
}

With these claims, the company can write a policy that matches the claims to allow access to the application, as follows.

permit(principal, action, resource)
when {
    // Returns true if the email ends in "@example.com"
    context.identity.email like "*@example.com" &&
    // Returns true if the user is part of the "finance" group
    context.identity.groups.contains("finance") &&
    // Returns true if the business unit is "corp"
    context.identity.business_unit == "corp" &&
    // Returns true if the level is greater than 6
    context.identity.level >= 6
};

Use case 3: Add a device trust provider to a policy

The other type of trust provider is a device trust provider. Verified Access supports two device trust providers today: CrowdStrike and Jamf. As detailed in the AWS Verified Access Request Verification Flow, for HTTP/HTTPS traffic, the extension in the web browser receives device posture information from the device agent on the user’s device. Each device trust provider determines what risk information and device information to include in the claims and how that information is formatted. Depending on the device trust provider, the claims are static or configurable.

In our example use case, with the evolution of the policy, you now add device trust provider checks to the policy. After you install the Verified Access browser extension on John’s computer, Verified Access receives the following claims from both the identity trust provider and the device trust provider, which uses the policy reference name “crwd”.

{
  "identity": {
    "email": "john@example.com",
    "groups": [
      "finance",
      "employees"
    ],
    "business_unit": "corp",
    "level": 8
  },
  "crwd": {
    "assessment": {
      "overall": 90,
      "os": 100,
      "sensor_config": 80,
      "version": "3.4.0"
    }
  }
}

With these claims, you can write a policy that matches the claims to allow access to the application, as follows.

permit(principal, action, resource)
when {
    // Returns true if the email ends in "@example.com"
    context.identity.email like "*@example.com" &&
    // Returns true if the user is part of the "finance" group
    context.identity.groups.contains("finance") &&
    // Returns true if the business unit is "corp"
    context.identity.business_unit == "corp" &&
    // Returns true if the level is greater than 6
    context.identity.level >= 6 &&
    // If the CrowdStrike agent is present
    ( context has "crwd" &&
      // The overall device score is greater or equal to 80 
      context.crwd.assessment.overall >= 80 )
};

For more information about these scores, see Third-party trust providers.

Use case 4: Multiple device trust providers

The final update to your policy comes in the form of multiple device trust providers. Verified Access provides the ability to match on multiple device trust providers in the same policy. This provides flexibility for your company, which in this example use case has different device trust providers installed on different types of users’ devices. For information about many of the claims that each device trust provider provides to AWS, see Third-party trust providers. However, for this updated policy, John’s claims do not change, but the new policy can match on either CrowdStrike’s or Jamf’s trust data. For Jamf, the policy reference name is “jamf”.

permit(principal, action, resource)
when {
    // Returns true if the email ends in "@example.com"
    context.identity.email like "*@example.com" &&
    // Returns true if the user is part of the "finance" group
    context.identity.groups.contains("finance") &&
    // Returns true if the business unit is "corp"
    context.identity.business_unit == "corp" &&
    // Returns true if the level is greater than 6
    context.identity.level >= 6 &&
    // If the CrowdStrike agent is present
    (( context has "crwd" &&
      // The overall device score is greater or equal to 80 
      context.crwd.assessment.overall >= 80 ) ||
    // If the Jamf agent is present
    ( context has "jamf" &&
      // The risk level is either LOW or SECURE
      ["LOW","SECURE"].contains(context.jamf.risk) ))
};

For more information about using Jamf with Verified Access, see Integrating AWS Verified Access with Jamf Device Identity.

Conclusion

In this blog post, we covered an overview of Cedar policy for AWS Verified Access, discussed the types of trust providers available for Verified Access, and walked through different use cases as you evolve your Cedar policy in Verified Access.

If you want to test your own policies and claims, see the Cedar Playground. If you want more information about Verified Access, see the AWS Verified Access documentation.

Want more AWS Security news? Follow us on Twitter.

AWS Security Blog