Now that this "WannaCry" malware is out there, the world's computer systems are vulnerable to a degree they haven't been before, unless people everywhere move quickly to install Microsoft's security patches.
It said the attacks were carried out with a version of WannaCry ransomware that encrypted files and prompted a demand for money transfers to free up the system.
This is already believed to be the biggest online extortion attack ever recorded, disrupting services in nations as diverse as the U.S., Russia, Ukraine, Spain and India. "The recent attack is at an unprecedented level and will require a complex worldwide investigation to identify the culprits", it said in a statement.
As part of the efforts to combat the attack, Microsoft, whose Windows software lies at the heart of the potential hacking vulnerability, released a software update available to those affected by the attack and others that could be potential targets.
He went on to say that, as a general rule of thumb, companies should always use a "robust" security solution, keep software up to date and limit the use of browser plugins.
Who was behind the attack?
Two security firms - Kaspersky Lab and Avast - said they had identified the malicious software behind the attack in upward of 70 countries, although both said the attack has hit Russian Federation hardest. Security experts say this attack should wake up every corporate board room and legislative chamber around the globe.
By Sunday morning Home Secretary Amber Rudd, who lead the government's public response to the NHS attacks, said that all but six NHS trusts IT systems were functioning again.
Mother's Day special: Handwritten letter is the best gift say viewers
Bring her some breakfast in bed , a cup of coffee or tea, allow her to relax and spend the day doing the things she enjoys most. And says another, "I want my husband to take the kids and let me go to lunch with girlfriends- or a movie".
The attack hit Britain's health service, forcing affected hospitals to close wards and emergency rooms. Patients were asked not to come to hospitals unless it was an emergency.
Security officials in Britain urged organizations to protect themselves by updating their security software fixes, running anti-virus software and backing up data elsewhere.
Cybersecurity firm Avast identified more than 75,000 ransomware attacks in 99 countries. Ministry spokeswoman Irina Volk was quoted by the Interfax news agency Saturday as saying the problem had been "localized" with no information compromised. "The numbers are going up, I am anxious about how the numbers will continue to grow when people go to work and turn (on) their machines on Monday morning".
Dr Krishna Chinthapalli, a neurology registrar at the National Hospital for Neurology and Neurosurgery in London, had warned that an increasing number of hospitals could be shut down by ransomware attacks in an article on the vulnerability of the NHS network in the "British Medical Journal" on Wednesday, two days before the major cyber-hack. Radio Slovenia said Saturday the Revoz factory in the southeastern town of Novo Mesto stopped working Friday evening to stop the malware from spreading.
Nissan's manufacturing plant in Sunderland, northeast England, was also affected by the cyber assault though "there has been no major impact on our business", a spokesman for the Japanese carmaker said.
It said the ransomware called "WannaCry" or "WannaCrypt" encrypts the computer's hard disk drive and then spreads laterally between computers on the same local area network (LAN). Microsoft has taken the unusual step of re-releasing security updates for some older versions of its Windows platforms to counter a massive global wave of cyber ransomware attacks.
A German ticket machine, a university laboratory in Italy and a number of Spanish firms - including telecoms giant Telefonica, power firm Iberdrola and utility provider Gas Natural - are among those hit by the outbreak. Deutsche Bahn said it deployed extra staff to busy stations to provide customer information, and recommended that passengers check its website or app for information on their connections.