How does CORS (Cross-Origin Resource Sharing) work

Cross-Origin Resource Sharing (CORS) is a security feature implemented in web browsers that allows web servers to specify which origins (domains) are permitted to access the resources on a web page.

Here's how it works:

  1. Origin: An origin is a combination of protocol (like HTTP), domain, and port (if specified). For instance, is an origin.
  2. Same-Origin Policy: By default, web browsers enforce a same-origin policy, which means a web page can only make requests to the same origin it was loaded from. This policy is a security measure to prevent certain types of attacks, like cross-site scripting (XSS).
  3. Cross-Origin Requests: When a web page tries to make a request (such as fetching data via AJAX) to a different origin, the browser blocks the request due to the same-origin policy.
  4. CORS Headers: CORS allows servers to specify which origins are allowed to access their resources. When a browser makes a cross-origin request, the server can include specific HTTP headers in its response to inform the browser whether the request is allowed or not.
    • Access-Control-Allow-Origin: This header specifies which origins are permitted to access the resource. For example, Access-Control-Allow-Origin: allows only that specific origin to access the resource.
    • Other CORS Headers: There are additional headers like Access-Control-Allow-Methods, Access-Control-Allow-Headers, etc., that servers can use to specify the allowed HTTP methods, headers, etc.
  5. Preflight Requests: For certain types of requests (e.g., those using methods other than GET, POST, or with custom headers), the browser first sends a preflight request (an OPTIONS request) to the server to check what methods and headers are allowed. The server responds with appropriate CORS headers indicating whether the actual request can proceed.
  6. Handling CORS: When developing web applications, developers need to ensure that their servers are configured to send the appropriate CORS headers to allow or deny cross-origin requests based on their requirements.

CORS is essential for allowing controlled access to resources across different origins while maintaining security on the web.

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