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Cross-Origin Resource Sharing (CORS)

Cross-Origin Resource Sharing (CORS) is a browser-enforced security mechanism that uses HTTP headers to grant a web application running at one origin access to selected resources from a different origin.

CORS is the structured exception to the Same-Origin Policy (SOP). When a frontend application at domain-a.com requests data from an API at api-b.com, the browser initiates a “Preflight” request (using the OPTIONS method). The server must then respond with specific Access-Control headers to validate the request. Mastering CORS configuration is essential for modern decoupled architectures, such as SPAs (Single Page Applications) communicating with headless microservices.

Frequently Asked Questions

A preflight request is triggered if the main request uses “non-simple” methods (like PUT or DELETE) or includes custom headers (like Authorization). It is the browser’s way of asking the server, “Is it safe to send this specific data from this specific origin?”

Yes. Since CORS is a browser security feature, server-to-server communication does not enforce it. A backend proxy can fetch the data from the third-party API and serve it to the frontend from the “same origin,” thereby bypassing the need for CORS headers on the external API.

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