Understanding the Same-Origin Policy and CORS

The Same-Origin Policy (SOP) serves as a cornerstone of web security, preventing scripts from one origin from accessing resources on another origin. An origin consists of the protocol, domain, and port combination. Without SOP, any website could read your Gmail, access your banking session, or steal data from internal corporate applications. CORS provides a controlled relaxation of this policy, allowing servers to explicitly permit specific cross-origin requests while maintaining security.

CORS operates through a system of HTTP headers that facilitate a handshake between browsers and servers. When a web page attempts to fetch resources from a different origin, the browser enforces CORS rules, potentially blocking the request unless the server explicitly allows it. This mechanism protects users while enabling legitimate cross-origin interactions required by modern web applications.