WHMCS, which provides billing and customer support tech to many web hosts, was comprehensively hacked on Monday and remains offline.

Hackers tricked WHMCS's own hosting firm into handing over admin credentials to its servers. The group that carried out the hack, UGNazi, subsequently extracted the billing company's database before deleting files, essentially trashing the server and leaving services unavailable in the process. The compromised server hosted WHCMS's main website and supported customers' installations of its technology.

UGNazi also gained access to WHMCS's Twitter account, which it used to publicise a series of posts on Pastebin that contained links to locations from which the billing firm's customer records and other sensitive data might be downloaded. A total of 500,000 records, including customer credit card details, were leaked as a result of the hack.

Card information was salted and hashed, but reports allege that a decryption key to recover the details was stored in clear text in the root directory of WHCMS's compromised server and also leaked. The billing firm warned that "credit card information although encrypted in the database may be at risk". Password records, by contrast, ought to be safe but WHCMS still recommends a password refresh as a precaution.

Hacktivists justified the attack by making unsubstantiated accusations that WHMCS offered services to shady characters, via an update to WHMCS's compromised Twitter feed:

Many websites use WHMCS for scams. You ignored our warnings. We spoke louder. We are watching; and will continue to be watching. #UGNazi

WHMCS was able to restore service on Monday night within hours of the attack - although the website has fallen offline again apparently to a distributed denial of service attack. WHMCS has yet to regain control of its Twitter feed.

The billing firm posted a blog post explained how the attack took place and apologised to its customers for the inconvenience cause by an interruption in their services. A spokesman wrote:

Following an initial investigation I can report that what occurred today was the result of a social engineering attack. The person was able to impersonate myself with our web hosting company, and provide correct answers to their verification questions. And thereby gain access to our client account with the host, and ultimately change the email and then request a mailing of the access details. This means that there was no actual hacking of our server. They were ultimately given the access details. This is obviously a terrible situation, and very unfortunate, but rest assured that this was no issue or vulnerability with the WHMCS software itself. We are immediately reviewing all of our hosting arrangements, and will be migrating to a new setup at the earliest opportunity.

WHMCS added that it had reported the breach on its systems to the FBI. ®