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.
Just don’t use Linux for this. Linux is meant to be customizable and configurable. If you want thoughtless drones working on cookie cutter setups, give them Windows or a Mac. Clipping Linux’s wings like like this should be a crime.
As someone else said, I’d go with an MDM vendor instead of trying to build something yourself.
The most secure thing would be to have the person connect to a remote server and do all their work on the remote server, essentially just using the laptop like a thin client.
This is the only reliable solution. To expand:
- Provide a Laptop with Windows on it, because that is easier to lockdown.
- apply desirable OS lock downs like blocking usb ports prevent storage devices, don’t give the user admin rights, etc.
- Setup a VPN server (openvpn should do) and configure the laptop with a VPN client. Configure the client so it blocks network connections that don’t go via the VPN. If you want to give them internet access you’ll need a proxy and firewall and DLP solution. At this point it all gets very complex and expensive.
The real answer is you are probably screwed without investing a bunch of time, effort, and cost.
You might get away with more basic security measures if the user has very limited IT knowledge.
I suggest getting legal advice before you give the user access to your data in the manner you intend.
Fundamentally, once someone has some of the data, they have that data, and you can make no guarantees to remove it. The main question you need to ask is whether or not you’re okay with limiting it to the data they’ve already seen, and what level of technical expertise they need to have to keep the data.
Making some assumptions for what’s acceptable as a possibility, and how much you want to invest, I’d recommend having the data on a network-mapped share, and put a daily enforced quota for their access to it. Any data they accessed (presumably as part of their normal duties) is their’s, and is “gone.” But if you remove their access, they can’t get any new data they didn’t touch before, and if they were to try and hoover up all the data at some point to copy it off, they’d hit their quota and lose access for a bit (and potentially send you an alert as well). This wouldn’t prevent them from slowly sucking out the data day after day.
If they only need to touch a small fraction of the customer data, and particularly if the sensitivity of the data goes down over time (data from a year ago is less sensitive than data from a day ago) this might be a decent solution. If they need to touch a large portion of the data, this isn’t as useful.
Edit: another nice bit is that you could log on the network share (at your location) which of the customer data they’re accessing and when. If you ever want to audit, and see them accessing things they don’t need, you can take action.
I think the next best solution is the VDI one, where you run a compute at your location, and they have to remote into it. If they screen capture, they’ll still save off whatever data they access, and if they have poor, or inconsistent, connection up your network it’ll affect their ability to do their job (and depending how far away they are it might just be super annoying dealing with the lag). On top of that, it’s dependent on how locked-down they need to be to do their job. If they need general Internet access, they could always attempt to upload the data somewhere else for them to pull it. If your corporate network has monitoring to catch that, you might be okay, but otherwise I think it’s a lot of downside with a fairly easy way to circumvent.
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.
I agree that you should assume you’re being monitored, but, while that helps against malware type exfiltration, it does little to stop someone who is determined to exfiltrate the data as there are a myriad of ways to do so that aren’t possible to monitor, such as simply taking a video of the screen whilst displaying the information.
Ultimately, the company has to trust the employee or not give them access to the sensitive data, there’s no middle ground.
This is fundamentally true. However it is possible to limit the bandwidth of data the employee can exfiltrate.
Assuming a privileged employee suddenly becomes a bad actor. Private-keys/certs are compromised, any kind of shared password/login is compromised.
In my case I have a legit access to my company’s web-certs as well as service account ssh-key’s, etc. If I were determined to undermine my company, I could absolutely get access to our HSM-stored software signing keys too. Or more accurately I’d be able to use that key to compile and sign an arbitrary binary at least once.
But I couldn’t for example download our entire customer database, I could get a specific record, I could maybe social engineer access to all the records of a specific customer, but there is no way I’d be able to extract all of our customers via an analog loophole or any standard way. The data set is too big.
I also wouldn’t be able to download our companies software source code in it’s entirety. Obviously I could intelligently pick a few key modules etc, but the whole thing would be impossible.
And this is what you are trying to limit. If you trust your employees (some you have to), you can’t stop them from copying the keys to the kingdom, but you can limit the damage that they can do, and also ensure they can’t copy ALL the crown jewels.
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.
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
The best option is to never trust anyone. Depending on how the info is supposed to be used, you can setup a website that does those important things with the sensitive data (stored on the server) without exposing the sensitive data to the user.
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’d add a dead man switch thing that, if the laptop doesn’t get the key in X days, it erases the safe partition anyway. Of course the company would want all that data backed up in their servers all the time.
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.
Ah, gotcha, I was thinking more in terms of software attacks than hardware, and that some vulnerability would come up at some point for them to get root access, at which point I think they’d be able to get the key one way or another. I’d imagine it also depends on how locked down the system can be based on the nature of their duties; arbitrary internet access makes shipping it off somewhere a bit easier. Another consideration would be that the drive could also be imaged, and if the key were ever recovered at a later date through whatever method/mistake/etc. the entirety of the data could be recovered.
But, yeah, definitely agree that that’s all moving well outside the bounds of disgruntled/opportunistic employee and more into the persistent adversary realm.
