SAN FRANCISCO — One of the world’s most evasive digital arms dealers is believed to have been taking advantage of three security vulnerabilities in popular Apple products in its efforts to spy on dissidents and journalists.

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 Thursday 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 Aug. 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 U.A.E. citizens.

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