Cross-origin resource sharing (CORS) is a security feature that enables secure communication between web applications hosted on different origins. In Node.js, CORS can be implemented securely to mitigate security risks such as data breaches and unauthorized access from third-party websites. Improper implementation of CORS can expose sensitive data to unintended origins or allow malicious websites to bypass same-origin policy protections. To implement CORS securely, developers should restrict allowed origins, use secure cookies and tokens for authentication, limit exposed headers and HTTP methods, and implement security headers outside of CORS. Testing CORS implementations and security is crucial to identify potential vulnerabilities and misconfigurations. Browser developer tools, Postman, custom scripts, and Snyk Code can be used to test CORS configurations and ensure the application is secure when it goes to production.