How to Tell if Someone is Using Your WiFi

WiFi is ubiquitous these days, with every coffee shop and library offering it up for free. People are used to being able to tap into the internet on their laptops for free when they’re out and about. So if they find an unsecured network—even one they don’t have permission to use—they may not hesitate to hop on yours.

If the WiFi network that you manage at your home or place of business is not secured, you may find unwelcome visitors riding your airwaves from time to time. If you’ve got it password-protected, there’s still a possibility of an unauthorized user gaining access.

Just how much of a problem that is depends, said Benjamin Miller, a WiFi specialist and author of the blog Sniff Wi-Fi: “Unwanted users on a WiFi network carry a risk varying between ‘none at all’ and ‘quite severe.’”

The most common problem is slower browsing speeds, especially if they are doing activities with high data demands. The most common risk is the unwanted user preventing authorized users of the network from accessing the Internet via Address Resolution Protocol (ARP) spoofing.

The most severe risk is for the user doing the spoofing to perform illegal acts on the Web from inside your network, which will show as coming from your IP address and potentially get you in legal or other trouble.

However, the risk of these incursions happening is fairly low if “client isolation” or some other peer-to-peer blocking protocol is in place and users’ device firewalls are enabled, Miller said.

Why would someone else be on my WiFi?

There are various reasons someone else may be on your WiFi. Here the main ones:

• Your network is unsecured and someone mistakenly connected. When an individual in your vicinity has a device WiFi-enabled, that device will search for open WiFi networks to join. If your network is not secured, it may connect to your WiFi network mistakenly, perhaps without the user even knowing it’s happening.
• Someone who’s been on it before is nearby. If a friend or neighbor has previously used your WiFi, including inputting your password, that same device will be able to automatically sign on to your secured WiFi if the computer or phone is close enough to your router.
• Someone intentionally connected to your unsecure network. Free internet access is the main reason for someone to ride your WiFi without your permission. If you leave your network open, someone nearby who can take advantage of the signal is likely to do so.
• Someone cracked your password. This is the worst-case scenario, in which someone has found a way to infiltrate your secured network. This is much more concerning than an unauthorized user tapping into your unsecure WiFi, since the person may be much more likely to have malicious intent.

How can I tell if someone is using my WiFi?

There are two main ways to see if someone is using your WiFi: using an app and accessing the router’s logs to check for strange media access control (MAC) addresses. Both methods have the drawback that they will show you solely the devices accessing the network at that moment.

“The only way I have seen apps or routers indicate that an unauthorized person is connected to a residential-grade Wi-Fi network is if the owner of the Wi-Fi network happens to look at the app or router interface while the unauthorized user is connected,” Miller said.

While this limits your sleuthing ability, it is still a useful tool for determining how secure your WiFi really is.

Checking via an app

Window users can download Wireless Network Watcher and Mac users can get Who Is On My WiFi, programs that list all the devices connected to your WiFi network at any given time.

Some names on the list may look unfamiliar, but remember that there may be things in your home beyond your laptops and tablets that are using your WiFi, like a smart thermostat, video doorbell or sound system. Try to match up each of these systems with a corresponding name.

Checking via router logs

Checking your wireless router logs is another way to check users or to get more info about the users you discovered using an app.

To find these logs, open a browser window and type your IP address into the address field. A box will ask you for a username and password for your router’s firmware. Your router will have default settings for this unless you set up your own. The default settings may be a username of admin and a password of password. If that doesn’t work, you may be able to use password-recovery to find it, or you can search for the brand of your router followed by “default username and password.”

Once you are able to log-in, navigate to an option with a name to the effect of “attached devices” or “client list” to find a list of all the devices that are accessing your WiFi. Account for ALL WiFi-enabled devices—from your smart sprinkler system to your cloud-connected lighting—before deciding that an unauthorized user is connected.

What should I do if I find someone connected to my WiFi?

Don’t overreact if you see an unknown user connected to your WiFi. Chances are it’s someone looking for a moment of free WiFi as they pause somewhere near your place.

“Personally, I just let them stay connected,” said Miller. “In my experience, unauthorized WiFi users are on my WiFi for a short period of time.”

If you want to ensure the users you see snagging your WiFi can’t keep doing so, the first step is to reset your router’s password—or establish one if you don’t currently have one.

To do so, log into your wireless router’s administrative controls as you did when checking your router logs. Find a menu called “wireless” or something along those lines, and look for “security settings” or a similar option. You’ll be able to set the type of encryption, the authentication mode, the pre-shared key format and the pre-shared key. The pre-shared key is your password. After you have changed these things, reboot your router.

Miller recommends using WPA2 encryption, personal authentication mode, a passphrase key format, and a long password.

“If I ever noticed an unauthorized user connecting for multiple days, I would use a WPA2 Personal passphrase, with a length of at least 20 characters, which is impervious to cracking via off-the-rack hacking tools,” he said.

Creating a strong password and maintaining your firewalls will go a long way toward ensuring that your network remains safe from unwanted use.

Protect your data

WiFi is a valuable commodity in this interconnected world, so it’s no surprise that you may find people looking for a free hookup. Allowing unauthorized users access to your WiFi comes with relatively low risk, though it’s a good idea to institute strong network security to ensure you’ll have no problems with anything unknown users might do on your network.