You can convert the .app file to an ISO using Terminal:
hdiutil create -o /tmp/HighSierra -size 8500m -volname HighSierra -layout SPUD -fs HFS+J
hdiutil attach /tmp/HighSierra.dmg -noverify -mountpoint /Volumes/HighSierra
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/HighSierra --nointeraction
hdiutil detach /Volumes/Install\ macOS\ High\ Sierra
hdiutil convert /tmp/HighSierra.dmg -format UDTO -o ~/Desktop/HighSierra.iso
mv ~/Desktop/HighSierra.iso.cdr ~/Desktop/HighSierra.iso
Apple does not offer a direct .dmg link via their website anymore, but the official InstallAssistant.pkg still validates and downloads. Macos High Sierra 10.13.6 Raw Download
Get it from Apple’s CDN (still active): You can convert the
http://updates-http.cdn-apple.com/2019/cert/061-39476-20191023-48f365f4-92b5-4f8e-9a89-3d7b3b14ebef/InstallMacOSHighSierra.pkg
🔗 Copy the link above – it downloads the full installer package directly from Apple’s servers.
Once downloaded, open the.pkg→ it will placeInstall macOS High Sierra.appinto your Applications folder. Apple does not offer a direct
Cause: An expired certificate. Apple stopped signing High Sierra in 2021. Fix: Disconnect from the internet. Open Terminal and set the date back to 2019:
date 0101010119
Then run the installer. After installation, re-enable automatic date/time.
Apple’s CDN may return a 404 for older direct links. In this case, use the softwareupdate Terminal method mentioned earlier. It is the only official "raw" pipeline still fully functional.