According to cybersecurity firm Avast, hackers exploited a known vulnerability in Microsoft Windows to lock scores of computers and demand a ransom of $300 worth of bitcoin, a digital currency that is hard to track, to restore access.
Edward Snowden, the former NSA contractor who leaked evidence of the agency's data collection program in 2013, has spoken out on Twitter to criticize the NSA for building this "dangerous attack tool".
"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".
"The investigation is at an early stage but we believe the malware variant is Wanna Decryptor", officials at NHS Digital said Friday in a statement.
"We see all the finger-pointing at the usual suspects, saying it's probably people in Russian Federation or China, but, to quote Sherlock Holmes, it's not really a good idea to guess without the evidence", Gazeley said. "As a result, we've got this terrible situation that NHS workers are facing today".
Hospitals in London, north-west England and central England have all been affected, according to the BBC. Deutsche Bahn said it was working to rectify the problem.
Krishna Chinthapalli, a doctor at Britain's National Hospital for Neurology & Neurosurgery who wrote a paper on cybersecurity for the British Medical Journal, said many British hospitals use Windows XP software, introduced in 2001.
Malware is a general term that refers to software that's harmful to your computer, said John Villasenor, a professor at the University of California, Los Angeles. "This is not targeted at the NHS".
G7 finance ministers meeting in Italy discussed the attacks and were expected to commit to stepping up worldwide cooperation against a growing threat to their economies.
Cybersecurity firm Avast said it tracked more than 75,000 ransomware attacks in 99 countries Friday. CNN has not independently confirmed that number.
Europol, the European Union's police agency, said the onslaught was at "an unprecedented level and will require a complex global investigation to identify the culprits".
Speaking ahead of the Cobra meeting in Whitehall at 2.30pm, she told Sky News: "It is disappointing that they have been running Windows XP - I know that the Secretary of State for Health has instructed them not to and most have moved off it".
Britain's National Cyber Security Centre and its National Crime Agency were looking into the United Kingdom incidents, which disrupted care at National Health Service facilities, forcing ambulances to divert and hospitals to postpone operations.
The East and North Hertfordshire NHS Trust was "experiencing significant problems with our telephone network", it said in an online statement.
In Brazil, the social security system had to disconnect its computers and cancel public access. Shadow Brokers said that they obtained it from a secret NSA server.
"For so many organizations in the same day to be hit, this is unprecedented", he said.
"The large-scale cyberattack on our NHS today is a huge wake-up call", said Jamie Graves, CEO of cybersecurity company ZoneFox. Patients were asked not to go to hospitals unless it was an emergency and even some key services like chemotherapy were cancelled. "First, there is no guarantee the criminals will release your data", Woodward said. Computers already affected will not be helped by the solution. "The only sensible way to tackle it is to "pull the plug" so that it can't spread any more until you can isolate the affected machines and work out a remediation plan".
British media had reported a year ago that most public health organisations were using an outdated version of Microsoft Windows that was not equipped with security updates.
"It is a terrible lesson about why using supported software, and keeping that software updated, is so important", he said.
Security experts said the attack appeared to be caused by a self-replicating piece of software that enters companies when employees click on email attachments, then spreads quickly internally from computer to computer when employees share documents.
This is Microsoft's answer to Chrome OS, and the new streamlined version of Windows 10 is being targeted mainly at students. Would you rather have a Surface Pro 5? Unlike other Surface models, it does not run the traditional version of Windows.
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