Understanding CORS: Cross-Origin Resource Sharing

CORS, or Cross-Origin Resource Sharing, is a crucial web security mechanism designed to enable controlled access to resources located outside of a script’s originating domain. It lets servers selectively relax the browser’s same-origin policy, which by default prevents scripts from reading responses from a different origin (scheme, host, and port) than the one that served the web page. MDN’s CORS guide covers every header in detail.

Why CORS Matters

The same-origin policy, while a critical security measure, can be overly restrictive, limiting the seamless integration of web services and APIs from various domains. CORS provides a way for servers to declare who can access their assets, thereby enabling cross-domain requests in a secure manner.

How CORS Works

When a web application attempts a cross-origin request, the browser automatically adds an Origin header to the outgoing request, indicating the domain of the web application. The server responds with CORS headers, such as Access-Control-Allow-Origin, declaring which origins may read the resource. The browser enforces the policy: if the headers do not permit the requesting origin, the browser blocks the script from reading the response. For non-simple... premium content.