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Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

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Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#141

Post by rstuart4133 »

comex wrote: post #60
als0 wrote: post #36

> using a fTPM would solve the problem. All TPMs support encrypted sessions to prevent these kind of MITM attacks. You use TPM2_StartAuthSession and specify encryption with each session command. But Bitlocker doesn't use one, which is epic fail. Microsoft need to fix it. Edit: For comparison, systemd uses encrypted sessions when using LUKS disk encryption with the TPM https://github.com/systemd/systemd/commit/acbb504…

This isn’t even a proper MitM attack, just passive sniffing. But, I ask as someone unfamiliar with TPM, how do authenticated sessions work? How does the OS prove its identity to the TPM in a way an attacker couldn’t spoof in a real MitM attack? Any secrets or keys stored by the OS side would have to reside unencrypted on disk, since it doesn’t have an encryption key yet. Or even if the OS verifies the TPM’s identity…

> How does the OS prove its identity to the TPM in a way an attacker couldn’t spoof in a real MitM attack? Any secrets or keys stored by the OS side would have to reside unencrypted on disk, since it doesn’t have an encryption key yet.

Your intuition is sound. The only way for the OS to prove it's identity is to have a secret only it knows, and prove to the TPM it knows it. TPM's do support that, but in this case the OS has nowhere that is robustly secure to store the secret.

Windows could store a obfuscate secret on disk, but it doesn't bother. To be fair, there probably isn't much point - if someone is willing to go to this much work, then it's very likely they would be willing to invest the additional effort to break the obfuscation.

This still gives you a level of protection you wouldn't have without the TPM. The disk can only be read when the TPM is present - so someone stealing disk, or walking away with a bit for bit copy of it won't get them very far. One place that's useful in cloud environments. If the cloud provided replaces a fail disk and doesn't wipe the old one - it's still useless unless someone unless they know what motherboard it was paid with.

Still, I think that's an anti-feature for a laptop. It means if the motherboard fails you've lost the data on the disk even though it's perfectly fine, and indeed that is the case with bitlocker. If you protect the disk with a password you entered on boot up it is immune to this sort of attack, and you can move it between machines. Win, win. That's what I do. But, I don't use Windows to do it.

> Or even if the OS verifies the TPM’s identity somehow

That can be solved using attestation. Attestation is just a secret TPM knows, signed by the manufacturer. Windows could choose to deal only with TPM's from manufacturers it trusts, and presumably a emulated one wouldn't be one of them. Secure boot should prevent you from modifying Windows to accept any manufacturer, so it's secure.

But I'd lay long odds Windows doesn't do this sort of verification.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#142

Post by dlgeek »

ongy wrote: post #49
brohee wrote: post #44

Earlier quoted context omitted.

At least TME-MK and its AMD equivalent are supposed to address in memory key stealing/memory bus snooping (even if it's still unclear to me how the key are generated/stored). There is still decapping and probing the CPU itself but given the size of features is that even remotely doable?

Yes and no. Mostly no IMO The memory encryption features are a solution to very specific problems. If the CPU is able to access the memory, then any exploit that gains the execution context of the legitimate user can also access the memory. If it doesn't, the normal memory access control should be enough. I'm iffy on how well they protect against the various side channels. Mostly because I haven't looked far enough i…

Should protect against warm boot attacks since the key is supposed to be reset on CPU reboot.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#143

Post by yencabulator »

o1y32 wrote: post #125
controversial97 wrote: post #23

Another write up from 2021 https://arstechnica.com/gadgets/2021/08/how-to-go-from-stole... One aspect of this is that some laptop manufacturers provide a setting to erase the TPM if the laptop is opened. You opened that laptop to see if you can add more RAM? Better hope you can access the bitlocker key or have a good backup.

Off topic -- The joke is that you can hardly find a laptop with upgradable RAM these days. Not even on some ThinkPad lines. Gaming laptops often do.

https://frame.work/ is there for you.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#144

Post by vladvasiliu »

GordonS wrote: post #104
vladvasiliu wrote: post #48

Earlier quoted context omitted.

> It's not compromised if you set a PIN/passkey, aka use it correctly. The issue is that you have to go out of your way to set it up this way. Last I checked, you couldn't just add a PIN. You had to enable it through GPO.

IIRC you can set a PIN without fiddling with security settings or GPOs, but if you want to use a password instead of a PIN, then you've got to jump through some hoops to enable that. It always struck me as a very, very strange design choice - why make it so difficult?!

Just checked on my up-to-date win 11 22h2 pro. I can't set a pin by default.

