Ransomware attack: What you need to know

"A number of countries and organizations have been affected". CNN has not independently confirmed that number.

The IT systems of NHS sites across the country on Friday appear to have been simultaneously hit, with a pop-up message demanding a ransom in exchange for access to the PCs, the Guardian reported. Now, the malicious software WanaCrypt0r 2.0 (aka WCry) is being used to carry out one of the biggest ransomware attacks of its kind.

Britain's national health service fell victim, its hospitals forced to close wards and emergency rooms and turn away patients. "The investigation is at an early stage but we believe the malware variant is Wanna Decryptor", officials at NHS Digital said in a statement.

"The end of the emergency doesn't mean the end of surveillance or assistance" to affected people, said Adeilson Cavalcante, secretary for health surveillance at Brazil's Health Ministry.

Hospitals in Scotland or Wales did not appear affected.

Several companies in Spain were also crippled by ransomware attacks.

In a statement, Kaspersky Labs said it was "trying to determine whether it is possible to decrypt data locked in the attack - with the aim of developing a decryption tool as soon as possible.".

Spain's Telefonica, a global broadband and telecommunications company, was among the companies hit.

Russian Federation appeared to be the hardest hit, according to security experts, with the country's Interior Ministry confirming it was struck.

The interior ministry said on its website that around 1,000 computers had been infected but it had localised the virus.

Security experts are still trying to get their arms around the problem.

Alan Woodward, visiting professor of computing at the University of Surrey, said there was evidence the ransomware was spreading using a Microsoft flaw exposed in a recent leak of information from US intelligence agencies.

FedEx reported that its Windows computers were "experiencing interference" from malware, but wouldn't say if it had been hit by ransomware.

The malicious software behind the onslaught appeared to exploit a vulnerability in Microsoft Windows that was supposedly identified by the National Security Agency for its own intelligence-gathering purposes and was later leaked to the internet.

He added, "It is a terrible lesson about why using supported software, and keeping that software updated, is so important".

According to it, ransomware spreads easily when it encounters unpatched or outdated software.

"Our society increasingly relies on interconnected systems to deliver key services such as health", he said.

"Of course it is important that we have set up the National Cyber Security Centre and they are able to work with the NHS organisations concerned and to ensure that they are supported and patient safety is protected". It said the attack did not specifically target the NHS. Routine appointments had been cancelled and ambulances were being diverted to neighbouring hospitals.

"We are very aware that attacks on critical services such as the NHS have a massive impact on individuals and their families, and we are doing everything in our power to help them restore these vital services", said Ciaran Martin, the body's chief executive.

Patrick Ward, a 47-year-old sales director, said his heart operation, which was scheduled for Friday, was cancelled at St. Bartholomew's Hospital in London. When he tried to access patient files on a computer, he couldn't find them - even though he knew they were there. "They told us there was a problem".

Some cyber security experts and privacy advocates said the massive attack reflected a flawed approach by the U.S. to dedicate more cyber resources to offence rather than defence, a practice they argued makes the internet less secure. The program recommends certain patients for treatment with specialists and has a two-week availability window before the treatment is canceled.

At Royal London Hospital, doctors who wanted to access patient scans to use as part of lessons for medical students could not do so, he said.