Here’s a comprehensive guide to obtain a token from Microsoft B2C OAuth 2.0 using Postman with the Implicit Grant Flow. You’ve already shared a lot of the required information, so we’ll build on that.

Microsoft B2C OAuth 2.0 using Postman.md

Microsoft B2C OAuth 2.0 using Postman

1. Prerequisites

Make sure you have the following:

• A registered application in Azure AD B2C.
• A Policy defined (like a sign-up/sign-in policy).
• A Client ID (Application ID) from your B2C app.
• A Scope related to your Web API.
• Postman installed.

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2. Azure AD B2C Configuration

Ensure that you’ve configured your B2C App with the Implicit Grant Flow in the Azure AD B2C portal. Here's a quick check of what should be configured in Azure AD B2C:

1. Register the Web App (OAuth 2.0 Client):

   • Go to Azure Portal > Azure Active Directory > App Registrations.

   • Select your B2C app.

   • Under Authentication, configure:

     • Platform: Web.
     • Redirect URI: Enter your app's redirect URI (for testing purposes, this could be https://oauth.pstmn.io/v1/browser-callback or a predefined URI in your B2C app).
     • Implicit Grant: Enable ID Token and Access Token if needed.

   • Under API Permissions, configure the scope related to your Web API, e.g., https://{tenant}.onmicrosoft.com/{web api app id uri}/{scope name}.

   • Note the Client ID and Tenant name, which you will use in Postman.

2. Scope: This will be the scope you are requesting when obtaining the access token. You can configure it in the Azure portal.

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3. Postman Setup

1. Launch Postman: Open Postman to configure the OAuth 2.0 flow.

2. Create a New Request:

   • Click New and select Request.

   • Name the request (e.g., OAuth 2.0 Implicit Flow).

   • Enter the Auth URL in the URL field:

     https://{tenant}.b2clogin.com/te/{tenant}.onmicrosoft.com/{policy}/oauth2/v2.0/authorize
     

     • Replace {tenant} with your actual tenant name.
     • Replace {policy} with the name of your B2C policy (e.g., b2c_1_signup_signin or b2c_1a_signup_signin).

3. Authorization Tab:

   • Go to the Authorization tab in Postman.
   • Set Type to OAuth 2.0.
   • For OAuth 2.0 Flow, select Implicit.

4. Configure OAuth 2.0 Settings:

   • Client ID: Paste your Application (Client) ID from your B2C app.

   • Client Secret: Leave this empty for Implicit flow (not needed).

   • Auth URL: Enter the URL you obtained earlier:

     https://{tenant}.b2clogin.com/te/{tenant}.onmicrosoft.com/{policy}/oauth2/v2.0/authorize
     

   • Scope: Enter your desired scope:

     https://{tenant}.onmicrosoft.com/{web api app id uri}/{scope name}
     

     • Replace {tenant} with your actual tenant.
     • Replace {web api app id uri} and {scope name} with the details of your API scope.

   • Redirect URL: Enter the redirect URL defined in your B2C app settings, e.g., https://oauth.pstmn.io/v1/browser-callback (or any other valid redirect URI).

   • State: Optional, but can be set to a random string for security purposes.

   • Response Type: Ensure this is set to id_token token (this is the standard response for Implicit flow).

   Example configuration:

   • Auth URL: https://contoso.b2clogin.com/te/contoso.onmicrosoft.com/b2c_1_signup_signin/oauth2/v2.0/authorize
   • Client ID: 12345678-abcd-1234-abcd-1234567890ab
   • Scope: https://contoso.onmicrosoft.com/my-api/scope.read
   • Redirect URI: https://oauth.pstmn.io/v1/browser-callback

5. Get New Access Token:

   • Once everything is configured, click on the Get New Access Token button in Postman.
   • This will redirect you to the B2C login page (depending on the policy you configured, it may prompt for sign-in or show a custom UI).
   • Log in with your credentials.
   • Postman should automatically capture the Access Token (and optionally an ID Token) after successful authentication.

6. Use the Token:

   • After you obtain the token, Postman will display the Access Token in a dialog.
   • You can then Use Token for subsequent requests.

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4. Making API Requests with the Token

After obtaining the Access Token, you can use it to make API calls that require authentication. Here’s how to do that:

1. Set Authorization Header:

   • In the Headers section of Postman, add the following:

     • Key: Authorization
     • Value: Bearer {access_token}

   • Replace {access_token} with the actual token you received from the Get New Access Token step.

2. Make the Request:

   • Set the request URL to the endpoint of the API you want to call.

   • For example:

     https://{api_url}/your-api-endpoint
     

3. Send the Request:

   • Click Send to make the request. If everything is set up correctly, you should receive a valid response from the API.

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5. Troubleshooting

Here are a few things to check if the process isn't working as expected:

• Redirect URI: Make sure that the redirect URI in Postman matches the one you set in Azure AD B2C. It’s case-sensitive.
• Scope: Verify that the scope is correctly defined in both your B2C app and Postman.
• Policy: Ensure the policy (e.g., b2c_1_signup_signin or b2c_1a_signup_signin) is correctly specified in the Auth URL.
• Browser Popup: Sometimes the authentication page may not load in Postman directly, but it will open in a browser. Ensure that the browser allows pop-ups.

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6. Final Thoughts

• Security: Implicit flow is typically used for client-side applications (e.g., Single Page Apps). Since it exposes the access token to the browser, be cautious about using this in production environments for highly sensitive applications.
• Access Token Expiry: Tokens obtained using Implicit flow usually have a short expiration time. Be prepared to handle token expiry and refresh tokens (although Implicit flow does not use refresh tokens, so the user would need to authenticate again).