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I have a Windows 10 desktop computer which is my main device. It's password-protected and I'm not concerned about it being physically accessed by bad actors.

I have a Windows 7 laptop with a fresh OS install and only a few programs installed. I'm using it as a dedicated computer for running a laser cutter. Both the laser cutter and the laptop are in the garage. The laptop user has no password, so anyone with physical access can boot it up and get into an admin-enabled user unrestricted. The laser cutter software, unfortunately, only works when run as administrator, so the user must be able to run programs as administrator.

Both computers are on the same home network, and in the same Workgroup.

I have a network-shared folder on each computer to allow me to easily send files to the laptop for use on the laser cutter without needing to go back and forth with a thumb drive. The idea is I can drop a file in the shared folder on my desktop PC, walk over to the laptop, and grab the file.

I want to have acquaintances from my local area be able to bring their own files, usually on thumb drive, and use them in the laser cutter.

I am not concerned if I have to nuke the laptop and install the OS again if it gets infected, but I am worried that an infection on the laptop could spread to my desktop through the shared folders. Is this something I need to worry about? Are there any other vectors whereby malware on the laptop could infect my desktop (without user action on the desktop, such as running an .exe from the shared folder)?

  • Seems the answer need update on EternalBlue ;-) – mootmoot Jul 11 '17 at 16:32
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First, this assumption is highly optimistic:

It's password-protected and I'm not concerned about it being physically accessed by bad actors.

You should. As soon as an evil guy has physical access to a machine, many things can happen:

  • at the simplest level, just boot from a CD or an USB key (UBCD or UBCD4WIN are great for that). That is normally enough to by-pass any password protection (and IMHO much simpler than attacking the desktop from the laptop)
  • at more complex levels are malware hardware devices such as a physical keyloggers (password is know next time you enter it), or USB devices simulating a keyboard

But your actual question is about what could happend to your desktop if malware are installed on the unprotected laptop.

This part highly depends on how your system is configured. The default installations assume that the local network is secured, and it is common for guest accesses to be allowed. That is not enough to execute something but that means that it is possible to read files from the computer - you know whether private information can be present, I do not.

Another possible way is more like social engineering. It consists of letting innocent looking files (images, videos, music) containing an infected payload targetted at common media readers in places where they could not notice them as suspect. As your main destop can read from the laptop, it would be possible that you inadvertantly load the malware on the desktop.

And as soon as it is on you local netword, it can spy all network traffic, perform Man In the Middle attacks through ARP cache poisoning (pretend to be your router for example), and various other network attacks. Not speaking for possible exploits in the OS itself on any service running on it.

TL/DR: The real problem here is the exposition of the desktop. Common security practices recommend that the higher is the exposition, the higher should be the security: only required services and applications, strong passwords and often changed. And if physical access is possible, no USB ports, no bootable device and a consistent control of all physical devices before use. And of course a physical lock to prevent box opening.

That being said, the security level must accomodate to the actual risk, and only you can know whether the cost (money and time) for securing that is worth it.

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I don't speak from experience, so I can't tell anything about how likely an attack might be. But just looking at possibility it seems to be a security risk; assessing the likelihood through experience is the only way to tell anything about the risk, though.

USB drives can have an autorun file on them just like CD or DVD that instructs the operating system to run an executable upon connection of the device. I believe at some point Microsoft fixed this vulnerability by ignoring the execution instruction in autorun files; I don't know whether this applies to Windows 7 yet. But either way, a user may only need a couple of seconds where you aren't looking to execute a file from the USB drive. Therefore one can assume that it is possible for others to entirely control this laptop, for example by bringing in a payload on the USB drive that enables them to remote-control the laptop or the payload acts on its own.

So now you have an infected device in your local network, and that is basically where I could end I guess. An infected device in your local network could pose as a DNS, re-routing any web requests done by any devices in your network (e.g. your desktop) or try to exploit a large number of possibly existing security breaches in your desktop or another device. Generally speaking: An attacker that has access to your local network has a huge number of possible attack vectors from which to choose that he wouldn't have if he could only see your router, coming from the internet side of the network.

So the problem is not the windows share primarily. Yes, he can place an infected PDF file in there, you load it up and you're done. But the really nasty part comes from giving strangers the opportunity to take over a local network device with administrator rights.

My advice would be to down-grade the user account on that laptop to a non-administrator account and just execute the software you need administrator rights for in the administrator scope. If someone wants to run malicious code this will then at least be restricted to normal user rights. And of course don't give them the administrator password but only enter it yourself.

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