This allows you to boot a different OS, but will still not allow you to hack the existing one, the PCR registers will be different if you try chain-loading it.

Also, if you store part of the full disk encryption key in the TPM, you are also safe, the TPM will not release it. But there is a risk of the user entering their password (if they have one) in a fake login screen. But the OS will fail booting after that screen, so you need some plausible story to trick the user into not reporting the incident to IT.

If you have a laptop where you can install Ubuntu on it, this attack allows you to install a modified (unsigned) Ubuntu on it. So not much has changed.

> so you need some plausible story to trick the user into not reporting the incident to IT.

Give them an authentic-looking "updating windows" screen after login, and reboot. 95% of users will bitch about Microsoft, instead of calling IT.