> Computer Config / Admin templates / Win components / Bitlocker / Os Drive / Require additional authentication at startup

> If you disable or do not configure this policy setting, users can configure only basic options on computers with a TPM.

This enables tpm + pin and / or password unlock. But only numeric pins. If you want alphanumeric, or "enhanced", pins, you have to enable a separate GPO in the same place: allow enhanced pins for startup.

> If you disable or do not configure this policy setting, enhanced PINs will not be used.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#145

Post by jrm4 »

jrm4 wrote: post #7

Bitlocker is the Windows one, that's essentially known to be compromised, right? Not saying that it's useless, might be fine for certain work environments, but I wouldn't rely on it for anything truly personal.

So again, I'm right and still downvoted. Anyway, I hadn't done much homework on it, but the thing that let me presume this pretty well was when Truecrypt was likely canaried and they told us to go to Bitlocker.

So, Hacker News, gonna do anything about your pro MS trolls? Would be nice.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#146

Post by cafeinux »

withinboredom wrote: post #67
cafeinux wrote: post #66

Earlier quoted context omitted.

Doesn't this method imply booting from an external disk, thus not decrypting the HDD, thus not being able to modify what needs to be modified in order to bypass the login screen?

If you can trick the computer into decrypting the disk, why does it matter which disk you actually boot from?

The point made earlier was that if you boot Windows, it automatically decrypts the disk, but if you boot from another disk, it doesn't. I don't know much in this, except that I think I know that the TPM has an "owner" and that only that owner can read and modify the content of the TPM.

If you could just plug your USB drive, boot from it and automatically decrypt the Windows partition to edit CMD.exe, I just see this whole Bitlocker and TPM thing as completely moot.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#147

Post by withinboredom »

cafeinux wrote: post #146
withinboredom wrote: post #67

Earlier quoted context omitted.

If you can trick the computer into decrypting the disk, why does it matter which disk you actually boot from?

The point made earlier was that if you boot Windows, it automatically decrypts the disk, but if you boot from another disk, it doesn't. I don't know much in this, except that I think I know that the TPM has an "owner" and that only that owner can read and modify the content of the TPM. If you could just plug your USB drive, boot from it and automatically decrypt the Windows partition to edit CMD.exe, I just see this…

I really hate to be the one to point this out, but its clear you didn't read the article since this is exactly what they did: boot windows, steal the key from the TPM, boot into different OS entirely, inject the key to decrypt the disk, replace sethc, reboot into windows and push shift five times ... and there you go.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#148

Post by withinboredom »

plonk wrote: post #85
brohee wrote: post #77

Earlier quoted context omitted.

BitLocker is mostly worthless if you don't also password UEFI, disable booting off any other disk than the main one, and enable SecureBoot.

If you booted anything but the OS that set up BitLocker, wouldn't the TPM refuse to release the key?

read the article.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#149

Post by cafeinux »

withinboredom wrote: post #147
cafeinux wrote: post #146

Earlier quoted context omitted.

The point made earlier was that if you boot Windows, it automatically decrypts the disk, but if you boot from another disk, it doesn't. I don't know much in this, except that I think I know that the TPM has an "owner" and that only that owner can read and modify the content of the TPM. If you could just plug your USB drive, boot from it and automatically decrypt the Windows partition to edit CMD.exe, I just see this…

I really hate to be the one to point this out, but its clear you didn't read the article since this is exactly what they did: boot windows, steal the key from the TPM, boot into different OS entirely, inject the key to decrypt the disk, replace sethc, reboot into windows and push shift five times ... and there you go.

I read the article (and skimmed some parts, as I'm not interested in the technical details of how the signals were decoded).

As I understood the conversation thread we're in, we were talking in the context of someone simply booting up the laptop, not someone opening it and plugging wires to tap the TPM bus.

Of course, once you've tapped the TPM with the technique described in the article you can do whatever you want with the disk, but in this case I don't see why you would bother bypassing the login, just mount the partition and get the data you need.

Edit: specifically in this case, my comment was a reaction to this but from psychphysic : > what does it matter if they sniff the key first? They always had access to the end result of they can boot the laptop => if you didn't sniff the key first, you can't decrypt the disk offline, and can't have access to CMD on the login screen. That's why it matters.

Re: Bypassing Bitlocker using a cheap logic analyzer on a Lenovo laptop

#150

Post by als0 »

gquere wrote: post #86
als0 wrote: post #43

Earlier quoted context omitted.

fTPMs (firmware TPMs) are also very bad advice, because they have been remotely exploited using only software techniques.

OTOH they can be patched and to exploit them locally you either need software code exec or do a physical side-channel attack which is miles beyond a simple bus snooping.

Hardware TPMs can also be patched.
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