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It is legal to download macOS High Sierra 10.13.6 directly from Apple or to copy it from a Mac you own. However, distributing the DMG file on public websites violates Apple’s software license agreement.
If you run a repair shop or a school lab, do not host the DMG on your own server for public download. Instead, point users to Apple’s official App Store link or use Apple’s softwareupdate method.
Pro tip: If you need multiple installs, download once from Apple, then create a bootable USB or a NetInstall image for your organization.
If you obtain the official installer:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyUSB --applicationpath /Applications/Install\ macOS\ High\ Sierra.app
You can download the macOS High Sierra 10.13.6 update from Apple's official website: