The Same-Origin Policy vs CORS
The browser Same-Origin Policy (SOP) blocks frontend scripts from reading cross-origin HTTP responses by default.
CORS is a set of HTTP response headers through which a server explicitly instructs the browser that a cross-origin client is authorized to read the response.
Dissecting Preflight OPTIONS Handshakes
When requests contain custom headers or JSON content types, browsers automatically send an HTTP OPTIONS preflight request before dispatching the real POST or PUT.
The server must respond to OPTIONS with HTTP 200/204 and valid Access-Control-Allow-Methods and Access-Control-Allow-Headers.
Step-by-Step Tutorial
Inspect DevTools Console and Network Headers
Check the exact Origin of your frontend and verify which required header is missing from the server response.
Handle HTTP OPTIONS on the Server
Configure reverse proxies (Nginx/Cloudflare) or backend routers to return 204 No Content for OPTIONS.
Enable Preflight Caching
Include Access-Control-Max-Age: 86400 to cache preflight decisions and avoid repeated OPTIONS roundtrips.
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Frequently Asked Questions
Why does cURL succeed when my web browser fails with a CORS error?
cURL and Postman are developer CLIs, not web browsers; they do not enforce the browser Same-Origin Policy sandbox.
Can I disable CORS in JavaScript frontend code?
No. CORS is enforced strictly by the client's browser security engine. Fixes require server configuration or a reverse proxy.