Understanding and Resolving CORS Preflight Request Issues

Introduction to CORS

Cross-Origin Resource Sharing (CORS) is a security feature implemented by web browsers to prevent websites from making requests to domains different than the one that served the website. This restriction, known as the "same-origin policy," prevents malicious sites from accessing sensitive data on another site without permission.

To overcome this limitation, CORS allows servers to specify who can access their resources and how they may be accessed by external sites. When a web application needs to make requests to an API hosted on a different domain, CORS policies must be properly configured to allow such interactions securely.

The Problem: Preflight Requests

In modern web development, especially when dealing with HTTP methods like POST, PUT, DELETE, or OPTIONS beyond simple GET requests, browsers initiate what is known as a "preflight request." This preflight request is an automatic browser behavior that uses the OPTIONS method to check if the actual request is safe to send.

The server must respond to this OPTIONS request with appropriate CORS headers. If it does not receive these headers, or if the response status code isn’t OK (200), the browser blocks the subsequent request for security reasons. This is often seen as a CORS policy error in the console logs:

has been blocked by CORS policy: Response to preflight request doesn’t pass access control check.

Configuring CORS on the Server

To resolve these errors, you need to configure your server to handle OPTIONS requests correctly and provide the necessary CORS headers. Below are some general steps applicable across various backend technologies.

Example in Go

If you’re using a Go server (as shown in the problem statement), here’s how you can set up your middleware to manage CORS:

Define Middleware: Create a function that sets the appropriate headers on each request.

func setupCORS(w http.ResponseWriter, req *http.Request) {
    w.Header().Set("Access-Control-Allow-Origin", "*")
    w.Header().Set("Access-Control-Allow-Methods", "POST, GET, OPTIONS, PUT, DELETE")
    w.Header().Set("Access-Control-Allow-Headers", "Accept, Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization")

    // Respond to the preflight request
    if req.Method == http.MethodOptions {
        w.WriteHeader(http.StatusOK)
        return
    }
}

func WithCORS(base http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        setupCORS(w, r)
        base.ServeHTTP(w, r)
    })
}

Integrate Middleware: Use this middleware in your main function where you set up the server.

func main() {
    // Your existing server setup code

    wrap := WithCORS(twirpHandler)
    log.Fatalln(http.ListenAndServe(":9707", wrap))
}

Example in ASP.NET Web API

If your server uses ASP.NET, enabling CORS is straightforward with the built-in support.

Install CORS Package: Use NuGet to add the necessary package.
```
Install-Package Microsoft.AspNet.WebApi.Cors
```

Configure CORS in WebApiConfig:

Open App_Start/WebApiConfig.cs and enable CORS globally or per controller.

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.EnableCors();

        // Other configurations...
    }
}

Apply CORS Attributes:

Apply the [EnableCors] attribute to controllers or methods as needed.

[EnableCors(origins: "http://example.com", headers: "*", methods: "*")]
public class MyController : ApiController
{
    // Controller actions...
}

Best Practices

Security: Avoid using wildcards (*) for Access-Control-Allow-Origin in production. Specify exact domains to minimize security risks.
Caching: Use the Access-Control-Max-Age header to specify how long a browser can cache preflight responses, reducing unnecessary requests.
Middleware Usage: Prefer middleware solutions (like Gorilla in Go or built-in ASP.NET support) for managing CORS settings consistently across your application.

Conclusion

Handling CORS is essential when developing web applications that interact with APIs hosted on different domains. By properly configuring your server to respond to preflight requests, you can avoid CORS-related issues and ensure smooth interactions between your frontend and backend systems.