Dell says skilled attackers have made a staggering $620,000 in the Dogecoin crypto-currency by exploiting vulnerable Synology network attached storage (NAS) boxes.

The clever hackers pulled off the largest heist of its kind by planting mining gear on the NAS boxes to borrow their computational might - many NAS now boast grunty multi-core CPUs - to seek out coins.

Scores of unpatched Synology boxes were infected and continued to mine Dogecoins for the assailants.

It took just two months for the attackers to accrue 500 million coins worth US$620,000, Dell Secureworks researcher Pat Litke (@LitkeP) wrote in a post.

"To date, this incident is the single most profitable, illegitimate mining operation," Litke wrote. "

"This conclusion is based in part on prior investigations and research done by [Secureworks], as well as further searching of the internet."

Secureworks' analysis suggests an experienced hacker, likely of German descent and using the alias Folio, was behind the Dogecoin mining spree.

In a brazen stunt, Folio stored the mining gear in a folder labelled PWNED, a move that could have foiled the plans earlier should forum warnings have been reported by the press.

Users first reported the attacks on web forums in February after noticing the folder and a drop in NAS performance due to the resource-sucking mining operation.

While the coin mining was not itself illegal, the act of hacking the NAS boxes and pilfering their compute resources was.

Yet users remained vulnerable to a string of more dangerous attacks due to the five-month-long exposure of Synology NASes to very serious vulnerabilities within the Linux-based DiskStation Manager. These included unauthenticated remote file downloading and a command-injection flaw.

Vulnerable servers could be found using only an advanced Google search (Google dorking) with keywords which could drop attackers right into exposed Synology NASes.

"Back in October of 2013, simply Googling for 'site:Synology.me' resulted in excess of one million results ... by going to 'something.Synology.me', the user is routed directly to their NAS," Litke said.

Awareness of the flaws grew and by March this year the SANS internet storm centre reported a spike in scans against port 5000 which was the default listener for Synology NASes.

The Reg approached Synology for comment, but the company had not replied as this article went to press. ®

Updated to add

A Synology spokesman has got in touch to say: "In September, under DSM [DiskStation Manager] 4.3 and 4.2, we were alerted to the Bitcoin and Dogecoin-mining malware. On September 23, our developers squashed the bug for those who updated their DSM.

"In February, we released a patch for DSM 5.0beta to resolve the issue. In February we also started getting a lot of support tickets for mining that happened on units that had not updated their DSM. The result: we made auto-updates the default behavior for the OS. We’ve been updating regularly, because we are now targets."