When you build websites or apps, sometimes they need to get information from other websites. But for safety, browsers have rules that can block this. These rules are called CORS.
If there's a CORS problem, it usually means the website you're asking for information from isn't letting your app talk to it.

This guide will explain what CORS is, why these problems happen, and how you can fix them easily, whether you're using tools like React, Vue, or Angular.

Introduction

When building modern web applications, especially those that interact with APIs, Cross-Origin Resource Sharing (CORS) errors are some of the most common — and often frustrating — hurdles developers face. This complete guide will help you understand what CORS is, why it causes issues, and how to fix CORS errors effectively in your projects.

What is CORS?

Cross-Origin Resource Sharing (CORS) is a security mechanism enforced by web browsers. It controls how resources hosted on one domain (origin) can be requested by a webpage from another domain.

For example, if your frontend app is running on https://myfrontend.com and you try to fetch data from https://api.mybackend.com, the browser checks if the backend server permits this cross-origin request. If it doesn’t explicitly allow it, the browser will block the request — leading to a CORS error.

In simple words:
CORS is like a security gatekeeper that says, “Hey, this frontend is trying to get something from a different backend — is that allowed?”

Why Do CORS Issues Happen?

CORS issues typically occur when:

• Your server doesn't include the correct CORS headers.

• Your frontend and backend are on different domains, ports, or protocols.

• You are making certain types of requests (like POST, PUT, DELETE, or requests with custom headers) that trigger a preflight request, and the server doesn’t respond properly.

• Some APIs by default block cross-origin requests for security reasons.

Common CORS Error Message

You might see errors like:

Translation:
Your frontend is trying to talk to a server, but the server didn’t say, “It’s okay for you to talk to me.”

How to Fix CORS Issues

Here’s a step-by-step guide depending on your environment:

1. Fix CORS on the Backend (Recommended)

The most proper way to fix CORS issues is to configure your server to allow requests from your frontend origin.

Node.js (Express.js) Example:

Other backend technologies:

• Django: Use django-cors-headers

• Flask: Use flask-cors

• Spring Boot: Use @CrossOrigin annotation

• ASP.NET: Configure CORS in Startup.cs

✅ Tip: Always configure CORS carefully in production — don’t allow * (all origins) unless absolutely necessary.

2. Use a Proxy Server (Temporary Solution)

During development, you can proxy API requests through your own server to avoid CORS issues.

• In React (Create React App), add this to package.json:

Now, instead of making API requests to the external domain, your app will send them to your local server, which then forwards them.

• In Vite (modern frontend tool):

3. Modify Frontend Only (Not Recommended)

There’s no way to completely fix CORS by changing frontend code alone because CORS is enforced by browsers — not your JavaScript. However, you can:

• Use JSONP (for old GET-only APIs) — not secure or recommended now.

• Use iframe hacks or browser plugins during development (e.g., “Allow CORS: Access-Control-Allow-Origin” Chrome extension) — again, not recommended for production.

4. Enable CORS at API Gateway Level

If you are using API gateways like AWS API Gateway, Cloudflare Workers, NGINX Reverse Proxy, or others, configure CORS there.

For example, in AWS API Gateway, you can enable CORS by:

• Opening the API Gateway console.

• Selecting your method (GET/POST).

• Enabling CORS in the settings.

Understanding Preflight Requests

Some requests — like those using PUT, DELETE, PATCH, or custom headers — trigger a preflight request.

• The browser sends an OPTIONS request first to check if it’s okay.

• If the server doesn’t respond with the proper CORS headers for the OPTIONS method, the real request will be blocked.

Fix: Make sure your server handles OPTIONS requests and returns correct CORS headers!

Important CORS Headers to Know

HeaderPurposeAccess-Control-Allow-OriginWhich domain(s) can access the resourceAccess-Control-Allow-MethodsWhat methods (GET, POST, etc.) are allowedAccess-Control-Allow-HeadersWhat headers can be used in the requestAccess-Control-Allow-CredentialsWhether to allow cookies/auth

Best Practices for Handling CORS

• Whitelist only trusted origins — don’t allow * in production.

• Log and monitor CORS errors on the client-side for debugging.

• Secure sensitive data even when CORS is configured — CORS is not a security mechanism for APIs; it’s a browser feature!

• Handle preflight (OPTIONS) requests properly.

• Test in production-like environments to avoid surprises.

Conclusion

Handling CORS issues might seem annoying at first, but with a clear understanding of how it works, solving them becomes easy. Always aim to fix CORS problems at the server level rather than relying on workarounds. Setting up proper CORS headers ensures your application stays secure, robust, and user-friendly.