CORS Error Explained: What It Is and How to Fix It

If you work with frontend development, you’ve probably seen this error:

"Access to fetch at 'https://api.example.com' from origin 'http://localhost:3000' has been blocked by CORS policy"

CORS errors are confusing, frustrating, and extremely common. In this article, we’ll explain what a CORS error really is and how to fix it step by step.

What Is CORS?

CORS stands for Cross-Origin Resource Sharing.

By default, browsers block requests made from one origin to another for security reasons.

An origin is defined by:

  • Protocol (http / https)
  • Domain
  • Port

If any of these differ, the browser considers it a different origin.

Why Browsers Block Cross-Origin Requests

This restriction exists to protect users from malicious websites.

Without CORS, a random website could:

  • Read your private data
  • Make authenticated requests on your behalf
  • Steal sensitive information

The Most Common CORS Error

No 'Access-Control-Allow-Origin' header is present on the requested resource.

This means the server did not explicitly allow requests from your frontend origin.

Important: CORS Is a Browser Problem

This is a critical point many developers miss:

CORS is enforced by the browser, not by the server.

If you call the same API using:

  • Postman
  • curl
  • A backend service

It usually works fine.

Preflight Requests Explained

Before sending certain requests, browsers send a OPTIONS request.

This is called a preflight request.

The server must respond with headers like:

Access-Control-Allow-Origin: http://localhost:3000
Access-Control-Allow-Methods: GET, POST, PUT
Access-Control-Allow-Headers: Content-Type, Authorization

If the preflight fails, the real request is never sent.

How to Fix CORS Errors (Backend)

Fix 1: Allow Specific Origins

The safest solution is to allow known origins.

Access-Control-Allow-Origin: https://yourfrontend.com

Fix 2: Enable CORS in Common Backends

Node.js (Express)

const cors = require('cors');
app.use(cors({ origin: 'http://localhost:3000' }));

Python (Flask)

from flask_cors import CORS
CORS(app, origins=["http://localhost:3000"])

ASP.NET Core

builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowFrontend",
        policy => policy.WithOrigins("http://localhost:3000")
                        .AllowAnyHeader()
                        .AllowAnyMethod());
});

How to Fix CORS Errors (Frontend)

Use a Proxy in Development

During development, proxies avoid CORS entirely.

Vite Example

server: {
  proxy: {
    '/api': 'http://localhost:5000'
  }
}

Do NOT Use no-cors Mode

Using mode: 'no-cors' hides the error but makes the response unusable.

This is not a real fix.

Common CORS Mistakes

  • Using * with credentials
  • Fixing CORS on the frontend only
  • Disabling CORS entirely in production

How to Debug CORS Errors

  • Check the browser Network tab
  • Inspect the preflight OPTIONS request
  • Verify response headers

Conclusion

CORS errors are not bugs — they are security features.

Once you understand how origins, headers, and preflight requests work, CORS becomes predictable and manageable.

Most CORS issues are solved by configuring the backend correctly.

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