How to use CORS Header Builder and Explainer

Enter the allowed origin, credential choice, methods and request-header names.
Supply the corresponding request origin, preflight method and header names to compare.
Read the generated header draft and model result, then test the actual server separately.

Example: CORS Header Builder and Explainer

Model a preflight with one explicitly allowed origin.

You add

Allowed origin and Supplied request origin: https://client.example; credentials: on; Allowed methods: GET, POST; Allowed headers and Supplied header names: content-type; Supplied method: POST.

You get

The model permits the supplied origin, method and header and produces an explicit Access-Control-Allow-Origin with Access-Control-Allow-Credentials: true. It does not use * for credentialed sharing.

Options

Share a response to a credentialed request: Models access to a credentialed response. It requires an explicit allowed origin in this builder.
Supplied non-safelisted header names: Enter non-safelisted header names, not values. The model compares those names with the explicit allowed-header set.

Supported inputs and limits

Local teaching model; no live request, server configuration change, origin authorization decision or security certification. One explicit HTTP(S) origin or non-credentialed wildcard. Credentialed wildcard origins are refused. Other browser/server restrictions can still block a request. At most 30 explicit methods and header names; Fetch-forbidden CONNECT, TRACE and TRACK are refused. Supply non-safelisted preflight header names explicitly: the tool does not infer them from values.

Where your input is processed

This tool processes your input in this browser. Your text and files are not uploaded to UseFreeTools. Check this tool's limits for anything it may save on your device.

CORS governs browser access to a response

These headers are not authentication or a network firewall. A permitted origin is a rule for browser response sharing, not evidence that a requester is trustworthy. Use an exact origin when modelling credentialed access; a wildcard origin cannot represent that credentialed case. Apply any policy only in the server context where it belongs.

A supplied preflight is not a captured request

The model compares the fields you entered and does not infer whether a real browser would preflight a request. It makes no HTTP call and cannot inspect deployed headers, caches or middleware. Check the real request/response pair before concluding that an application’s cross-origin workflow works.

Questions about CORS Header Builder and Explainer

Does CORS authenticate a visitor?

No. It controls whether a browser exposes a cross-origin response. Authentication and authorization are separate server responsibilities.

Why is a wildcard refused with credentials?

Credentialed sharing requires an explicit origin. A wildcard Access-Control-Allow-Origin cannot authorize that browser response.

Does a passing simulation prove my server works?

No. It compares entered settings only. Your real response, browser state, redirects and network behavior remain untested.

Project manager: Tony Hines · Content updated 3 October 2026 · Report a problem

CORS Header Builder and Explainer

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