Investigators discovered that a company called the NSO Group, an Israeli outfit that sells software that invisibly tracks a target's mobile phone, was responsible for the intrusions. The NSO Group's software can read text messages and emails and track calls and contacts. It can even record sounds, collect passwords and trace the whereabouts of the phone user.

In response, Apple on has released a patched version of its mobile software, iOS 9.3.5. Users can get the patch through a normal software update.

Apple fixed the holes 10 days after a tip from two researchers — Bill Marczak and John Scott Railton, at Citizen Lab at the University of Toronto's Munk School of Global Affairs — and Lookout, a San Francisco mobile security company.

"We advise all of our customers to always download the latest version of iOS to protect themselves against potential security exploits," said Fred Sainz, a company spokesman.

In interviews and manuals, the NSO Group's executives have long boasted that their spyware worked like a "ghost," tracking the moves and keystrokes of its targets, without leaving a trace. But until this month, it was not clear how exactly the group was monitoring its targets, or who exactly it was monitoring.

A clearer picture began to emerge on August 10, when Ahmed Mansoor, a prominent human rights activist in the United Arab Emirates, who has been tracked by surveillance software several times, began receiving suspicious text messages. The messages purported to contain information about the torture of UAE citizens.

Mansoor passed the messages to researchers at the Citizen Lab, who confirmed they were an attempt to track him through his iPhone.

This latest effort was far more sophisticated than what was found aimed at his devices before. The researchers found it was connecting to 200 servers, several of them registered to the NSO Group. Strewn throughout the spyware code were references to Pegasus, the name of an NSO Group spyware product.

Citizen Lab brought in Lookout to help examine the code. Together, they discovered that the spyware relied on three previously unknown iOS vulnerabilities, called "zero days" because Apple didn't know about them and had zero days to patch them.

In many cases, the NSO Group had designed its tools to impersonate those of the Red Cross, Facebook, Federal Express, CNN, Al-Jazeera, Google and even the Pokemon Company to gain the trust of its targets, according to the researchers.

"NSO Group was very professional, and very effective in staying silent," said Mike Murray, a vice president at Lookout.

In recent years, zero day flaws have been traded among hackers, brokers, companies like the NSO Group, and spy agencies and law enforcement networks eager for ways to break into devices.

Flaws in Apple's iOS software are sold at a premium. Last year, a similar zero-day exploit in Apple's iOS software was sold toZerodium, a Washington buyer and seller of zero-days, for $US1 million.

Earlier this year, James Comey, the director of the FBI, announced that his agency had paid hackers who found a way for the FBI to crack into an iPhone used by one of the shooters in last year's mass killings in San Bernardino, California. Neither the hackers nor the FBI have told Apple how this was accomplished.

Apple's software update patches the NSO Group's exploits, but it is unclear whether the company has patched the vulnerabilities used by the FBI to crack into its iPhone. Apple recently began a "bug bounty" program to pay hackers who report vulnerabilities in its systems.

Among the other NSO targets, besides Mansoor, were Rafael Cabrera, a Mexican journalist, who broke a story on conflicts of interest among Mexico's ruling family. In several cases, NSO Group's tools had been crafted to target users in Yemen, Turkey, Mozambique, Mexico, Kenya and the UAE.

Zamir Dahbash, an NSO Group spokesman, said in an email, "The company sells only to authorised governmental agencies, and fully complies with strict export control laws and regulations."

Dahbash added that NSO Group does not operate any of its systems and requires that its customers use its products in a "lawful manner.""Specifically," he said, "the products may only be used for the prevention and investigation of crimes."

He would not say if the software is used by government agencies in the UAE or Mexico.

In 2014, NSO Group sold a majority stake to San Francisco-based private equity firm Francisco Partners Management for $US120 million. Francisco Partners declined to comment.

Mansoor said in an interview that the discoveries were a sad reminder that no matter what he does to protect his devices and digital security, he will continue to be a target for companies that provide this sort of spying technology.

"I guess I am their regular customer," he said. "I am the guinea pig."

Bill Marczak, the researcher at Citizen Lab who has been helping Mansoor protect his digital security, said that the surveillance experienced by Mansoor is likely to expand.

"The targeting of these activists and dissidents is a taste of what's to come," Marczak said. "What they're facing today will be faced by ordinary users tomorrow."

 – The New York Times