The main page or website is not affected by this breach. Hacker managed to upload a html file "b0x.html" in the image directory of WHMCS documentation website(docs.whmcs.com).
Hacker didn't leave any message other than "b0x" in the defacement page. Whenever main page of a website is not defaced, it will take time to admin to notice it. So, we are still able to see the defacement at "http://docs.whmcs.com/images/b0x.html".
Hacker also provided mirror of the defacement " http://zone-h.org/mirror/id/21518159".
This is not the first time WHMCS being victim of hackers attack. In 2012, the infamous UGNazi hacker group break into WHMCS using Social Engineering attack.
"Our system admin team just evaluated the server and b0x.html had a timestamp dating back to 2012. At the current time it is our belief that this was the result of a previous vulnerability related to mediawiki and no defacement has taken place." WHMCS representatives told Softpedia.