CORS Header Generator — Access-Control Headers

Introduction

You ship a React frontend on https://app.example.com and an API on https://api.example.com. Fetch works in Postman but the browser shows "CORS policy blocked" — because browsers enforce cross-origin rules that CLI tools ignore. Fixing CORS means returning the right Access-Control-* response headers on both preflight OPTIONS and actual requests. Copy-pasting Stack Overflow snippets with * in production creates credential leaks; typos in Allow-Headers break Authorization tokens.

Certoflow's CORS Header Generator formats a standard three-header block from your allowed origin input. Defaults include GET, POST, OPTIONS methods and Content-Type, Authorization allowed headers. Everything runs locally — paste origins freely while debugging staging URLs. Pair with Query String Builder for full request URLs, HTTP Status Code Lookup when preflight returns unexpected codes, and JSON Formatter for readable API response bodies once CORS succeeds.

What this tool does

Input	Behavior
Allow origin	Single origin URL, or * for any origin (default example: https://example.com)
Generate	Produces multi-line header block
Copy	Clipboard-ready text

Generated output format:

Access-Control-Allow-Origin: <your origin>
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Content-Type, Authorization

Methods and allowed headers use fixed defaults in the generator function — customize manually after copy if your API needs PUT, DELETE, or custom headers like X-Request-Id.

How it works

export function generateCorsHeaders(origin: string, methods = "GET, POST, OPTIONS"): string {
  const o = origin.trim() || "*";
  return [
    `Access-Control-Allow-Origin: ${o}`,
    `Access-Control-Allow-Methods: ${methods}`,
    "Access-Control-Allow-Headers: Content-Type, Authorization",
  ].join("\n");
}

Empty origin after trim falls back to *. The UI does not expose method or header editors — developers edit the copied text for advanced scenarios. No network simulation or preflight request is sent; this is a formatting aid, not a CORS debugger proxy.

Real-world examples

Express middleware bootstrap

New Node API needs CORS before frontend integration. Generate headers with your staging origin, translate into cors package options or manual res.setHeader calls. Test from the browser while watching HTTP Status Code Lookup for 204 preflight responses.

nginx reverse proxy snippet

Paste generated lines into documentation for ops teammates configuring add_header directives. Remember nginx requires always flag for error responses in some setups — the generator provides names and values, not server-specific syntax.

Multiple origins documentation

Certoflow outputs one origin per generation. Document each deployment target separately — production https://app.example.com, staging https://staging.example.com. Wildcard * is convenient for public read-only APIs without credentials.

API technical spec appendix

Include generated header blocks in OpenAPI descriptions or README deployment sections so consumers know which browser origins are supported.

Contrasting with credentialed requests

When fetch uses credentials: 'include', Access-Control-Allow-Origin cannot be * — generate explicit origins and add Access-Control-Allow-Credentials: true manually to your server; the generator focuses on the core trio.

Common mistakes

Using * with cookies or Authorization. Browsers reject wildcard when credentials are included. Specify exact origins.

Forgetting OPTIONS handlers. Preflight sends OPTIONS first. Your server must respond with the same CORS headers, not only on GET/POST.

Omitting custom headers from Allow-Headers. If clients send X-Api-Key, add it to the copied header list — defaults cover only Content-Type and Authorization.

Mismatching origin trailing slashes. https://example.com and https://example.com/ may differ. Match the browser's actual Origin header exactly.

Assuming generated headers alone fix CORS. Server must actually emit them on responses. The tool does not configure infrastructure.

Copying production origins into public tickets. Certoflow is local, but origin URLs can reveal unreleased hostnames. Redact when sharing.

Ignoring HTTPS mixed content. CORS errors sometimes mask HTTP/HTTPS mismatches — verify scheme and port in the origin field.

Use cases

Full-stack developers unblocking local frontend-to-API development.

Technical writers documenting required response headers for partner APIs.

DevOps engineers drafting gateway configuration checklists.

Students learning why browsers enforce CORS differently than curl.

QA testers verifying expected header names during API audits.

Backend contractors standardizing header spelling across microservices.

FAQ

Does this send test requests?

No. It formats header text only. Test with your browser devtools Network tab.

Can I allow multiple origins?

Generate once per origin. Servers typically echo the request Origin when it matches an allowlist — implement that logic in code.

What methods are included by default?

GET, POST, and OPTIONS.

Can I add PUT or DELETE?

Edit the copied Access-Control-Allow-Methods line after generation.

Is * safe for production?

Only for fully public APIs without credentials. Prefer explicit origins for authenticated apps.

Are headers uploaded?

No. Generation is entirely client-side.

What headers are allowed by default?

Content-Type and Authorization.

How does this relate to Query String Builder?

Build encoded query strings for API URLs; CORS headers govern whether browsers may read responses from those URLs cross-origin.

Does this configure nginx or Express automatically?

No. Copy values into your framework or server configuration.

What status code should OPTIONS return?

Often 204 No Content — see HTTP Status Code Lookup for reference.