Ondrej Holzman Apple released a relatively low-key update to its iOS 9 system on Thursday, but version 9.3.5 is very important. It represents a key security update for the entire system.
"iOS 9.3.5 brings an important security update to your iPhone and iPad that is recommended for all users," writes Apple, which was supposed to release the fix just ten days after the Israeli firm NSO Group drew attention to the bug in the system. She specializes in tracking cell phones.
According to available reports, it is not entirely clear how significantly the Israelis penetrated iOS 9, but according to The New York Times they created software that allowed them to read messages, emails, calls, contacts and other data.
Despite the discovered security holes discovered by Bill Marczak and John Scott-Railton, it was supposed to even record sounds, collect passwords and track the location of users. So Apple strongly recommends installing the latest iOS 9.3.5. It is possible that this is the last update for iOS 9 before the arrival of iOS 10.
A more detailed article (in English) is here https://motherboard.vice.com/read/government-hackers-iphone-hacking-jailbreak-nso-group
updating to this version blew up my iPhone 4s…..straight to repair, firmware was blocking the update from completing