Use the official App Store link to download the latest High Sierra installer .

If you specifically need a .rdr image for a virtual machine or a restoration project:

: You can find the macOS High Sierra installer on the Mac App Store using a direct link in Safari.

Once you have a legitimate Install macOS High Sierra.app (version 10.13.5):

: Sometimes the App Store downloads a small ~20MB file instead of the full ~5GB installer. To fix this, ensure you are running the latest version of macOS Sierra before downloading, or use the gibMacOS script from GitHub to fetch the full recovery or installer files.

: Apple provides manual download links for the macOS High Sierra 10.13.5 Update and the Combo Update . These are PKG files used to update an existing High Sierra installation to version 10.13.5.