Haven't got round to testing fully yet, but have looked at the manual installation method described by 'fth0' and all it describes is how to apply the necessary certificate files from the latest Test Build Guest Additions ISO prior to installation, so I can't see that it would pose any additional 'security risk'.
I haven't found what was meant by using "the appropriate option" in the VirtualBox installer, so still puzzling over what Klaus meant by that in his post. Most of my VirtualBox use (almost 10 years) has been for running Windows/Linux on macOS Hosts, so it feels a bit like breaking new ground with the Windows Host process!
