CORS Header Generator: Free Online CORS Header Builder — How-To Guide

Try it free: CORS Header Generator

If your API works in curl or Postman but your frontend gets blocked, you are almost certainly looking at a CORS problem. The browser refuses to let JavaScript read a response from another origin unless the server sends the right Access-Control-* headers. Hand-writing those headers from memory is error-prone — one wrong value and the preflight fails silently. The CORS Header Generator builds the common response headers for you, with copyable output you can paste straight into your server config.

What the Tool Does

The generator creates the standard CORS response headers a server must return so browsers allow cross-origin requests: allowed origins (Access-Control-Allow-Origin), allowed methods (Access-Control-Allow-Methods), allowed request headers (Access-Control-Allow-Headers), and related preflight settings.

It is a planning and reference tool for anyone configuring CORS on Express, Next.js route handlers, Cloudflare Workers, Nginx, API gateways, or serverless functions. It helps you see exactly what your server should return — it does not bypass browser security. CORS is enforced by the browser, and only the target server can truly fix a CORS error.

How to Use It, Step by Step

1. Open the tool. Go to the CORS Header Generator in your browser. No signup, no install.
2. Enter your allowed origins. List the exact origins that should be allowed to call your API — for example, https://app.example.com. Be specific: CORS origin matching is exact, including scheme and port, so http://localhost:3000 and https://app.example.com are different origins.
3. Choose the methods and headers. Select the HTTP methods your frontend uses (GET, POST, PUT, DELETE, and so on) and any custom request headers it sends (such as Authorization or X-Request-ID). These go into the Access-Control-Allow-Methods and Access-Control-Allow-Headers values.
4. Copy the generated output. The tool produces the full set of Access-Control-* headers in a copyable block. Paste it into your server code or config as a starting point, then adjust caching and credentials options for your setup.
5. Test in the browser, not just curl. Reload your frontend app and watch the network tab. curl never sends preflight OPTIONS requests, so it can look like everything works while the browser still blocks the real request.

Practical Tips

• Never use * with credentials. If your API uses cookies or Authorization headers, Access-Control-Allow-Origin: * will not work — browsers reject it. Use explicit origins for private APIs.
• * is only fine for public APIs. If the API is open data with no credentials, * is the simple, correct choice. Keep the wildcard away from anything private.
• Debug preflight failures with the OPTIONS response. Preflight fails when the OPTIONS response does not include the required origin, method, or header permissions. Check that response first — the fix is usually one missing value, not a rewrite of your whole config.

FAQ

Can frontend code fix CORS?
No. CORS must be configured by the server that returns the response. No amount of client-side JavaScript can override the browser's enforcement.

Can I use * for every API?
Only for public APIs without credentials. Private APIs should use explicit allowed origins, and any API that relies on cookies or authorization headers must not use *.

Why does preflight fail?
Preflight fails when the OPTIONS response does not include the required CORS method, origin, or header permissions. Regenerate the headers, double-check each origin is exact, and make sure custom headers your app sends are listed as allowed.

Where do I put the generated headers?
Anywhere your server or edge layer sets response headers: Express middleware, a Next.js route handler, a Cloudflare Worker response, an Nginx add_header block, or your API gateway's response config.

Privacy Note

The generator runs 100% in your browser — nothing you type is uploaded to a server and no files ever leave your device. You can build headers for internal or client APIs without exposing your origin list or endpoint details to anyone.