Restricting CORS Access

Cross-origin resource sharing (CORS) is a mechanism that allows access to a web application to be filtered by specific client domains. This mechanism is especially useful if a call to an external resource, such as a request to a function, needs to be done from a standard web page.

Web browsers enforce a security policy known as the same-origin policy that restricts JavaScript code running on a web page to only access resources from the same origin (domain, protocol, and port) as the web page itself. This policy helps prevent malicious scripts from accessing sensitive data from other websites.

However, there are legitimate use cases where web applications need to access resources from different origins. CORS provides a way to relax the same-origin policy selectively and allow cross-origin requests under controlled circumstances.

When a web browser makes a cross-origin request, it sends an HTTP header called the Origin header that indicates the origin of the requesting web page. The server receiving the request can respond with appropriate CORS headers to specify which origins are allowed to access its resources.