Cyber criminals could be using your computer to cash in while it sits idle at home.
Research by cyber security providers Symantec has found New Zealand ranks 29th globally as a destination for crypto-miners.
Symantec technology strategist Mark Shaw says most people don't even know their computers are infected.
"These hackers are infecting people's machines and using the processing power to generate digital currency. But instead of you receiving the benefits of all those resources that are consumed… it's going straight back to the attackers."
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A key sign of infection is computers running much slower while completing tasks like opening documents.
"Let's say you're opening up your Office application and it's taking three or four times longer - there may well be something else taking place on that machine… A general slowdown on your machine is something to consider."
The number of attempted cryptocurrency-related hacks globally has risen by 8500 percent in three months, Symantec says. They followed a rapid rise in the value of currencies like bitcoin in the latter half of 2017.
"Make sure you've got good security software in place."
Symantec's research also showed the average amount demanded by ransomware criminals more than halved to €522 (NZ$890). Ransomware locks up your computer until you pay the hackers money.
Despite a number of high-profile attacks like Wannacry and Petya, Symantec says there was less innovation in ransomware in 2016, suggesting criminals are focusing their efforts elsewhere - on cryptocurrencies, most likely.
Mobile malware is on the rise, with new variants increasing by 54 percent. An average 24,000 malicious mobile apps were blocked every day, Symantec says.
A big problem is that only 20 percent of Android devices typically run on the latest version of the software. Unlike Apple iPhones - which are all made and updated by a single company - each Android device is reliant on its manufacturer for updates, and there are dozens of different manufacturers.
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