Apple's newest iPhone operating system is set to land on Monday, but iPhone users should still use the weekend to update their phones before it drops, in order to install a critical security patch.

On Sept. 13, Apple released security updates for its iPhones, iPads, Apple Watches and Mac computers that close a vulnerability reportedly exploited by invasive spyware built by NSO Group, an Israeli security company. 

A security note for iOS 14.8 and iPadOS 14.8 said some harmful PDFs could take advantage of Apple's popular operating systems. "Processing a maliciously crafted PDF may lead to arbitrary code execution," the note read. "Apple is aware of a report that this issue may have been actively exploited." 

Apple also released WatchOS 7.6.2, MacOS Big Sur 11.6 and a security update for MacOS Catalina to address the vulnerability. The patches came a day before Apple's splashy fall event that introduced new versions of iPhones and iPads, along with the latest Apple Watch. The company used the event to say that iOS 15 and iPadOS 15 would generally be available for free download starting Sept. 20. 

The security fix, earlier reported by The New York Times, stems from research done by a public interest cybersecurity group called Citizen Lab that found a Saudi activist's phone had been infected with Pegasus, NSO's best-known product. According to Citizen Lab, the zero-day, zero-click exploit against iMessage, which it nicknamed ForcedEntry, targets Apple's image rendering library and was effective against the company's iPhones, laptops and Apple Watches. 

Read more: Check if your iPhone is infected with Pegasus spyware with this free tool

Citizen Lab, based at the University of Toronto, says it determined NSO used the vulnerability to remotely infect devices with its Pegasus spyware, adding that it believes the exploit has been in use since at least February. It urged all Apple users to immediately update their operating systems.

"Ubiquitous chat apps have become a major target for the most sophisticated threat actors, including nation state espionage operations and the mercenary spyware companies that service them," Citizen Lab said in a report. "As presently engineered, many chat apps have become an irresistible soft target."

Apple touted its security measures during its fall rollout of devices, which is one of the company's most important annual events. Saying that privacy is "built in from the beginning," Apple added that iOS 15 will block trackers and prevent monitoring of email, among other safety provisions.

Apple thanked Citizen Lab for providing a sample of the exploit, which the iPhone maker said wasn't a threat to most of its users.

"Attacks like the ones described are highly sophisticated, cost millions of dollars to develop, often have a short shelf life, and are used to target specific individuals," Ivan Krstić, who runs Apple's security engineering and architecture operations, said in a statement. "While that means they are not a threat to the overwhelming majority of our users, we continue to work tirelessly to defend all our customers, and we are constantly adding new protections for their devices and data."

In July, researchers found evidence of attempted or successful installations of Pegasus on 37 phones of activists, journalists and businesspeople. All but three of the devices were iPhones. Some of the people appear to have been targets of secret surveillance through Pegasus, software that's supposed to be used to pursue criminals and terrorists. The spyware is reportedly capable of accessing and recording texts, videos, photos and web activity as well as passively recording and scraping passwords on a device. 

NSO released a statement that didn't directly address Apple's update but said it "will continue to provide intelligence and law enforcement agencies around the world with life saving technologies to fight terror and crime."

The company, which licenses surveillance software to government agencies, says its Pegasus software helps authorities combat criminals and terrorists who take advantage of encryption technology to go "dark." Pegasus runs secretly on smartphones, providing insight into what their owners are doing. Other companies provide similar software.

CEO Shalev Hulio co-founded the company in 2010. In addition to Pegasus, NSO offers other tools that locate where a phone is being used, defend against drones and mine law enforcement data to spot patterns.

NSO has been implicated in other hacks, including the high-profile hack of Amazon founder Jeff Bezos in 2018. In the same year, a Saudi dissident sued the company for its alleged role in hacking a device belonging to journalist Jamal Khashoggi, who was murdered inside the Saudi embassy in Turkey.