SAN FRANCISCO - Researchers at Citizen Lab found that NSO Group, an Israeli spyware company, had infected Apple products without so much as a click.

Apple on Monday issued emergency software updates for a critical vulnerability in its products after security researchers uncovered a flaw that allows highly invasive spyware from Israel’s NSO Group to infect anyone’s iPhone, iWatch or Mac computer without so much as a click.

Apple’s security team has been working around the clock to develop a fix since Tuesday, after researchers at Citizen Lab, a cybersecurity watchdog organization at the University of Toronto, discovered that a Saudi activist’s iPhone had been infected with spyware from NSO Group.

The spyware, called Pegasus, used a novel method to invisibly infect an Apple device without the victim’s knowledge for as long as six months. Known as a “zero click remote exploit,” it is considered the Holy Grail of surveillance because it allows governments, mercenaries and criminals to secretly break into a victim’s device without tipping them off.

Using the zero-click infection method, Pegasus can turn on a user’s camera and microphone, record their messages, texts, emails, calls — even those sent via encrypted messaging and phone apps like Signal — and send it back to NSO’s clients at governments around the world.

“This spyware can do everything an iPhone user can do on their device and more,” said John-Scott Railton, a senior researcher at Citizen Lab, who teamed up with Bill Marczak, a senior research fellow at Citizen Lab, on the finding.

In the past, victims only learned their devices were infected by spyware after receiving a suspicious link texted to their phone or email. But NSO Group’s zero-click capability gives the victim no such prompt, and enables full access to a person’s digital life. These capabilities can fetch millions of dollars on the underground market for hacking tools.

An Apple spokesman confirmed Citizen Lab’s assessment and said the company planned to add spyware barriers to its next iOS 15 software update, expected later this year.

NSO Group has long drawn controversy. The company has said it sells its spyware to only governments that meet strict human rights standards. But over the past six years, its Pegasus spyware has turned up on the phones of activists, dissidents, lawyers, doctors, nutritionists and even children in countries like Saudi Arabia, the United Arab Emirates and Mexico.

In July, NSO Group became the subject of intense media scrutiny after Amnesty International, the human rights watchdog, and Forbidden Stories, a group that focuses on free speech, teamed up with a consortium of media organizations on “The Pegasus Project” to publish a list they said contained some 50,000 people — including hundreds of journalists, government leaders, dissidents and activists — selected as targets by NSO’s clients.

The consortium did not disclose how it obtained the list and it was unclear whether the list was aspirational or whether the people were actually targeted with NSO spyware.