I have multiple older devices on Sierra and Catalina. How can I protect them in they cannot upgrade to is Big Sur 11.6?
Mac mini, macOS 10.15
apple made a security update for catalina yesterday, they don't only support their latest version
Apple security updates - Apple Support
high sierra does not seem to be supported any more Which versions of macOS (OS X) are supported - Technote TN-O11 (powermapper.com)
If you're referring the Israli's Pegasus spyware it's only for iPhones and Android phones. It's not for desktop computers.
As a small addition to what Rudegar said, and that may assuage your fears a little: this dreaded threat that was countered by the recent software updates is one that is unlikely to be used against us anonymous users. It is a very expensive piece of software used to target specific individuals.
Thanks. Several replies confirm.
Protection from spywsre
