How Does CORS Work?

CORS, or Cross-Origin Resource Sharing, is a security feature implemented by web browsers to control how web pages in one domain can request and interact with resources from another domain. In simpler terms, it helps prevent unauthorized access to data on different websites, enhancing the overall security of web applications.

When a web page makes a request for a resource, such as an image or API data, from a different domain, the browser enforces the Same-Origin Policy by default. This policy restricts the browser from loading the resource if the requesting domain is different from the domain serving the resource. CORS comes into play to relax this policy and allow controlled access between different origins.

To enable CORS, a web server must include specific HTTP headers in its responses. The two key headers are "Access-Control-Allow-Origin" and "Access-Control-Allow-Methods." The former specifies which domains are allowed to access the resource, while the latter defines the HTTP methods (like GET, POST) permitted for the cross-origin request. These headers inform the browser about the permissions granted by the server, allowing it to decide whether to proceed with the request or block it for security reasons.

For instance, if a website at domain A wants to fetch data from domain B, the server at domain B must include the appropriate CORS headers in its responses. Without these headers, the browser will block the request, preventing potential security threats.

In summary, CORS is a crucial mechanism for maintaining the security of web applications by regulating cross-origin requests. It involves the exchange of specific HTTP headers between the client and server, indicating which domains can access resources and what types of requests are permitted. This helps ensure that only trusted sources can access and interact with data across different origins, preventing malicious activities and enhancing the overall safety of the web.