A ransomware attack had an impact on tens of thousands of computers around the world Friday. A ransomware program called WannaCry infects computers and then locks them, demanding $300 in bitcoins to unlock them. The attack infected computers in 99 countries including Britain where the National Health Service was seriously impacted. From the BBC:

Some hospitals and GPs cannot access patient data, after their computers were locked by a malicious program demanding a payment worth £230.

There is no evidence patient data has been compromised, NHS Digital has said.

The BBC understands up to 39 NHS organisations and some GP practices have been affected.

In a separate BBC story, one NHS worker claimed patients would “almost certainly suffer and die” as a result of the problems created by the ransomware. Other impacted systems included the Russian Foreign Ministry, some German rail stations, and a Spanish telecom firm.

The NY Times reports the ransomware used in this international attack appears to be one that was revealed after a hacker group published information taken from the NSA a few months ago.

The hacking tool used on Friday was ransomware, a kind of malware that encrypts data, locks out the user and demands a ransom to release it. Security experts say the tool exploited a vulnerability in Microsoft systems that was discovered and developed by the National Security Agency of the United States.

The ransomware, known as WannaCry, was leaked by a group calling itself the Shadow Brokers, which has been dumping stolen N.S.A. hacking tools online since last year. Microsoft rolled out a patch for the vulnerability in March, but hackers apparently took advantage of the fact that vulnerable targets — particularly hospitals — had yet to update their systems or had ignored advisories from Microsoft to do so.

Lior Div of cyber security firm Cybereason tells McClatchy, “I believe this is the largest (global attack) in the effect it is having.” Similarly, Chris Camacho of security firm Flashpoint tells the NY Times, “When people ask what keeps you up at night, it’s this.”