Computers running on Windows 7 accounted for the largest proportion of machines infected with the WannaCry ransomware, according to recent statistics by security firms. According to the BBC, more than 97% of the infections Kaspersky Lab has seen were on machines running on Windows 7, while security ratings firm BitSight said 66% of the infections it saw were machines running on it.

Windows 7, WannaCry, Windows XP
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64-bit Windows 7 was the worst hit

The WannaCry outbreak started by infecting some vulnerable machines and has now infected more than 200,000 computers around the world. WannaCry started spreading in mid-May and has now become one of the largest cyber-attacks in history.

Figures from Kaspersky Lab showed that Windows 7, particularly the 64-bit edition, was the worst affected and was responsible for the fast and wide spread of the cyber-attack, reports the BBC. The 64-bit edition is used widely by large organizations, according to figures from Kaspersky.

Several organizations appear to have been caught because they failed to apply a patch that was issued by Microsoft in March. The patch blocked the vulnerability exploited by WannaCry. German rail firm Deutsche Bahn, Spanish telecom firm Telefonica, logistics firm FedEx, French automaker Renault, 61 NHS organizations, and the interior ministry of Russia were all caught by the WannaCry ransomware.

Security experts also found that the ransomware spread by seeking out vulnerable machines on the net by itself, the BBC adds. There have been no reports yet of anyone who paid and had some or all of their data restored by the hackers behind the attack. The WannaCry ransomware demanded a payment of about $300 or £230 in bitcoins after encrypting files. Over $99,448 (296 payments) have been paid in bitcoins to the gang behind the attack, according to the BBC.

Windows XP WannaCry hit count “insignificant”

In the U.K., NHS trusts were the most-hit organizations. Many systems failed to upgrade from Windows XP, the version of Microsoft’s OS that debuted in 2001, and this was considered one of the main reasons they were hit hard by the rasomware.

However, Costin Raiu from Kaspersky Lab revealed that infections of Windows XP by the WannaCry worm were not that significant (less than one in a thousand).

Raiu, director of Kaspersky’s global research and analysis team, tweeted: “#WannaCry infection distribution by the Windows version. Worst hit – Windows 7 x64. The Windows XP count is insignificant.”

2008 R2 Server clients accounted for more than 1% of infections.

Raiu said that the Windows XP computers that were compromised were likely infected manually for testing purposes. The researchers believe that Windows XP was largely unaffected by the ransomware because the PCs crashed before the virus could take hold.

Windows 7 is the most popular version of the OS, and it runs on almost four times as many systems as Windows 10 worldwide. So it makes sense that Windows 7 systems were the hardest hit, considering that the more recent Windows versions were not vulnerable to WannaCry. However, still there are questions about the role played by Windows XP in the spread of the ransomware.