Ensure that your WHMCS installation, along with all plugins, modules, and themes, is regularly updated to the latest versions. Updates often include security patches that address vulnerabilities.
Enforce strong password policies for both admin and client accounts. Consider implementing two-factor authentication (2FA) for an added layer of security.
Implement a firewall to filter incoming and outgoing traffic, blocking potential threats before they reach your WHMCS site.
Limit access to important portions of your WHMCS site, such as the admin dashboard and server configuration files, to just authorized users.
Perform frequent security audits on your WHMCS website to discover and remedy any potential vulnerabilities or flaws.