How to Use the CORS Policy Explainer & Header Builder
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1
Select your allowed origin (wildcard *, specific domain, or request origin).
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2
Choose allowed methods (GET, POST, PUT, DELETE, OPTIONS, PATCH).
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3
Configure allowed headers and credentials.
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4
Copy the complete CORS response headers.
Practical Example & Use Case
Common Real-World Scenario
Fixing a React frontend getting CORS errors from an Express API at a different subdomain.
Technical Notes & Limitations
Browser Engine Architecture
Generates correct CORS response headers. CORS is enforced by the browser — API server must be configured.
Frequently Asked Questions
What causes a CORS error?
When a browser-based script at origin A requests server B, and server B does not return the correct Access-Control-Allow-Origin header.
Can I just use Access-Control-Allow-Origin: *?
Wildcard works for public APIs but cannot be used with Access-Control-Allow-Credentials: true.