Security Updates
Does macOS High Sierra 10.13.6 still receive security updates?
iMac 21.5″, macOS 10.13
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macOS High Sierra is no longer supported with security updates by Apple. The latest update for macOS High Sierra was macOS High Sierra 10.13.6 Security Update 2020-006, which was released in November 2020.
Apple security updates - Apple Support
The earliest version of macOS currently supported by Apple is macOS Catalina, which should receive security updates until Fall 2022. For tighter security and even more features, you should download macOS Big Sur or macOS Monterey if your Mac can support these operating systems.