🛡️CORS Header & Preflight Response Generator Documentation & Guide

Web & Developer

Cross-Origin Resource Sharing (CORS) is a browser-enforced security mechanism governed by the W3C/WHATWG Fetch Standard that enables servers to selectively loosen the strict Same-Origin Policy (SOP). By returning explicit HTTP response headers, backends specify which client origins, HTTP verbs, and headers are permitted to interact with protected resources. This generator produces battle-tested, syntax-checked configurations for Nginx, Express, Cloudflare Workers, Apache, and Caddy with built-in preflight collision detection.

📋How to Configure Production CORS Headers

Select Server Platform: Choose your reverse proxy or backend runtime (Nginx, Express.js, Cloudflare Workers, Apache, or Caddy).
Define Allowed Origin: Enter the exact origin of your web application (e.g. https://app.example.com) or choose wildcard * for public read-only APIs.
Select Permitted Methods: Check allowed HTTP verbs. Ensure OPTIONS remains enabled so browsers can perform preflight handshakes.
Configure Credentials & Cache: Toggle Allow Credentials if your client transmits cookies or Authorization headers, and adjust Access-Control-Max-Age to minimize redundant OPTIONS queries.
Generate & Copy: Click 🛡️ Generate CORS Config to review the syntax and copy directly to your server configuration.

⚙️Technical Details & Preflight Handshake Anatomy

Understanding browser CORS evaluation trees prevents production outage loops and security vulnerabilities:

• Simple vs Preflight Requests: Requests using methods other than GET, POST, or HEAD, or containing headers other than Accept, Accept-Language, or Content-Type (with values other than text/plain, multipart/form-data, or application/x-www-form-urlencoded), trigger an automated browser OPTIONS preflight.

• The Wildcard + Credentials Trap: The Fetch specification strictly forbids Access-Control-Allow-Origin: * when Access-Control-Allow-Credentials: true. Browsers reject this combination with a fatal error to prevent malicious cross-origin credential harvesting.

• Max-Age Browser Caps: Chromium caps Access-Control-Max-Age at 7,200 seconds (2 hours); Firefox caps it at 86,400 seconds (24 hours). Setting higher numbers safely clamps to the client ceiling.

• Expose Headers Requirement: By default, client JavaScript can only inspect CORS-safelisted response headers (Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, Pragma). Custom headers like X-RateLimit-Remaining require explicit enumeration in Access-Control-Expose-Headers.

💡Production Best Practices

Avoid Blind Origin Reflection: Never blindly reflect the client Origin request header back into Access-Control-Allow-Origin alongside credentials. Always validate incoming origins against an explicit server-side allowlist.
Return 204 No Content for OPTIONS: Preflight requests contain no response body. Returning HTTP status 204 saves server bandwidth and avoids downstream processing overhead.
Cache-Control on Static Assets: When serving fonts or images across origins, ensure you pair CORS headers with Vary: Origin so intermediate CDN caches don't serve un-authenticated or mis-attributed cached responses.

100% Client-Side Privacy Guarantee:All rule generation, origin parsing, and configuration compilations execute 100% locally within your browser sandbox. Zero internal API hostnames, private domain origins, or infrastructure tokens are ever transmitted across external networks.

CORS Header & Preflight Response Generator