CORS (Cross-Origin Resource Sharing) is a security feature implemented in web browsers that controls how resources on a web server can be requested from a different origin (domain, protocol, or port) than the server's own. Modern web applications often make API requests to servers hosted on different domains (cross-origin requests). For security reasons, browsers restrict such requests unless the server explicitly allows them. This prevents Cross-Site Request Forgery (CSRF) and other malicious attacks. When a browser makes a cross-origin request, it first sends a preflight request (using the HTTP OPTIONS method) to verify whether the server allows the actual request. The server responds with specific HTTP headers, such as: * **Access-Control-Allow-Origin** - Specifies which domains can access the resource. * **Access-Control-Allow-Methods** - Lists allowed HTTP methods (e.g., GET, POST). * **Access-Control-Allow-Headers** - Specifies permitted custom headers. If the server’s response includes the appropriate headers, the browser proceeds with the actual request. Otherwise, the request is blocked. CORS is a standard that helps address the following with APIs. * **Security** - Prevents unauthorized or malicious scripts running in a browser from accessing sensitive data. * **Controlled Access** - Allows you to define and enforce which clients (origins) can interact with your API. * **Flexibility** - Supports modern web applications and Single-Page Applications (SPAs) that often communicate with external APIs. * **Compliance** - Ensures adherence to web security standards, reducing vulnerabilities to attacks. * **Better User Experience** - Enables seamless integration with third-party services or APIs without compromising security. CORS is essential for APIs exposed to web applications to enable secure cross-origin communication while preventing unauthorized access. Properly configuring CORS headers ensures your API remains both functional and protected.