Britain's home secretary says about one in five National Health Service trusts have been hit by the global ransomware cyberattack, and that all but six are now back to normal.
A young cybersecurity researcher has been credited with helping to halt the ransomware's spread by accidentally activating a so-called "kill switch" in the malicious software.
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Private security firms identified the ransomware as a new variant of "WannaCry" that had the ability to automatically spread across large networks by exploiting a known bug in Microsoft's Windows operating system.
NHS Digital said the attack "was not specifically targeted at the NHS and is affecting organizations from across a range of sectors".
The so-called "Wannacry" virus notably attacks Windows XP, which in principle has not been supported by Microsoft since 2014.
In addition, he said, the following action could be taken immediately: remove the system from network, do not use flash/pen drive/external drives on the system to copy files to other systems, format the system completely, and contact NITDA's Computer Emergency Readiness and Response Team for assistance.
The attack itself was conducted using a revised version of a known ransomware strain "WannaCry" that exploits a specific Windows bug, security experts claim, adding that Taiwan, Ukraine, and Russian Federation were among the hardest-hit targets of the attack. The national railway said it was attacked but operations were unaffected.
The malicious software behind the onslaught appeared to exploit a vulnerability in Microsoft Windows that was supposedly identified by the US National Security Agency for its own intelligence-gathering purposes and was later leaked to the internet.
Although Microsoft released a security patch for the flaw earlier this year, many systems have yet to be updated, researchers said.
"It had a countdown clock ticking down, stating that all data would be deleted unless a payment was received within that timeframe", he said.
On Wednesday, hackers disrupted the websites of several French media companies and aerospace giant Airbus.The hack happened four weeks before a British general election in which national security and the management of the state-run National Health Service are important issues.
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".
"It is a disgusting lesson about why using supported software, and keeping that software updated, is so important", he said.
Deutsche Bahn says that departure and arrival display screens at its stations were hit Friday night by the attack. The virus is usually covertly installed on to computers by hiding within emails containing links, which users are tricked into opening.
The effects of the attack on Turkey are unclear. "This is not targeted at the NHS, it's an worldwide attack and a number of countries and organisations have been affected", British Prime Minister Theresa May said.
"In fact, 97% of the NHS trusts and hospitals. are working as normal".
She added: "Where the patient data has been properly backed up, which has been in most cases, work can continue as normal because the patient data can be downloaded and people can continue with their work".
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