Washington - Computers in dozens of countries - from British hospitals to Russian government ministries and French multinational companies - have been hit by what experts are calling an unprecedented mass cyber attack using ransomware.
Experts were scrambling to determine who was behind the attack, which exploited a security flaw in older versions of Microsoft's Windows operating software.
Here is what we know so far about the cyber ransom attacks:
Computers around the globe were hacked beginning on Friday using a security flaw in Microsoft's Windows XP operating system, an older version that was no longer given mainstream tech support by the US giant.
The so-called WannaCry ransomware locks access to user files and demands money -- in the form of the virtual currency Bitcoin - in order to decrypt them.
How many countries were affected?
Certainly dozens, but precise numbers vary.
The Finland-based cyber security company F-Secure said 130 000 systems in more than 100 countries had been affected. Kaspersky Lab said it had recorded incidents in 74 countries, mostly in Russia, but noted that its visibility "may be limited and incomplete."
High-profile victims include hospitals in Britain, the Spanish telecoms firm Telefonica, French carmaker Renault, US package delivery firm FedEx, Russia's interior ministry and German rail operator Deutsche Bahn.
How did the attack spread worldwide?
Experts said the ransomware programme appears to support dozens of languages, showing that the hackers wanted to corrupt networks worldwide.
The virus spread quickly because the culprits used a digital code believed to have been developed by the US National Security Agency - and subsequently leaked as part of a document dump, according to researchers at the Moscow-based computer security firm Kaspersky Lab.
F-Secure's chief research officer Mikko Hypponen said Russia and India were particularly hard-hit because many there were still using Windows XP.
Who was behind the attack?
So far, unknown. Security agencies in affected countries were scrambling to find out.
The US security firm Symantec said the attack appeared to be indiscriminate.
How can users protect their computers?
Microsoft took the unusual step of reissuing security patches first made available in March for Windows XP and other older versions of its operating system.
Kaspersky said it was seeking to develop a decryption tool "as soon as possible."
How much ransom was asked?
Victims were asked for payment of $300 in Bitcoin. Given the attack's widespread nature, even such a small sum would stack up quickly.
Payment is demanded within three days or the price is doubled, and if none is received within seven days, the files will be deleted, according to the screen message.
Experts advise users not to pay, as it would only encourage the attackers.
It was not immediately clear how much, if any, ransom had been paid in connection with the widespread attack.
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