CORS Success and Failure in ASP.NET Core

In this post we will see why does cross-origin requests (CORS) fail or succeed in ASP.NET Core.

Cross-Origin Resource Sharing (CORS) errors in ASP.NET Core typically occur due to security restrictions that are enforced by browsers to protect users' data from being accessed by unauthorized websites. Here’s a breakdown of why CORS fails or succeeds in ASP.NET Core and how it can be managed:

Why CORS Fails

Same-Origin Policy: Browsers enforce the "Same-Origin Policy" as a security measure, which blocks requests originating from different origins (domain, protocol, or port) by default. For instance, if your frontend (JavaScript running in a browser) is served from https://example-client.com and it tries to access an API endpoint hosted on https://example-api.com, this will trigger a CORS preflight request.

Missing CORS Policy: If your ASP.NET Core API does not explicitly define a CORS policy allowing requests from the origin of the calling site, the browser will block the request.

Request Method or Headers Not Allowed: Even if CORS is enabled, certain request methods (e.g., PUT, DELETE) or headers (like custom headers) might not be allowed in your policy. The browser sends a preflight request using the OPTIONS HTTP method to check if the API allows the desired method and headers. If the API does not allow them, the CORS request fails.

Why CORS Succeeds

Configured CORS Policy: In ASP.NET Core, you can configure a CORS policy that explicitly specifies allowed origins, headers, methods, and other settings. By doing this, your API signals to the browser that it permits requests from the specified origins. For example:

services.AddCors(options =>
{
    options.AddPolicy("AllowSpecificOrigin",
        builder => builder.WithOrigins("https://example-client.com")
                          .AllowAnyMethod()
                          .AllowAnyHeader());
});

Applying this policy allows the API to respond successfully to requests from https://example-client.com.

Preflight Request Passed: The browser’s preflight request (if sent) is accepted by the server based on the CORS configuration. Once the server responds with the necessary headers indicating allowed origins, methods, and headers, the browser allows the main request to proceed.

Permissive Policy: Sometimes, a more permissive policy, such as allowing any origin with AllowAnyOrigin(), will allow all cross-origin requests. This setup can be useful for open APIs but should be used cautiously to avoid security risks.

Enabling CORS in ASP.NET Core

To enable CORS in your ASP.NET Core application, add the configuration in Startup.cs:

Register the CORS service in ConfigureServices:

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(options =>
    {
        options.AddPolicy("MyPolicy",
            builder => builder.WithOrigins("https://example-client.com")
                              .AllowAnyMethod()
                              .AllowAnyHeader());
    });

    services.AddControllers();
}
Apply the CORS policy in Configure:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseCors("MyPolicy");

    app.UseRouting();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}

By carefully defining the origins, headers, and methods allowed, you control which requests succeed, ensuring that only trusted origins can access your resources.