Although a patch to protect against attacks such as Friday's had been released by Microsoft (MSFT), many computer systems hadn't been upgraded.
In the United Kingdom, the bug locked up IT systems and phone lines at National Health Service (NHS) hospitals, causing officials to discourage people from visiting unless they need emergency treatment.
Among the other nations hit by the attack, dubbed "Wannacry", are Spain, Russia, Ukraine and India, reported the securelist website. They can also use a firewall to block unrecognized internet traffic from accessing the networking ports the Server Message Block uses.
The government has said there are no reports of any Irish companies caught up in the attack so far. Cyber security experts say the brunt of the attacks were felt in Russian, including the country's largest mobile phone company.
Cyberint, subordinated to the Romanian Intelligence Service, said Friday it thwarted a cyberattack to a government institution, without saying when it occurred, following notification from North Atlantic Treaty Organisation and the Romanian foreign intelligence agency.
However, the WannaCrypt ransomware worm has been found to be spreading by targeting out-of-date systems, prompting Microsoft to now make the security update available for all platforms, including those receiving custom support only - Windows XP, Windows 8, and Windows Server 2003.
"The intelligence community should develop strong procedures that when such tools leak, they immediately give relevant information to software developers and security vendors so protections can be developed before attacks are seen in the wild", Bambenek said.
On Friday, a security researcher inadvertently created a "kill switch" to help stop the spread of this ransomware.
It remained unclear how many organizations had already lost control of their data to the malicious software - and researchers warned that copycat attacks could follow.
The software used in the latest attacks is called WannaCry, or Wanna Decryptor, and exploits a vulnerability in the Windows operating system. There are now signs that the ransomware attack has subsided thanks to a kill switch, discovered by a 22-year-old in the UK.
Mikko Hypponen, its chief research officer, calls it "the biggest ransomware outbreak in history".
Kalember says it is spreading rapidly, making it hard to identify "patient zero" and attribute the attack to a particular hacker group.
The Spanish government said several companies had been targeted in ransomware cyberattack that affected the Windows operating system of employees' computers.
Earlier, a number of hospitals across England were forced to divert emergency patients after being hit by a suspected cyber attack.
State agencies and major companies around the world were left reeling by the attacks which blocked access to files and demanded ransom money, forcing them to shut down their computer systems.
Well, the latest massive global cyberattack overnight may give you the best reason of all to hit "update" when prompted.
Pictures posted on social media showed screens of NHS computers with images demanding payment of $300 (AU$409) in Bitcoin, saying: "Oops, your files have been encrypted!"
"But our immediate priority as a government is to disrupt the attack, restore affected services as soon as possible, and establish who was behind it so we can bring them to justice".