Allow two-factor authentication for both client areas and admin as it helps to add an extra security layer.
Implementing strong password policies can help to prevent hacking. This policies enables users to create complex password with a mix of letter, numbers, and special characters.
Use IP whitelisting to enable only trusted IPs to access the sensitive areas like the backlisting and whitelisting.
SSL/TLS encrryption is properly configured to secure data transfer bettwen the users and your WHMCS installation.
Secure your server environment by implementing best strategues like regular security patches, firewall configuration, and disabling unnecessary services.
Set strict directory and file permissions to prevent illegal access to sensitive files that contains customer data or configurations.