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Networkingintermediate
How CORS Works
A browser rule that lets a server opt in to cross-origin reads
Reviewed by Abhijith N M · 24 October 2026 · Editorial standards
The Simple VersionNo jargon
Browsers enforce the same-origin policy: JavaScript on site A cannot read responses from site B unless B allows it. CORS is B's way of allowing specific origins, methods and headers. For a non-simple request, the browser sends a preflight OPTIONS request first and waits for approval. CORS is not server-side access control. It protects the browser user from a malicious page reading another site's data; it does not stop curl, a proxy or a server-to-server call.
How CORS Works — Visual Breakdown
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Frontend
(nexgencoders.com)
Browser
(Security Guard)
API Server
(api.example.com)
Sequence of Events
Why It Matters in Production
The common failure is a correct API blocked by a missing Access-Control-Allow-Origin header, often because a proxy dropped it. Credentials make the rules stricter: you cannot use a wildcard origin with cookies, and the response must include Access-Control-Allow-Credentials. Preflights add a round trip, so cache them with Access-Control-Max-Age. Do not fix CORS by disabling the browser check. If an endpoint is public, say so explicitly; if it is private, authenticate the caller.
Implementation & Syntax
Python
# FastAPI CORS setup (the right way)
from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(
CORSMiddleware,
allow_origins=[
"https://nexgencoders.com", # production
"http://localhost:3000", # local dev
],
allow_credentials=True,
allow_methods=["GET", "POST", "PUT", "DELETE"],
allow_headers=["Authorization", "Content-Type"],
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Who sends the CORS headers that allow or deny a request?
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Written and edited by the nexGenCoders team. Spotted a mistake? Tell us and we will fix it.