DevToolkit

CORS Header Generator / Checker

Browser only

CORS Header Generator / Checker

Build CORS headers and platform snippets locally in your browser, then check for wildcard origin, credentials, preflight, and dynamic origin issues before you copy anything into production.

Allowed originUse * or one origin per line. Localhost is allowed for development.

Allowed headersComma or newline separated, for example Content-Type, Authorization, X-Requested-With.

Expose headersOptional. Use comma or newline separated names.

Allow credentialsIf true, the client fetch/XHR call must also opt in.

Max age secondsFor example 86400. Leave blank to omit the header.

Output targetChoose one format or generate all of them.

This helper runs entirely in your browser and does not upload the policy you type here.

Deep dive

More context for CORS Header Generator / Checker

Useful when you want to shape a browser-safe CORS policy before you wire it into an API, proxy, or edge function.

Overview

What is this tool?

It builds CORS response headers and platform snippets locally in your browser, then best-effort checks wildcard origins, credentials, preflight settings, and dynamic-origin cases that can break in production.

Output

Example output

Raw headers

Access-Control-Allow-Origin: <dynamic request origin>
Vary: Origin
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Headers: Content-Type, Authorization, X-Requested-With
Access-Control-Expose-Headers: X-Request-Id, X-RateLimit-Remaining
Access-Control-Allow-Credentials: true
Access-Control-Max-Age: 172800

Nginx map block

map $http_origin $cors_allow_origin {
    default "";
    "https://app.example.com" $http_origin;
    "https://admin.example.com" $http_origin;
}

location / {
    add_header Access-Control-Allow-Origin $cors_allow_origin always;
    add_header Vary "Origin" always;
}

Express middleware

function corsMiddleware(req, res, next) {
  const origin = req.headers.origin;
  if (origin && allowedOrigins.includes(origin)) {
    res.setHeader("Access-Control-Allow-Origin", origin);
    res.setHeader("Vary", "Origin");
  }
  next();
}

When to use it

Common use cases

Check a browser API policy before you paste headers into a proxy or framework config.
Generate a starter config for Nginx, Caddy, Express.js, Cloudflare Workers, or Next.js headers.
Review wildcard origin, credentials, and preflight behavior before you deploy a public endpoint.

Navigation

Explore related workflows

Keep moving through the collection, workflow, and adjacent tools that usually belong with this page.

Part of collection

Jump to the broader tool set that covers the same problem space.

HTTP & Web Debugging Tools

Tools for parsing headers, generating redirects, checking CORS, and debugging common website configuration issues.

Related workflow

Continue with the tool chain that usually goes together here.

Debug website headers and CORS

Parse response headers, review security protections, generate CORS snippets, and prepare redirect rules for web apps.

HTTP Headers Parser HTTP Security Headers Checker Redirect Rules Generator HTTP Status Code Lookup MIME Type Lookup

Related tools

Open the closest adjacent tools without leaving the current context.

HTTP Security Headers Checker Nginx Reverse Proxy Generator Caddyfile Generator Redirect Rules Generator HTTP Headers Parser JSON Formatter / Validator HTTP Status Code Lookup MIME Type Lookup URL Encoder / Decoder

Answers

FAQ

Can I use wildcard origin with credentials?

No. That combination is unsafe, and the tool calls it out because browsers and servers need a tighter policy.

Why do multiple origins need dynamic handling?

A CORS response can only send one Access-Control-Allow-Origin value. When you allow more than one origin, the server usually has to echo the request Origin dynamically.

Should the client also change its fetch settings?

Yes. If you enable credentials, the browser request also has to opt in with credentials or withCredentials.

Does it replace backend CORS logic?

No. It is a best-effort helper that gives you a starting point, but you still need to verify the behavior in your actual browser, backend, and proxy stack.

Disclaimer

Disclaimer / limitation note

This is a best-effort CORS helper, not a full browser, backend, or proxy validator. CORS behavior depends on the browser, the server framework, reverse proxies, caching layers, and preflight handling. Review origin, credentials, and preflight behavior carefully before you ship a production policy, and do not rely on wildcard origin plus credentials to fix a fast-moving error. All generation and checking happen locally in your browser and are not uploaded to a server.

Privacy

Privacy note

All parsing and generation happen locally in your browser. No backend request is made, no policy text is uploaded, and DevToolkit does not store the values you enter here.