Security firm Kaspersky Lab has recorded more than 45,000 ransomware attacks in the last 10 hours, most of which were targeting Russia. The ransomware, “WannaCry,” has been spreading through a Microsoft Windows exploit called “EternalBlue,” which Microsoft released a patch for in March, reports CNN.  

Users of infected machines have six hours to pay the ransom, and the cost goes up every few hours, says Kurt Baumgartner, principal security researcher at Kapersky Lab.

The ransomware was used to target 16 National Health Service (NHS) organizations in the UK and Spanish telecom company Telefonica.

The virus can spread to other computers on the same wireless network as an infected computer, and experts are strongly recommending that users patch their systems as soon as possible.