CORS in C# APIs: What It Is and How to Fix It

?? What is CORS?

CORS (Cross-Origin Resource Sharing) is a security feature in web browsers that prevents unauthorized domains from accessing resources on a different domain. By default, browsers block cross-origin requests unless explicitly allowed by the server.

?? Example of a Blocked CORS Request:

• Frontend (React, Vue, Angular) runs on: https://localhost:3000
• API runs on: https://api.example.com
• When the frontend makes an API call, the browser blocks it due to different origins.

?? Common CORS Error in Console:

??? How to Fix CORS in a C# API (ASP.NET Core)

To allow cross-origin requests, we need to enable CORS policy in our API.

? Solution 1: Enable CORS in Program.cs

Modify your ASP.NET Core API to allow requests from specific origins.

? Solution 2: Allow CORS Per Controller or Action

If you don’t want to enable CORS globally, use [EnableCors] on specific controllers or actions.

? Solution 3: Handle CORS for Authentication Requests

If your API uses authentication (JWT or cookies), AllowCredentials() is required.

Example with JWT Authorization

? Solution 4: Debug CORS Issues in Azure

If your API is deployed in Azure, make sure to:

1. Enable CORS in Azure App Service (Azure Portal → App Service → CORS).
2. Use Application Logs (D:\home\LogFiles) to debug API failures.
3. Check HTTPS & Preflight Requests by adding OPTIONS endpoint:

Conclusion

?? CORS errors happen because browsers block cross-origin requests by default

. ?? Fix it by configuring CORS in ASP.NET Core (UseCors()).

?? Always allow only necessary domains for security.

?? Use AllowCredentials() if sending authentication tokens.

Now your API should work smoothly across different domains! ???? #CSharp #ASPNetCore #WebSecurity #CORS #API