Cross-Origin Resource Sharing (CORS) is one of the most commonly misconfigured HTTP mechanisms. AI chatbots regularly produce incorrect or insecure CORS headers — this tool generates production-ready headers and server config with security validation.
CORS Configuration
Generated Output
HTTP Response Headers
Server Configuration
Preflight (OPTIONS) Request
Multi-Framework
Generate config for Nginx, Apache, Express.js, Flask, and Cloudflare Workers — copy-paste ready.
Security Validation
Detects dangerous combinations like wildcard + credentials or missing preflight headers.
Preflight Simulation
See exactly what browsers send as OPTIONS and what your server must respond with.
100% Private
All processing is in your browser. No data sent to any server. No signup, no tracking.
What Is CORS?
Cross-Origin Resource Sharing (CORS) is a browser security mechanism that controls how web pages from one origin can request resources from a different origin. Without CORS headers, browsers block cross-origin requests by default (Same-Origin Policy).
CORS errors are among the most common and frustrating issues developers face. The browser console shows errors like No 'Access-Control-Allow-Origin' header is present, but the fix depends entirely on your server stack and security requirements.
This generator produces production-ready headers and framework-specific code snippets that you can copy directly into your server configuration.
How to Use
- Choose your Allowed Origin — wildcard (*) for public APIs, specific URLs for authenticated endpoints
- Select the HTTP methods your API supports
- Add allowed headers — include every header your frontend sends
- Toggle Allow Credentials if your API uses cookies or Authorization headers
- Set Max-Age to cache preflight responses (86400s = 24h recommended)
- Click Generate and copy the headers or framework-specific code
Frequently Asked Questions
What does CORS stand for?
Cross-Origin Resource Sharing. It's a browser security mechanism that uses HTTP headers to tell browsers whether a web application running at one origin can access resources from a different origin.
Why can't I use * with credentials?
When Access-Control-Allow-Credentials: true is set, the Access-Control-Allow-Origin header cannot be *. Browsers explicitly block this combination for security reasons. You must specify exact origins.
What is a preflight request?
Before certain cross-origin requests, browsers send an HTTP OPTIONS request (preflight) to check if the actual request is safe to send. The server must respond with appropriate CORS headers. Preflight is triggered by non-simple methods (PUT, DELETE, PATCH) or custom headers.
What's the difference between 401 and 403 with CORS?
Neither is a CORS error. CORS errors appear in the browser console as network errors, not as HTTP status codes. A CORS failure blocks the browser from reading the response entirely, regardless of the HTTP status.
Can I use localhost in CORS origins?
Yes, but browsers treat http://localhost:3000 and http://127.0.0.1:3000 as different origins. For development, include all variants your frontend uses.
What is Vary: Origin?
If your server dynamically sets Access-Control-Allow-Origin based on the request's Origin header, you should include Vary: Origin so caches don't serve the wrong CORS headers to different origins.
Why does AI get CORS configuration wrong?
AI chatbots often produce CORS configs that use wildcard with credentials, omit preflight handling, or suggest application-level fixes for what is fundamentally a server response header issue. They also struggle with framework-specific syntax variations. This tool produces tested, production-ready config.
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