Spyware crafted by a sophisticated group of hackers-for-hire took advantage of a flaw in the popular WhatsApp communications program to remotely hijack phones, the company said late Monday.

The Financial Times identified the actor as Israel’s NSO Group, with WhatsApp describing the hackers as “a private company that has been known to work with governments to deliver spyware.” A spokesman for the Facebook subsidiary later said: “We’re certainly not refuting any of the coverage you’ve seen.”

The malware was able to penetrate phones through missed calls alone via the app’s voice calling function, the spokesman said. An unknown number of people were infected with the malware, which the company discovered in early May, said the spokesman, who called the hack “a very scary vulnerability.”

Read more at NBC News.