Securing a web application Protect Your Data And Your Users' Data.

Use HTTPS: HTTPS encrypts the data sent between the client and the server, protecting it from eavesdroppers.
Sanitize User Input: Always sanitize user input to protect against SQL injection and cross-site scripting (XSS) attacks. Never trust user input.
Use Secure Cookies: Use the Secure and HttpOnly flags on cookies to protect them from being intercepted or accessed by client-side scripts.
Implement Content Security Policy (CSP): CSP can help prevent XSS attacks by controlling which domains the browser is allowed to load resources from.
Use CORS carefully: Cross-Origin Resource Sharing (CORS) allows you to control which domains can access your resources. Be careful not to set it to * for sensitive resources.
Keep Your Software Up-to-Date: Always keep your server software, programming languages, frameworks, and libraries up-to-date to ensure you have the latest security patches.
Limit Rate of Requests: Implement rate limiting to prevent brute force attacks.
Use Proper Error Handling: Don't reveal sensitive information in error messages.
Use Authentication and Authorization: Always verify the identity of users and what they are allowed to access. Consider using JWT for stateless authentication.
Encrypt Sensitive Data: Always encrypt sensitive data like passwords and credit card numbers, both in transit and at rest.

Security is not a one-time task. It should be part of your regular design and development process.