Improving the security of a Node.js app requires following several best practices. First, always use HTTPS to encrypt data in transit and protect against man-in-the-middle attacks. Input validation and sanitization are critical to prevent common attacks like SQL injection or cross-site scripting (XSS). Use libraries like `validator` to sanitize and validate user inputs. Ensure that errors are handled securely—don’t expose sensitive information through error messages. It’s also important to keep your dependencies up to date by regularly checking for vulnerabilities using tools like `npm audit`. Additionally, implement security headers (such as `helmet` in Express.js) and ensure that user authentication and session management follow secure patterns (e.g., JWT or OAuth). By following these steps, you can greatly enhance the security of your Node.js application.