(NEXSTAR) — Apple released an emergency software update Monday after it discovered a vulnerability that would allow hackers to infect iPhones, iPads, Apple computers and watches without a user even clicking a malicious link. The spyware detected could open up an Apple device to data theft and eavesdropping.

The flaw was detected by researchers at the University of Toronto’s Citizen Lab, who noticed spyware from the world’s most infamous hacker-for-hire firm, NSO Group, had infected the iPhone of a Saudi activist.

It was the first time a so-called “zero-click” exploit had been caught and analyzed, said the researchers, who found the malicious code on Sept. 7 and immediately alerted Apple. They said they had high confidence the Israeli company NSO Group was behind the attack, adding that the targeted activist asked to remain anonymous.

The flaw discovered by Citizen Lab affected all of Apple’s operating systems, the researchers said. Although security experts say that average iPhone, iPad and Mac user generally need not worry — such attacks tend to be highly targeted.

Still, Apple said in a blog post it was issuing a security update for iPhones and iPads because a “maliciously crafted” PDF file could lead to them being hacked. It said it was aware that the issue may have been exploited and cited Citizen Lab.

Users are encouraged to check if they have automatic software updates enabled in their devices’ settings. If not, they should consider running the update manually.

“Do you own an Apple product? Update it today,” John Scott-Railton, a researcher at Citizen Lab, told the New York Times.

Malicious image files were transmitted to the activist’s phone via the iMessage instant-messaging app before it was hacked with NSO’s Pegasus spyware, which then opens a phone to eavesdropping and remote data theft, Marczak said. It was discovered during a second examination of the phone, which forensics showed had been infected in March. He said the malicious file causes devices to crash.

NSO Group did not immediately respond to an email seeking comment.

Researcher John Scott-Railton said the news highlights the importance of securing popular messaging apps against such attacks. “Chat apps are increasingly becoming a major way that nation-states and mercenary hackers are gaining access to phones,” he said. “And it’s why it’s so important that companies focus on making sure that they are as locked down as possible.”

The researchers said the discovery also exposes — again — that NSO’s business model involves selling spyware to governments that will abuse it, not just to law enforcement officials chasing cybercriminals and terrorists, as NSO claims.

“If Pegasus was only being used against criminals and terrorists, we never would have found this stuff,” said Citizen Lab researcher Bill Marczak

Facebook’s WhatsApp was also allegedly targeted by an NSO zero-click exploit In October 2019, Facebook sued NSO in U.S. federal court for allegedly targeting some 1,400 users of the encrypted messaging service with spyware.

In July, a global media consortium published a damning report on how clients of NSO Group have been spying for years on journalists, human rights activists, political dissidents — and people close to them, with the hacker-for-hire group directly involved in the targeting.