A hacked WHMCS installation is a nightmare. Here is the checklist for hardened "Full Work."
The first time you log into the Admin Area, a Getting Started Wizard appears. This wizard is your blueprint. It guides you to configure: whmcsfull work
Before installing, ensure your server meets the latest PHP and MySQL requirements. You can find detailed steps on the WHMCS Installation Guide . A hacked WHMCS installation is a nightmare