The macOS High Sierra 10.13.5 update is an incremental release that introduced critical features like Messages in iCloud
From the Mac App Store: macOS High Sierra Apple still provides old OS versions via direct links, but you may need a compatible Mac and Apple ID.
hdiutil convert /tmp/HighSierra.dmg -format UDTO -o ~/Desktop/HighSierra.iso
The update, released by Apple Support on June 1, 2018, was a pivotal mid-cycle release for the High Sierra operating system . While primarily focused on under-the-hood refinements, it introduced a significant feature that altered how users managed data: Messages in iCloud . Key Features and Stability
This is the safest, most legal method.
Check the file’s integrity by comparing its SHA-1 hash. For a genuine ISO created from Apple’s installer, hashes will vary depending on conversion method, but you can check the original InstallESD.dmg inside Apple’s installer:
Due to Apple’s deprecation of High Sierra (no longer receiving security updates), the Internet Archive hosts many legacy ISOs. If you choose this route: