CORS Headers Builder & Validator

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CORS Headers Builder & Validator

Build correct CORS (Cross-Origin Resource Sharing) headers with a visual configurator. Set allowed origins, methods, headers, and credentials, then get the generated HTTP headers and ready-to-use code snippets for Express.js, Nginx, Apache, Django, FastAPI, Flask, and Go. Real-time validation catches common CORS misconfigurations before they reach production.

Cómo utilizar

Configure your CORS policy using the form controls: choose origin mode (wildcard, specific origins, or reflect), select allowed HTTP methods, add allowed headers, and enable credentials if needed. The tool generates the correct Access-Control-* headers instantly, shows validation warnings for common mistakes, and provides copy-ready code snippets for 7 server frameworks. Use the CORS explainer section for a quick refresher on how CORS works.

Características

Visual CORS Configurator — Set Access-Control-Allow-Origin (wildcard, specific origins, or reflect), methods (GET/POST/PUT/PATCH/DELETE/OPTIONS/HEAD), custom headers, credentials, max-age, and expose-headers.
Real-Time Header Generation — See the complete CORS response headers and preflight (OPTIONS) headers update instantly as you configure.
7 Server Framework Snippets — Copy-ready code for Express.js, Nginx, Apache (.htaccess), Django (django-cors-headers), FastAPI (CORSMiddleware), Flask (flask-cors), and Go (net/http). Each snippet is dynamically generated from your configuration.
Configuration Validation — Color-coded warnings catch common CORS mistakes: wildcard + credentials (invalid), missing Content-Type header with POST, missing OPTIONS method for preflight, short max-age increasing preflight traffic.
Quick-Add Header Buttons — One-click buttons for common headers: Content-Type, Authorization, X-Requested-With, Accept, Origin.
Preflight Headers — Separate display of preflight (OPTIONS) response headers when they differ from regular CORS headers.
CORS Explainer — Collapsible reference covering simple vs preflight requests, what triggers preflight, the role of each header, and why credentials + wildcard is invalid.

CORS Headers Explained

Access-Control-Allow-Origin: Which origins can access the resource. Wildcard (*) allows all, or specify exact origins.
Access-Control-Allow-Methods: Which HTTP methods are permitted for cross-origin requests.
Access-Control-Allow-Headers: Which request headers the client is allowed to send.
Access-Control-Allow-Credentials: Whether the browser should include cookies/auth. Cannot be used with wildcard origin.
Access-Control-Max-Age: How long (seconds) browsers cache preflight results.
Access-Control-Expose-Headers: Which response headers the browser can access from JavaScript.

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What is CORS and why do I need it?

CORS (Cross-Origin Resource Sharing) is a browser security mechanism that controls which websites can make requests to your server. By default, browsers block cross-origin requests (e.g., your frontend at app.example.com calling your API at api.example.com). CORS headers tell the browser which origins, methods, and headers are allowed. Without correct CORS configuration, your API will work from server-to-server but fail when called from a browser — one of the most common issues in web development.

Why can’t I use wildcard (*) with credentials?

When Access-Control-Allow-Credentials is true, the browser requires the server to specify the exact origin — not wildcard (*). This is a security measure: if a server says ‘any origin can access this resource with credentials (cookies, auth tokens),’ that would allow any website to make authenticated requests on behalf of the user. Browsers enforce this restriction to prevent credential theft. Instead of wildcard, use ‘Reflect Origin’ mode — the server reads the request’s Origin header and echoes it back if it’s in the allowed list.

What is a preflight request?

A preflight request is an automatic OPTIONS request the browser sends before certain cross-origin requests to check if the server allows them. Preflight is triggered when: the request uses methods other than GET/HEAD/POST, includes custom headers (like Authorization), or uses Content-Type values other than form data or plain text. The server must respond to OPTIONS with the appropriate Access-Control-Allow-* headers. If the preflight fails, the browser blocks the actual request. The Access-Control-Max-Age header controls how long browsers cache preflight results.

Which server framework snippet should I use?

Choose based on your backend technology: Express.js for Node.js APIs, Nginx for reverse proxy/static file servers, Apache for traditional hosting (.htaccess), Django for Python Django projects (uses django-cors-headers package), FastAPI for modern Python APIs (built-in CORSMiddleware), Flask for Python Flask apps (uses flask-cors), and Go for Go net/http servers. Each snippet is generated from your exact configuration — just copy and paste into your project. For containerized deployments, you may need to configure CORS at the reverse proxy level (Nginx/Apache) rather than the application level.

CORS Headers Builder & Validator

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