Hi all, we are hiring a remote worker and will be supplying a laptop to them. The laptop will be running a Debian variant of Linux on it.
We are a small shop and this is the first time we have entrusted somebody outside of our small pool of trusted employees.
We have sensitive client data on the laptop that they need to access for their day-to-day work.
However, if something goes wrong, and they do the wrong thing, we want to be able to send out some kind of command or similar, that will completely lock, block, or wipe the sensitive data.
We don’t want any form of spying or tracking. We are not interested in seeing how they use the computer, or any of the logs. We just want to be able to delete that data, or block access, if they don’t return the laptop when they leave, or if they steal the laptop, or if they do the wrong thing.
What systems are in place in the world of Linux that could do this?
Any advice or suggestions are greatly appreciated? Thank you.
You’ll first want to lock down the laptop with using the TPM so it only boots kernels signed by you, and also encrypt the drive using the TPM as the locking key so the key is only ever available to a kernel you signed. From there you’ll probably want to use dm-verity to also verify the integrity of the system or at least during the boot process.
Then, on top of that, once online and the machine is still authorized to access that data, you download a key from a server under your control to unlock the rest of the drive (as another partition). And log those accesses of course.
Then, when you want to revoke access to it, all you have to do is stop replying with the key whenever requested. That just puts a ton of hurdles to overcome to access the data once the server stops handing the key. They would have to pry out the key from the TPM to unlock the first stage and even be able to see how it works and how to potentially obtain the key. They could still manage to copy the data out while the system is fully unlocked and still trusted, which you can make a lot harder by preventing access to external drives or network shares. But they have physical access so they kind of have the last word if they really really really want to exfiltrate data.
This is the best you can do because it’s a passive: you stop supplying the unlock key so it’s stuck locked encrypted with no key, so the best they can do is format the laptop and sell it or use it for themselves. Any sort of active command system can be pretty easy to counter: just don’t get it online if you suspect the kill signal is coming, and it will never come, and therefore never get wiped. You want that system to be wiped by default unless your server decides it’s not.
I’m not the most up to speed on TPM’s, but does it have the capability to directly do network access in order to pull the key? Otherwise, you’re going to need the regular OS to get it to the TPM somehow, in which case that’s the weak link to pull the key instead of ripping it from the TPM itself.
And once they have the key once, how do you enforce them having to re-request it? Is there a reason that that point they couldn’t just unplug from the Internet (if even necessary) and copy the entirety of that drive/partition somewhere else?
No, that’s why signed kernel+initramfs+LUKS+dm-verity: protect the boot process all the way into userspace where you do have network access. From there you can request the TPM signs messages with a preloaded key it will only allow using it if you went through the whole secure boot process. It’s exactly what Android does with Play Integrity and the strong integrity flag.
That way you can prove to the server that the computer is still secured and untampered with up to that point, which means the script that deals with the periodic checkin should be running untampered as well. If you’ve secured down the Linux install appropriately, it should be impossible for the user to gain enough privileges to request the key again from the TPM or extract the data key out of the mounted filesystem. That also means you can trust the system to block mounting any drives, force VPN on, make sure your MDM runs, all that stuff.
You can reset the BIOS, boot from USB, all that stuff still, but then it would also wipe the TPM and so the OS no longer bootable, and obviously no signed TPM messages either so even if you find the script and how it works, at that point you don’t have the ability to sign the messages so the server won’t give you the data partition’s key either. The moment you tamper with it, it breaks the trust chain and the keys are gone. Can’t flip a single bit on the system and boot partitions without the checks failing.
It’s not bulletproof, some laptops you can sniff the TPM bus in minutes due to design flaws, but in theory as long as the hardware holds it’s pretty secure. And obviously you can always just take a picture of the screen, no avoiding that. But it puts enough hurdles it’ll stop most opportunistic exfiltration. One bad move and you wipe the keys, so you better know exactly what you’re dealing with or you have one embarassing and incriminating email to write to IT to have them reprovision the keys.
If the data is sensitive just give them a cheap whatever machine and have them connect to a vdi. That way the data never leaves your estate and means you don’t have to worry as much about the device being lost/stolen. If this isn’t an option I’d strongly recommend looking into an MDM solution for your devices.
This is the correct answer, while it may have more up front costs. It’ll save in the long run, especially if the company has growth potential.
An Anydesk license is not that expensive
Realistically the best option here is to not have the data in the laptop. So they would remote into a machine you control to access the data, or something of the sort. Regardless the laptop should have full disk encryption so if it gets stolen no data is accidentally leaked.
Other than that the best way I can think of is giving the user a non-root account and have the laptop connect to tailscale automatically so you can always ssh into it and control it if needed. But this is not ideal, because a malicious person could just not connect to the internet and completely block you from doing anything. This is true for almost any sort of remote management tool you would be able to find.
First part is what I was thinking - have the sensitive data on a server for the laptop to access. Of course an unscrupulous user can always save any data they want. But it’s totally normal for companies to give temp employees access to their internal data. Their standard method of protection is to make the person sign an NDA. The threat of expensive legal action deters most people from violating it.
ScaleFusion does this.
+1 for going with a third party on something like this. Your small shop is an expert in whatever they’re doing, don’t try and recreate someone else’s buisness thinking it will be easy
There is a fundamental issue with this approach: the rogue employee has already copied the data to a USB drive by the time you try to wipe it.
If the data is confidential, you either need to set up standard disk encryption and trust the employee, or not let them access it in a way it can be bulk copied. For instance, might it be possible for them to use a webapp that you control access to or a remote desktop type setup?
A lot of employers (at least the larger ones) block USB drives and have software to monitor for data exfiltration - monitoring where files are copied to, usage of copy/paste in browsers, etc.
You should always assume that your work devices are being monitored.
