Worried? That might seem a bit strong, but some concern is certainly reasonable.
The short answer to your question is yes, it’s quite possible that someone on her computer with less than honorable intent could cause you grief in several ways.
Let’s look at how.
It’s all about trust
We need to start by understanding exactly what it means to share your internet connection.
One of the reasons using a router is so important is that it provides a firewall between an untrusted network, such as the internet, and a trusted or safe network, such as the local network on which you connect all of your computers.
You implicitly trust all the computers connected to your local network on the “trusted” side of the router. You know you’ve taken whatever steps are appropriate to keep them secure, and you understand the expertise and knowledge of the users of those computers. The bottom line is that you have a reasonable level of safety associated with those computers being allowed to communicate with each other.
The most important side effect of being able to trust all of the computers on the trusted side of the router is that you don’t need to take a lot of extra steps to protect those computers from each other. The biggest example might be that you probably don’t need to have a firewall enabled on every computer; the router’s firewall protecting them all from the un-trusted internet is enough.
Sharing your internet
By giving your neighbor a password, I assume you mean you are giving her your existing Wi-Fi password and she is connecting using a wireless connection. (The alternative would be to string a cable to her house, and I’m pretty sure you’re not doing that. )
That means that her computer is connecting to the trusted side of your router.
This changes that trust dynamic dramatically.
As you’ve said, you don’t really know how much you can trust your neighbor, or perhaps the other people in her house.
In other words, you just allowed an un-trusted connection to the formerly trusted side of your router.
What could possibly go wrong?
There are several ways a non-trusted computer or computer user could cause issues for you.
- It could infect your computers with malware.
You don’t really know how up-to-date your neighbors are on things like computer security. For all you know, their machines have never been updated, have never run anti-malware software, and are liberally infected with all forms of viruses and spyware. Since you’ve been able to trust all of the computers on the trusted side of your router up to this point, you probably didn’t bother to put firewalls on each individual computer. As a result, they are potentially vulnerable to a network-based threat coming from your neighbors’ poorly maintained computers.
- It could be used to access your computer.
It’s possible your neighbor could access your computers directly. If you have remote desktop enabled, they might be able to log in to your machine. Fortunately, remote desktop isn’t enabled by default.
- It could be used to peek at your data.
A more likely scenario is that your neighbor would be able to access files on your machines using Windows networking. Exactly how much of your computer’s data is visible will vary depending on how you set up your network. The issue, of course, is that you likely set it up assuming all machines on the “safe side” could be trusted. In the absolute worst case, it’s conceivable your neighbor could access all of the files on all of your computers. While that’s a bit unlikely, a more common scenario is that your neighbor could access “some” of your files – where “some” once again depends on your specific network configuration.
- It could get you in trouble.
We’ve all heard news reports of this; it is possible you could be legally liable for the activity of your neighbor over your internet connection. For example, if your neighbor were to begin downloading copyrighted material, such as music or movies, or downright illegal material, such as some forms of pornography, that could be traced back to your internet connection, for which you are ultimately liable. (Caveat: I am not a lawyer, but it’s my understanding that in most jurisdictions, you are liable. Seek appropriate counsel if you’re not sure.)
- It could slow you down.
There’s the inconvenience factor to take into account. Allowing your neighbor access to your internet could potentially impact your own ability to use it. If they regularly perform large downloads, stream lots of video, or otherwise use it heavily, you could see your own speed slow down significantly. If they get blocked or banned from certain sites, it’s possible that the block could be against your IP address, and thus apply to you as well.
What to do?
Needless to say, the easiest – and safest – answer is to politely decline.
Perhaps you can explain that you have heard some horror stories, and don’t want to put yourself or your neighbor at the risk of experiencing any kind of problem or misunderstanding.
If you want to allow your neighbor to access the internet through your connection, you should at a minimum look to lower your risk. For example, I would make sure to enable the Windows firewall on all of the computers connected to your LAN to protect you from some of the threats of malware or data snooping.
If you have a router that supports what’s often called “guest access”, it does the isolation we’re looking for automatically. Simply give your neighbors access to that “guest” network. This doesn’t address liability or bandwidth issues, but it can be a safe way to share your connection.
If you don’t have a router that supports guest access, another secure solution is to invest in a second router.
The internet would connect to router #1, and your neighbor would connect to router #1.
Router #2 would be connected to the LAN side of router #1, and all of your computers would be connected to router #2. Router #2 maintains our un-trusted/trusted demarcation, in which all of your computers can remain on a trusted local network and everything that is untrusted (including both your neighbor and the internet at large) are on the other side of that router.
Unfortunately, it can be a bit of work to set up correctly, and doesn’t do anything to address the liability or bandwidth issues. It does, however, protect the computers on your local network from any malicious activity on the part of your neighbor, intended or otherwise.