Cyber security experts are warning a new computer virus, Adylkuzz, looks set to infect more computers than last week's WannaCry attack.
The new threat actually predates WannaCry, with reports of infections as early as late April, but the way it works behind the scenes means it's stayed under the radar until now.
Instead of locking up your files and demanding a ransom like WannaCry, Adylkuzz works by quietly installing itself in the background and running software to mine a digital currency called Monero, similar to Bitcoin.
And it's working. One expert says Adylkuzz would be earning its creators a five-figure sum every day, growing as the worm spreads.
"As disruptive as WannaCry has been to vulnerable organisations, more deadly attacks that don't announce their presence, like the cryptocurrency miner Adylkuzz, go undetected," Brian Vecci from Varonis told CNET.
The only clue the average computer user might have that their machine has been infected is it might appear to be running sluggishly.
Adylkuzz uses the same flaw in Microsoft Windows that WannaCry exploited. Users who have updated their operating systems since March should be protected.
Microsoft even issued a security patch for Windows XP, a 16-year-old operating system it no longer maintains, but remains popular on low-powered, older computers.
The software both hacks use, known as EternalBlue, is believed to have been developed by the US National Security Agency. It was leaked by a hacker group in mid-April.
The best way to protect yourself from hacks like WannaCry and Adylkuzz remains not opening emails and attachments from unknown sources, and keeping your computer's operating system up to date.
Variants on both WannaCry and Adylkuzz have been detected on the internet, 'black-hat' hackers trying to stay a step ahead of their do-gooder 'white-hat' counterparts.