Protect an API in Azure API Management using OAuth 2.0 authorization with Azure Active Directory

1. Prerequisites
2. Overview
3. Register an application in Azure AD to represent the API
4. Configure a JWT validation policy to pre-authorize requests

In this article, you'll learn high level steps to configure your Azure API Management instance to protect an API, by using the OAuth 2.0 protocol with Azure Active Directory (Azure AD).

For a conceptual overview of API authorization, see Authentication and authorization to APIs in API Management.

Prerequisites

Prior to following the steps in this article, you must have:

• An API Management instance
• A published API using the API Management instance
• An Azure AD tenant

Overview

Follow these steps to protect an API in API Management, using OAuth 2.0 authorization with Azure AD.

1. Register an application (called backend-app in this article) in Azure AD to protect access to the API.

   To access the API, users or applications will acquire and present a valid OAuth token granting access to this app with each API request.

2. Configure the validate-jwt policy in API Management to validate the OAuth token presented in each incoming API request. Valid requests can be passed to the API.

Details about OAuth authorization flows and how to generate the required OAuth tokens are beyond the scope of this article. Typically, a separate client app is used to acquire tokens from Azure AD that authorize access to the API. For links to more information, see the Next steps.

Register an application in Azure AD to represent the API

Using the Azure portal, protect an API with Azure AD by first registering an application that represents the API.

For details about app registration, see Quickstart: Configure an application to expose a web API.

1. In the Azure portal, search for and select App registrations.

2. Select New registration.

3. When the Register an application page appears, enter your application's registration information:

   • In the Name section, enter a meaningful application name that will be displayed to users of the app, such as backend-app.
   • In the Supported account types section, select an option that suits your scenario.

4. Leave the Redirect URI section empty.

5. Select Register to create the application.

6. On the app Overview page, find the Application (client) ID value and record it for later.

7. Under the Manage section of the side menu, select Expose an API and set the Application ID URI with the default value. If you're developing a separate client app to obtain OAuth 2.0 tokens for access to the backend-app, record this value for later.

8. Select the Add a scope button to display the Add a scope page:

   1. Enter a new Scope name, Admin consent display name, and Admin consent description.
   2. Make sure the Enabled scope state is selected.

9. Select the Add scope button to create the scope.

10. Repeat the previous two steps to add all scopes supported by your API.

11. Once the scopes are created, make a note of them for use later.

Configure a JWT validation policy to pre-authorize requests

The following example policy, when added to the <inbound> policy section, checks the value of the audience claim in an access token obtained from Azure AD that is presented in the Authorization header. It returns an error message if the token is not valid. Configure this policy at a policy scope that's appropriate for your scenario.

• In the openid-config URL, the aad-tenant is the tenant ID in Azure AD. Find this value in the Azure portal, for example, on the Overview page of your Azure AD resource. The example shown assumes a single-tenant Azure AD app and a v2 configuration endpoint.
• The value of the claim is the client ID of the backend-app you registered in Azure AD.

XML

<validate-jwt header-name="Authorization" failed-validation-httpcode="401" failed-validation-error-message="Unauthorized. Access token is missing or invalid.">
    <openid-config url="https://login.microsoftonline.com/{aad-tenant}/v2.0/.well-known/openid-configuration" />
    <audiences>
        <audience>{audience-value - (ex:api://guid)}</audience>
    </audiences>
    <issuers>
        <issuer>{issuer-value - (ex: https://sts.windows.net/{tenant id}/)}</issuer>
    </issuers>
    <required-claims>
        <claim name="aud">
            <value>{backend-app-client-id}</value>
        </claim>
    </required-claims>
</validate-jwt>

 Note

The preceding openid-config URL corresponds to the v2 endpoint. For the v1 openid-config endpoint, use https://login.microsoftonline.com/{aad-tenant}/.well-known/openid-configuration.

For information on how to configure policies, see Set or edit policies. Refer to the validate-jwt reference for more customization on JWT validations. To validate a JWT that was provided by the Azure Active Directory service, API Management also provides the validate-azure-ad-token policy.

Authorization workflow

1. A user or application acquires a token from Azure AD with permissions that grant access to the backend-app.

2. The token is added in the Authorization header of API requests to API Management.

3. API Management validates the token by using the validate-jwt policy.

   • If a request doesn't have a valid token, API Management blocks it.

   • If a request is accompanied by a valid token, the gateway can forward the request to the API.