"Given the potential impact to customers and their businesses, we made the decision to make the security update for platforms in custom support only, Windows XP, Windows 8, and Windows Server 2003, broadly available".
Hackers exploited vulnerabilities in the most widely used operating system in the world: Microsoft Windows. Microsoft had "patched", or fixed it, in updates of recent versions of Windows since March, but many users did not apply the software fix.
"The ransomware appeared to exploit a vulnerability in Microsoft Windows that was purportedly identified by the U.S. National Security Agency for its own intelligence-gathering purposes".
The risk of being infected by ransomware can be reduced significantly by taking the usual computer security steps, such as ensuring patches and updates are installed as they are released by software firms.
"But it is important to understand that the way these attacks work means that compromises of machines and networks that have already occurred may not yet have been detected, and that existing infections from the malware can spread within networks".
The ransomware, which locked users' files and demanded payment to allow access, spread to 150 countries, including Russian Federation, the USA and China.
Some federal agencies have moved more quickly than others to stamp out Windows XP, said R. David Edelman, an Obama administration official who advised the White House on technology matters.
The most disruptive attacks were reported in Britain, where hospitals and clinics were forced to turn away patients after losing access to computers on Friday.
Europol, the European Union's policing agency, said the attack remains an "escalating threat" whose numbers "are still going up" after a brief slowdown on Friday.
MalwareTech, who wants to remain anonymous, was hailed as an "accidental hero" after registering a domain name to track the spread of this ransomware, which actually ended up halting it.
Elsewhere in Europe, the attack hit companies including Spain's Telefonica, a global broadband and telecommunications company. Finance chiefs from the Group of Seven rich countries will commit on Saturday to join forces to fight the growing threat of worldwide cyber attacks, according to a draft statement of a meeting they are holding in Italy.
One of the biggest-ever ransomware attacks continues to take computers hostage.
Ransomware is a type of malware that blocks access to a computer or its data and demands money from the user to release it. The temporary fix initially helped slow down the rate of infected computers.
"Our immediate priority as a government is to disrupt the attack, restore affected services as soon as possible, and establish who was behind it".
Ironically, large scale attacks such as these do have the effect of highlighting the threat of malware attacks and cybersecurity in general.
Ryan Kalember, senior vice president at Proofpoint Inc., said that millions of devices could be vulnerable if they have not applied security patches over the weekend.