This is honestly an extremely expensive (in terms of skills, maintenance, chance of messing up) solution for a small shop that doesn’t mitigate at all the threats posed.
You said correctly, the employee has the final word on what happens to the data appearing on their screen. Especially in the case of client data (I.e., few and sensitive pieces of data), it might even be possible to take pictures of the screen (or type it manually) and all the time invested in (imperfect) solutions to restrict drives and network (essentially impossible unless you have a whitelist of IPs/URLs) goes out the window too.
To me it seems this problemi is simply approached from the wrong angle: once the data is on a machine you don’t trust, it’s gone. It’s not just the employee, it’s anybody who compromises that workstation or accesses it while left unlocked. The only approach to solving the issue OP is having is simply avoiding for the data to be stored on the machine in the first place, and making sure that the access is only for the data actually needed.
Data should be stored in the company-controlled infrastructure (be in cloud storage, object storage, a privileged-access workstation, etc.) and controls should be applied there (I.e., monitor for data transfers, network controls, etc.). This solves both the availability concerns (what if the laptop gets stolen, or breaks) and some of the security concerns. The employee will need to authenticate each time with a short-lived token to access the data, which means revoking access is also easy.
This still does not solve the fundamental problem: if the employee can see the data, they can take it. There is nothing that can be done about this, besides ensuring that the data is minimised and the employee has only access to what’s strictly needed.
If you find a reliable way to allow for people to use data without being able to copy it: Patent it right away, the entertainment industry will be paying big time for it.
Same for erasing the laptop: You can only erase something as long as you can talk to the machine in some way to instruct it to clean itself up. The guy with the machine in hand can just turn off wifi to stop it from receiving the message…
That’s why I focused on platform security. You can’t fully eliminate the possibility, but you can make it as hard as possible to pull off. TPM+dm-verity is to make it hard for the user to even look at how it works and prevent filesystem modifications that would give them root access to intercept the key loading mechanism.
The part where the laptop continuously check and refreshes the key is to address the second part about just turning off WiFi: make it so if you do that, you have N hours to break the system open before it reboots itself and you’ve lost access to the key for good. This can also depend on hardware-backed checks like TPM measurements and signature, to make sure the data key is only handed over to the expected environment.
It’s the same fundamental principles as Android and Play Integrity: use the security processor to attest the state of the device before the server agrees to send you stuff over. It’s been worked around via leaked keys mostly because Google is lenient for older devices, but the actual secure enclave hasn’t been broken yet.
The point is for the security to hold long enough the key’s gone before they can get it, and without the key the laptop is effectively wiped. That’s plenty for the overwhelming majority of “employee got fired and is pissed off and wants to retaliate”, and the best that can be done without going remote desktop/VDI.
VM behind a VPN with a firewall that blocks everything except the rdp protocol and no sudo access?
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
Unrelated, but their website has tablets (as in type of computer) translated as pills in Russian
Nice
Useful for standardized management of fleets, but requires personnel to maintain and configure it, but I don’t think it’s very effective (or feasible - I doubt they will even join the call for a 1-device contract) for what OP needs.
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.
You’re assuming you’ll have a network connection and that sensitive data is all in the same place.
Short of remotely unlocking an encrypted disk on every single boot… and even then…
- use a non sudo user for the user
- somehow get the IP address of that laptop all the time. There are dynDNS solutions like this where the client just needs to automatically download a certain file daily and you know his IP, my implementation is here.
- have ssh access to root with a ssh key. The usual hardening, fail2ban, block using passwords
- open the port for ssh on the clients system
If something goes wrong, login via ssh (you know the dynamically changing IP) and remove a directory or the entire user.
You cannot avoid that a user would copy files from there to a usb stick. Well you could, by using usbguard. Works really well in my experience, just prevent nonsudo users from adding new devices.
And then you need to prevent the user from booting another system, or taking out the SSD and reading it. TPM and boot lock is the right thing here.
Your ability to SSH in the machine depends on the network connectivity. Knowing the IP does nothing if the SSH port is not forwarded by the router or if you don’t establish a reverse tunnel yourself with a public host. As a company you can do changes to the client device, but you can’t do them on the employee’s network (and they might not even be connected there). So the only option is to have the machine establish a reverse tunnel, and this removes even the need for dynamic DNS (which also might not work in certain ISPs).
The no-sudo is also easier said than done, that means you will need to assist every time the employee needs a new package installed, you need to set unattended upgrades and of course help with debugging should something break. Depending on the job type, this might be possible.
I still think this approach (lock laptop) is an old, ineffective approach (vs zero-trust + remote data).
You could implement a ssh tunnel every time it is online. Then you just use a reverse tunnel through that connection.
Yeah, that’s what I wrote too, but that is still a very fragile way. For once, you depend on a network connections, or in the local firewall not blocking you etc.
Reactive, on-demand ssh is something you can do for tech support, not for security imho.
True, forgot about that.
Alternatively yeah some system to load the data online, autodelete after a while of not logging into something.
But the question really is “why?”
Disk encryption should deal with everything. Secure boot and usbguard are useful anyways.
Disk encryption is a control against lost or stolen device and malicious physical access (kinda). Storing the data elsewhere is more a control (or the basis for controls) against malicious insiders.
