CORS Header Builder

What is CORS Header Builder?

CORS Header Builder generates Cross-Origin Resource Sharing configuration for common web servers and frameworks. It provides a visual interface for setting Access-Control-Allow-Origin, Allow-Methods, Allow-Headers, Expose-Headers, Max-Age, and Allow-Credentials. The tool outputs ready-to-use configuration for Express.js middleware, Nginx server blocks, Apache .htaccess, and raw HTTP headers.

Backend developers configuring API servers, DevOps engineers setting up reverse proxy headers, and frontend developers debugging cross-origin request failures. The tool runs entirely in your browser — your data never leaves your device.

How to Use CORS Header Builder

1. Enter the allowed origins — specific domains, or * for any origin (not recommended with credentials)
2. Select the allowed HTTP methods — GET, POST, PUT, PATCH, DELETE, OPTIONS
3. Specify allowed request headers like Authorization, Content-Type, and X-Request-ID
4. Configure expose headers, max-age for preflight caching, and credentials support
5. Copy the generated configuration for your server — Express, Nginx, Apache, or raw headers

Common Use Cases

• Configuring CORS headers for a REST API that serves multiple frontend domains
• Setting up Nginx CORS rules for a reverse proxy in front of a microservice
• Generating Express.js CORS middleware configuration for a Node.js API server
• Debugging CORS preflight failures by reviewing and adjusting header configurations
• Configuring CORS for APIs that need to support credentials (cookies or Authorization headers)

Pro Tips

• You cannot use Access-Control-Allow-Origin: * together with Access-Control-Allow-Credentials: true — browsers reject this combination. You must echo the specific requesting origin instead
• Set Access-Control-Max-Age to 86400 (24 hours) to cache preflight responses — this eliminates the extra OPTIONS request on subsequent API calls from the same origin
• The preflight OPTIONS request is only triggered for non-simple requests — GET with standard headers never preflight, but POST with Content-Type: application/json always does

Why Use This Tool?

• Privacy-first: All processing happens locally in your browser. No data is transmitted to any server.
• No installation required: Works in any modern browser on desktop or mobile.
• Completely free: No account, no credit card, no usage caps.
• Fast and reliable: Powered by modern Web APIs for near-instant results.

Related Concepts

Learn more about related topics: Preflight request, Same-origin policy, OPTIONS method, Access-Control headers.

Frequently Asked Questions