CORS Header Builder Guide

Cross-Origin Resource Sharing failures often come from one inconsistent combination: a wildcard origin with credentials, a missing preflight method, or an allowed-header list that does not match the request.

How to use the CORS Header Builder

1. Enter the exact browser origin that should receive access.
2. List the methods and request headers the endpoint supports.
3. Enable credentials only when cookies or browser credentials are genuinely required, then copy the response headers.

A practical example

If credentials are enabled, Access-Control-Allow-Origin must echo a specific permitted origin rather than *. The builder blocks the incompatible wildcard combination and adds Vary: Origin for cache correctness.

The CORS Header Builder runs locally in the browser, so the input is available for the calculation without being uploaded by this tool.

Checks before you use the output

• CORS is a browser access policy, not authentication.
• Validate requested origins against an allowlist before reflecting them.
• Apply the headers to preflight responses as well as the actual response where required.

Use the result as part of a review

Assemble compatible CORS response headers for a known browser client. The output is designed to make a small task faster, but it should still be checked against the requirements of the destination system, document, or decision.

Privacy and safe sample data

Processing happens in the current browser tab. Even so, remove passwords, authorization values, customer records, and other confidential data before copying results into chat, tickets, or public examples.