Request an access token in Azure Active Directory B2C

Άρθρο
01/11/2024

An access token contains claims that you can use in Azure Active Directory B2C (Azure AD B2C) to identify the granted permissions to your APIs. To call a resource server, the HTTP request must include an access token. An access token is denoted as access_token in the responses from Azure AD B2C.

This article shows you how to request an access token for a web application and web API. For more information about tokens in Azure AD B2C, see the overview of tokens in Azure Active Directory B2C.

Note

Web API chains (On-Behalf-Of) is not supported by Azure AD B2C - Many architectures include a web API that needs to call another downstream web API, both secured by Azure AD B2C. This scenario is common in clients that have a web API back end, which in turn calls another service. This chained web API scenario can be supported by using the OAuth 2.0 JWT Bearer Credential grant, otherwise known as the On-Behalf-Of flow. However, the On-Behalf-Of flow is not currently implemented in Azure AD B2C. Although On-Behalf-Of works for applications registered in Microsoft Entra ID, it does not work for applications registered in Azure AD B2C, regardless of the tenant (Microsoft Entra ID or Azure AD B2C) that is issuing the tokens.

Prerequisites

Create a user flow to enable users to sign up and sign in to your application.
If you haven't already done so, add a web API application to your Azure Active Directory B2C tenant.

Scopes

Scopes provide a way to manage permissions to protected resources. When an access token is requested, the client application needs to specify the desired permissions in the scope parameter of the request. For example, to specify the Scope Value of read for the API that has the App ID URI of https://contoso.onmicrosoft.com/api, the scope would be https://contoso.onmicrosoft.com/api/read.

Scopes are used by the web API to implement scope-based access control. For example, users of the web API could have both read and write access, or users of the web API might have only read access. To acquire multiple permissions in the same request, you can add multiple entries in the single scope parameter of the request, separated by spaces.

The following example shows scopes decoded in a URL:

scope=https://contoso.onmicrosoft.com/api/read openid offline_access

The following example shows scopes encoded in a URL:

scope=https%3A%2F%2Fcontoso.onmicrosoft.com%2Fapi%2Fread%20openid%20offline_access

If you request more scopes than what is granted for your client application, the call succeeds if at least one permission is granted. The scp claim in the resulting access token is populated with only the permissions that were successfully granted.

OpenID Connect scopes

The OpenID Connect standard specifies several special scope values. The following scopes represent the permission to access the user's profile:

openid - Requests an ID token.
offline_access - Requests a refresh token using Auth Code flows.
00000000-0000-0000-0000-000000000000 - Using the client ID as the scope indicates that your app needs an access token that can be used against your own service or web API, represented by the same client ID.

If the response_type parameter in an /authorize request includes token, the scope parameter must include at least one resource scope other than openid and offline_access that will be granted. Otherwise, the /authorize request fails.

Request a token

To request an access token, you need an authorization code. The following is an example of a request to the /authorize endpoint for an authorization code:

GET https://<tenant-name>.b2clogin.com/<tenant-name>.onmicrosoft.com/<policy-name>/oauth2/v2.0/authorize?
client_id=<application-ID>
&nonce=anyRandomValue
&redirect_uri=https://jwt.ms
&scope=<application-ID-URI>/<scope-name>
&response_type=code

Replace the values in the query string as follows:

<tenant-name> - The name of your Azure AD B2C tenant. If you're using a custom domain, replace tenant-name.b2clogin.com with your domain, such as contoso.com.
<policy-name> - The name of your custom policy or user flow.
<application-ID> - The application identifier of the web application that you registered to support the user flow.
<application-ID-URI> - The application identifier URI that you set under Expose an API blade of the client application.
<scope-name> - The name of the scope that you added under Expose an API blade of the client application.
<redirect-uri> - The Redirect URI that you entered when you registered the client application.

To get a feel of how the request works, paste the request into your browser and run it.

This's the interactive part of the flow, where you take action. You're asked to complete the user flow's workflow. This might involve entering your username and password in a sign in form or any other number of steps. The steps you complete depend on how the user flow is defined.

The response with the authorization code should be similar to this example:

https://jwt.ms/?code=eyJraWQiOiJjcGltY29yZV8wOTI1MjAxNSIsInZlciI6IjEuMC...

After successfully receiving the authorization code, you can use it to request an access token. The parameters are in the body of the HTTP POST request:

POST <tenant-name>.b2clogin.com/<tenant-name>.onmicrosoft.com/<policy-name>/oauth2/v2.0/token HTTP/1.1
Host: <tenant-name>.b2clogin.com
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code
&client_id=<application-ID>
&scope=<application-ID-URI>/<scope-name>
&code=eyJraWQiOiJjcGltY29yZV8wOTI1MjAxNSIsInZlciI6IjEuMC...
&redirect_uri=https://jwt.ms
&client_secret=2hMG2-_:y12n10vwH...

If you want to test this POST HTTP request, you can use any HTTP client such as Microsoft PowerShell.

A successful token response looks like this:

{
    "access_token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImtpZCI6Ilg1ZVhrN...",
    "token_type": "Bearer",
    "not_before": 1549647431,
    "expires_in": 3600,
    "expires_on": 1549651031,
    "resource": "f2a76e08-93f2-4350-833c-965c02483b11",
    "profile_info": "eyJ2ZXIiOiIxLjAiLCJ0aWQiOiJjNjRhNGY3ZC0zMDkxLTRjNzMtYTcyMi1hM2YwNjk0Z..."
}

When using https://jwt.ms to examine the access token that was returned, you should see something similar to the following example:

{
  "typ": "JWT",
  "alg": "RS256",
  "kid": "X5eXk4xyojNFum1kl2Ytv8dl..."
}.{
  "iss": "https://contoso0926tenant.b2clogin.com/c64a4f7d-3091-4c73-a7.../v2.0/",
  "exp": 1549651031,
  "nbf": 1549647431,
  "aud": "f2a76e08-93f2-4350-833c-965...",
  "oid": "1558f87f-452b-4757-bcd1-883...",
  "sub": "1558f87f-452b-4757-bcd1-883...",
  "name": "David",
  "tfp": "B2C_1_signupsignin1",
  "nonce": "anyRandomValue",
  "scp": "read",
  "azp": "38307aee-303c-4fff-8087-d8d2...",
  "ver": "1.0",
  "iat": 1549647431
}.[Signature]

Next steps

Learn about how to configure tokens in Azure AD B2C

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