How to include recent security patch (APFS plain password vulnerability) into already downloaded macOS High Sierra installer image?
You can't merge an update into an older macOS installer app easily. So you have to re-download the full installer to get the most current version. Check that you don't get the restricted (20 MB) installer only app.

After downloading the first full installer (without installing it), move it to a folder and create a dmg choosing the folder (or alternatively zip the app).

Then install the upgrade or trash the macOS installer.app.

After Apple announces and publishes a new installer (not necessarily as a point release update), download it again. Repeat the steps above. If you don't need the old installer.dmg anymore, trash it. I usually keep them.



Under certain circumstances (often after moving the macOS installer.app to an arbitrary folder) downloading the new release will replace the old macOS installer in-place.



To check the build version grep the the file /Contents/Info.plist for DTSDKBuild:

grep DTSDKBuild -A1 .../Install\ macOS\ High\ Sierra.app/Contents/Info.plist


The golden master's build version was 17A362a.

The first installer's build version available to the public was 17A364.

The latest build version as of today is 17A400 (small installer) or 17A403 (large installer).