The WannaCry ransomware attack mainly took advantage of a flaw in Windows code and mostly hit Windows 7 systems.

The attack hit approximately 300,000 systems around the world in 150 countries and asked for a ransom of $300. 98% of the affected systems were running Windows 7. About 1.5 percent of infections hit 2008 R2 Server clients, while Windows XP infections were "insignificant," according to Costin Raiu, director of research for Kaspersky Lab.

Microsoft released Windows 7 back in 2009 and it is still the most popular version still running. A patch was released in March to protect against the flaw, but many people and organizations did not apply it.

"Those who are running our free antivirus software or have Windows Update enabled are protected," said a Microsoft spokesperson.

Source: CNet