How secure am I on a VPN?

Yes, they might possibly be able to see things on your computer. Naturally
it depends on how you’ve set it up.

The good news, though, is that it should be possible to reestablish a decent
level of security while still connecting to the VPN.

A VPN, or Virtual Private Network, is as it name implies: a way to set up
connectivity among two or more computers privately. In fact, even though the
computers may be miles apart, except for the speed of the connection it’s much
like having those computers all in the same room connected together through a
hub or switch.

And that’s the best way to think of a VPN: as a slower, virtual LAN with all
the machines connected to each other.

That’s actually a very powerful concept. For example as I sit here in my
office I can connect to, share files with, share printers with, and otherwise
use the computers at my wife’s business 10 miles away. I do that across the
internet through the VPN I’ve set up. All of our machines on that VPN can “see”
each other just as if they were actually here on my home network.

Naturally there’s a risk.

Since I control all the machines at both ends of this VPN, I know who and
what I’m dealing with. If you’re connecting to a VPN from work you may have no
idea who or what is at the other end, or what they’re doing.

In that sense, connecting to a VPN might be considered the same as allowing
several untrusted machines to connect to your local network. You don’t know
what they might do. Other machines on that VPN might be infected with viruses,
or your corporate IT department might be inventorying all the machines on the
network.

Perhaps that co-worker you annoyed last week might is trying to see what’s
on your machine. If you have file sharing enabled, it’s quite possible that he
or she can. Since you’re all connected to the same (virtual) network your file
shares may be visible to other members of the network.

The solution? Quite simple, actually. If there are machines you may not
trust on the VPN you’re connecting to, just make sure to turn your
firewall.

I’d consider the built-in Windows Firewall sufficient for this, but do be
careful to make sure that things like file sharing are appropriately blocked.
Some firewalls will allow you to configure file sharing to be enabled locally,
but blocked on the VPN. Or, naturally, you could just turn off Windows File
sharing entirely if that makes sense for your situation.

In your shoes, doing something along those lines I’d feel just fine about
connecting up to that VPN.