Resolving Access Token Issues in ASP.NET Core Web API with Azure B2C Authentication

Learn how to troubleshoot `unauthenticated errors` when using Azure B2C tokens with ASP.NET Core Web API. Step-by-step guide provided!
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This video is based on the question stackoverflow.com/q/74479164/ asked by the user 'ManuBera' ( stackoverflow.com/u/20439892/ ) and on the answer stackoverflow.com/a/74530869/ provided by the user 'ManuBera' ( stackoverflow.com/u/20439892/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Unauthenticated Errors in ASP.NET Core Web API with Azure B2C

When building a Web API in ASP.NET Core that is secured with Azure B2C authentication, it's not uncommon to encounter issues related to access tokens—especially the dreaded 401 Unauthorized error. If you're facing an issue where your API returns this error despite receiving a valid access token, you're not alone. In this guide, we'll walk through common pitfalls and how to resolve them, ensuring you get your API working smoothly.

Understanding the Problem

You have a Web API registered within an Azure B2C tenant, and you're successfully obtaining an access token using the Resource Owner Password Credential (ROPC) grant flow. However, when you attempt to interact with your API using this token, you are met with a 401 Unauthorized error. What's going wrong here? Here are some points to consider:

The token appears valid when checked on sites like jwt.io but warns of an "Invalid Signature."

The Web API's service configuration may not be set up correctly.

There could be formatting issues with the token itself when being passed in your requests.

Step-by-Step Solution

Let's troubleshoot and fix this issue by addressing the common problems that can occur in this situation.

1. Token Acquisition

You obtain your token with the following snippet:

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Key Point: Ensure you're receiving a clean token without any unwanted characters. For this specific scenario, it’s important to check the token format. Any leading or trailing whitespace or special characters can invalidate its usability. If your token shows unwanted characters (like "), trim them off before returning it:

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2. Sending the Token in HTTP Requests

When constructing HTTP requests, ensure the token is correctly added to the Authorization header:

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Key Note: Again, if your token has unwanted characters, it could result in a failure. Always check the formatting before making the request.

3. Configuring the Web API

In your Startup.cs, you must ensure proper authentication setup. Here’s a typical setup for integrating Azure B2C:

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Make sure that the parameters passed in your configuration are correct and include proper values for Instance, TenantId, ClientId, and the policy names.

4. Check appsettings.json Configuration

Verify your appsettings.json for correctness. Ensure that the policies, domains, and other identifiers are correctly specified:

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Final Thoughts

Encountering a 401 Unauthorized error can be frustrating, especially for those new to Azure, authorization, and Web APIs. However, by carefully examining how tokens are processed and ensuring your configuration settings are correct, you can resolve these issues effectively.

If you find yourself feeling lost amidst all these configurations, don’t hesitate to reach out for help or consult official Azure documentation.

Remember, troubleshooting is often part of the development process, and each error is an opportunity to learn!

So, now that you have resolved your access token issues, you can move forward confidently in your API development journey. Happy coding!