this post was submitted on 27 Aug 2024
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The thing is... If someone has access to your system enough to replace your bootloader, they could probably just slip a USB keylogger between your keyboard and computer. Or set up a small hidden camera. Or plug all your devices into a raspberry pi to spoof the login screen.
It strikes me as odd that people assume that an attacker with a few hours physical access is going to bother going down the "change the bootloader" route when there are other, easier routes available.
Ironically, the only practical use case I can see for Secure Boot is when you have a dual boot setup where you don't trust one of the OSes. Which I'm betting wasn't Microsoft's intention at all.
Just because they can do X doesn't mean you shouldn't protect against Y.
Just as an example scenario, say border guards took my laptop out of my eyesight. A camera or USB keylogger won't do anything in that case. Hijacking my bootloader though potentially gives them access to my machine without me having any clue.
Secure Boot is useful and worth setting up. But everyone has to decide their own level of comfort when it comes to security.
You need to correctly set it up tho. ie. remove all default keys because some were discovered ages ago anyway.
They could open up the laptop to insert a small device that reads the usb header. Or just replace the guts of the laptop with something else. Or replace one of the usb leads in your bag with one with a tracker. Or sell a usb-c cable with a tracker for cheap in the gift shop.
There's a bunch of other ways to compromise your system and some might be easier than putting a backdoored bootloader on your device.
Also, if it's the TSA, they could almost certainly create a bootloader that was signed by Microsoft to replace any existing one.
Basically xkcd 538 as always.
No point in putting locks on your house, because an attacker can just drive their car through your front door.
The attacks you mention have their own ways of being detected: usually eyeballs. But eyeballs can't help you against something hiding in your bootloader. So Secure Boot was made.
And I don't really follow your dual boot claim. If you don't trust one of the OSes, and you boot it up on your hw, you're already hosed. At that point it can backdoor your bootloader and compromise your other OS. Secure Boot prevents malicious OSes from being booted, it can't help you if you willingly boot a malicious OS.
Okay, so my original post I think I was thinking of TPM rather than Secure Boot. TPM would protect against a hostile OS, I think. I think most of my greviences mentioned above are to do with TPM rather than Secure Boot. But they still apply to either.
There are ways to put keyloggers in devices stealthily to where they're not perceptible to a normal person. For example, they could replace one of the USB leads in your bag with one that transmits keystrokes over bluetooth. If you're at home, a maid could just plug a keylogger behind your desktop because most people don't check behind their system when they boot it.
It all feels like a wierd threat model to me. You can either assume that physical access is or isn't covered by it. If you are worried about physical access (beyond your device being stolen), there is a lot more you need to do to secure it. "It's better than nothing" isn't really that convincing to me with regards security models; either you're protected or you aren't.
I'm actually not sure what TPM can guard against, but I think you're right, I think if a malicious OS borked with the bootloader, TPM would catch it and complain before you decrypt the other OS.
Yeah, physical access usually means all bets are off, but you still lock your doors even though a hammer through a window easily circumvents it. Because you don't know what the attacker is willing to do/capable of. If you only ever check for physical devices, you'll miss the attack in software, similarly if you only rely on Secure Boot you'll miss any hardware based attacks. It's there as a tool to plug one attack vector.
Also, my guess is the most common thing this protects against are stupid employees plugging a USB they found in the parking lot into their PC. If they do it while the OS is running, IT can have a policy that blocks it from taking action. But if they leave it there during a reboot, IT is otherwise helpless.