Transcript for Unprecedented global cyberattack spread ransomware to five continents
I'm Tom llamas. The cyberattack rolling across the globe. Targeting governments, hospitals and major companies. Security experts racing to fight back. Hackers striking around 100 look at that. Countries including right here in the U.S. Locking up tens of thousands of computers, demanding ransom to free them up. Tonight the computer wiz who may have stumbled on a way to disable the attack and the international hunt to catch the culprits before they strike again. ABC's Brian Ross starts us off. Reporter: The tally of victims tonight includes FedEx and the United States and Russia and railroads and Germany and factories and phone companies across Europe and hospitals in great Britain where surgeries were cancelled. Ambulances turned away and lives put at risk. It's a heinous crime. This hacking because they're putting people's lives at risk. Reporter: The international cyberattack titled want to cry was announced when tens of thousands of targets simultaneously received this message that their computers were frozen. Oops, your important files are encrypted followed by to pay a ransom of $300 or more to keep their files from being erased permanently. The reason this is hitting so many computers at once is they discovered a vulnerability in the most popular operating system in the world in Microsoft windows and they're taking advantage of it. It's one that Microsoft delivered a solution for but a lot of people haven't used it. Reporter: As the attacks spread to more than a 100 countries the damage was contained today for the moment. When a computer programmer in great Britain stumbled on to a kill switch based on the work of this computer programmer in Indiana who talked with us by Skype. Once the kill switch was activated the malware would do nothing. It would not carry out its infection or take for ransom all the files on that computer. Reporter: Law enforcement and intelligence authorities around the world led by Britain's new computer security squad where we were recently given rare access are working to track down whoever was responsible. With Russian organized crime considered a leading suspect. People trying to steal money. The digital world gives people an opportunity to do that. Reporter: Ironically experts believe the attack was carried out with the help of a program first developed by U.S. Intelligence to compromise the computers of terrorists and foreign adversaries. They lost it. Somebody stole the information, published it on the internet and now is being used against victims in the United States and elsewhere. And Brian, we heard your report about the kill switch but the problem isn't over? Reporter: That's right. U.S. Officials believe American companies dodged the bullet in this case because most had upgraded their computers to close the vulnerability and the quick activation of the kill switch. Also helped. Officials say they believe the hackers are preparing a second wave of attacks without a kill switch that could come any time soon Tom. What's advice from our experts to our viewers to protect themselves? Reporter: Anybody who uses the Microsoft system should download any upgrades including the patch that covers this vulnerabili vulnerability. Tom.
This transcript has been automatically generated and may not be 100% accurate.