Cross-Origin Resource Sharing (CORS) is a browser security mechanism that controls whether a web page from one origin may call an API on a different origin. The server declares what it permits through Access-Control response headers; the browser enforces those rules before exposing the response to JavaScript.

What is a CORS preflight request?

For requests that are not simple — custom headers, methods like PUT or DELETE, or JSON content types — the browser first sends an OPTIONS preflight asking what is allowed. The server answers with the permitted methods and headers, and a max-age telling the browser how long it may cache that answer before asking again.

Can I use a wildcard origin with credentials?

No. The CORS spec forbids combining Access-Control-Allow-Origin set to * with Access-Control-Allow-Credentials set to true. If your API sends cookies or auth headers, you must echo back the specific requesting origin, not a wildcard. This builder warns you when that combination is selected.

Why list origins per environment?

Your local, staging, and production front ends usually live on different origins. Documenting allowed origins per environment prevents the common mistake of leaving a localhost origin allowed in production, which would let a developer's machine talk to live data.

Is my configuration sent anywhere?

No. Both the policy document and the response headers are generated in your browser from your inputs. Nothing is uploaded, so you can plan CORS for internal APIs privately.

What is the CORS Policy Documentation Builder?

Creates a human-readable CORS policy document plus the matching response headers, listing allowed origins per environment, permitted HTTP methods, allowed headers, credentials handling, and preflight cache behavior. It runs free in your browser on Gera Tools, with nothing uploaded.

CORS Policy Documentation Builder

Name: CORS Policy Documentation Builder
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The CORS Policy Documentation Builder produces two things from one set of inputs: a plain-language policy document for your team and the exact Access-Control-* response headers your server must send. CORS is one of the most misconfigured security mechanisms in web development — too open and authenticated responses are exposed to any origin, too strict and the front end breaks with opaque errors. Writing the policy down and generating the headers from the same source keeps them in sync.

Why document CORS policy separately from the code?

CORS configuration lives in server middleware — an Express cors() option, an API Gateway setting, a Nginx header block — and is rarely reviewed in the same place twice. A documented policy answers three questions that code alone often does not:

Who approved this configuration? A wildcard origin on a public API may be fine; a wildcard on an API that receives auth tokens is a security finding that needs an explicit sign-off.
Are localhost origins allowed in production? This is the single most common accidental exposure — a dev origin left in the allowlist after a copy-paste.
Is this configuration actually correct for the spec? The wildcard + credentials combination is always invalid regardless of framework settings.

How it works

You declare the allowed origins (per environment: local, staging, production), the HTTP methods and request headers to permit, whether the API uses credentials, and a preflight max-age. The tool renders a policy document explaining each choice in plain language, then emits the matching response headers:

Access-Control-Allow-Origin
Access-Control-Allow-Methods
Access-Control-Allow-Headers
Access-Control-Allow-Credentials
Access-Control-Max-Age

It enforces the spec rule that a wildcard origin cannot be combined with credentials, flagging that contradiction so you cannot accidentally ship an invalid policy.

Worked example: authenticated API for a SaaS front end

Settings:

Production origin: https://app.example.com
Staging origin: https://staging.app.example.com
Local origin: http://localhost:3000
Methods: GET, POST, PUT, DELETE
Headers: Content-Type, Authorization
Credentials: yes
Preflight max-age: 600 seconds

Generated headers (production):

Access-Control-Allow-Origin: https://app.example.com
Access-Control-Allow-Methods: GET, POST, PUT, DELETE
Access-Control-Allow-Headers: Content-Type, Authorization
Access-Control-Allow-Credentials: true
Access-Control-Max-Age: 600

The server must dynamically check the incoming Origin header against its allowlist and echo back only the matching origin — not hardcode the value — so that the staging and local configs work when those requests arrive.

Common mistakes this prevents

Wildcard with credentials (always invalid, always flagged here).
Localhost in production allowlist — keep per-environment origins separate and explicitly documented.
Missing Authorization in Allow-Headers — the most frequently forgotten header on authenticated APIs, causing every credentialed request to fail preflight.
Max-age too short — 0 seconds means a preflight before every single CORS request, adding latency at scale.

Nothing you enter leaves your browser — the document and headers are generated entirely in JavaScript.