Apply sanitization techniques and strict input validation like SQL injections, crose site scripting (XSS), and command injection
Use secure communication protocols like HTTPS to encrypt data transfer between the WHMCS module and external servers or APIs.
You can easily implement authentication mechanisms to verify the identity of users accessing the WHMCS module.
Also make sure that the WHMCS module is securely and configured by following recommended security guidelines offered by WHMCS and other relevant authorities
Also follow industry-standard coding conventional and leverage security-focused frameworks and libraries.
Additionally, employ cryptographic hasing algorithms to securely store sensitive information and passwords.