3 ms·

> which btw allows you to also resign it Are you sure? I havent tried, but this disagrees: https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/wi

by shawnz 7y ago

> which btw allows you to also resign it Are you sure? I havent tried, but this disagrees: https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-8.1-and-8/dn747883(v=win.10)?redirectedfrom=MSDN https://docs.microsoft.com/en-us/previous-versions/windows/i... "Windows boot components: BootMgr, WinLoad, Windows Kernel Startup. Windows boot components verify the signature on each component. Any non-trusted components will not be loaded and instead will trigger Secure Boot remediation. " > DB entries can also contain SHA-256 hashes OK, fair enough, but it still doesn't really solve the problem because an attacker could just copy your modified bootmgr to be able to steal and use your workstation. In order for this to work you'd also have to add some kind of additional checks which we can now do with an open source version of the bootloader.

p_l 7y ago
The signatures are, IIRC, checked in order, so a resigned BootMgr won't impact verification of WinLoad, etc. As for abusing signed bootloader, the full process depends on verifying that the signed bootloader appropriately handles TPM, which is then also coupled with an encrypted disk, which in turn works to prevent loading unsanctioned code. Of course, it's possible to defeat this, but the idea is to frustrate the efforts and raise the cost of an attack, as well as give you a longer timeframe to deal with an attack.
- shawnz 7y ago
  Those mechanisms only prevent an attacker from decrypting the disk. The scenario I'm envisioning is where the attacker steals your workstation, wipes the disk and reinstalls Windows.
  - p_l 7y ago
    That's not part of the threat model - the encrypted data is lost, the point of the defences are to prevent damage to things accessible from that specific machine, not to prevent stealing the machine.
    - shawnz 7y ago
      That's not part of Microsoft's threat model. Which is exactly the problem! It could be done, if only Microsoft supported that use case. But with an open source bootloader, we could make the necessary changes ourselves.
      - p_l 7y ago
        No, that's the threat model that most clients that require high security have (the machine is often nigh-infinitely cheaper than the data and access rights), and MS, surprise, explicitly insists (in difference to secure boot spec) that owner of the physical machine is, well, it's owner and can reinstall, resell, etc.
        shawnz 7y ago
        I don't see how any of this is relevant. All I said was that it's an interesting possibility which has now been made feasible. Who cares about the threat model that most high security clients have? If they don't want it then I'm not talking about those clients. Besides: the prevalence of terrible software like Lojack/CompuTrace in the enterprise just goes to show that many clients actually do care about physical theft scenarios. Also consider how basically every modern mobile device now provides factory reset protection.

by shawnz 7y ago

p_l 7y ago

The signatures are, IIRC, checked in order, so a resigned BootMgr won't impact verification of WinLoad, etc. As for abusing signed bootloader, the full process depends on verifying that the signed bootloader appropriately handles TPM, which is then also coupled with an encrypted disk, which in turn works to prevent loading unsanctioned code. Of course, it's possible to defeat this, but the idea is to frustrate the efforts and raise the cost of an attack, as well as give you a longer timeframe to deal with an attack.

shawnz 7y ago
Those mechanisms only prevent an attacker from decrypting the disk. The scenario I'm envisioning is where the attacker steals your workstation, wipes the disk and reinstalls Windows.
- p_l 7y ago
  That's not part of the threat model - the encrypted data is lost, the point of the defences are to prevent damage to things accessible from that specific machine, not to prevent stealing the machine.
  - shawnz 7y ago
    That's not part of Microsoft's threat model. Which is exactly the problem! It could be done, if only Microsoft supported that use case. But with an open source bootloader, we could make the necessary changes ourselves.
    - p_l 7y ago
      No, that's the threat model that most clients that require high security have (the machine is often nigh-infinitely cheaper than the data and access rights), and MS, surprise, explicitly insists (in difference to secure boot spec) that owner of the physical machine is, well, it's owner and can reinstall, resell, etc.
      - shawnz 7y ago
        I don't see how any of this is relevant. All I said was that it's an interesting possibility which has now been made feasible. Who cares about the threat model that most high security clients have? If they don't want it then I'm not talking about those clients. Besides: the prevalence of terrible software like Lojack/CompuTrace in the enterprise just goes to show that many clients actually do care about physical theft scenarios. Also consider how basically every modern mobile device now provides factory reset protection.