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ZDNET's key takeaways

• Wi-Fi slowing down, or unfamiliar devices on your network? You may have an intruder.
• There are various ways to quietly confirm your suspicions.
• Here's how to verify whether someone is using your Wi-F, and how to stop it.

Surveillance continues creeping into our daily lives. Facial recognition cameras litter the streets; we are constantly monitored online, and it's rare to attend a social event without a camera being waved in our faces. 

However, the prospect of such surveillance entering our homes can be even more disturbing. Have you ever noticed an unfamiliar device connecting to your home network? Or is your bandwidth decreasing rapidly? If someone is lurking on your Wi-Fi, that's a serious invasion of your privacy, not to mention unfair, as you are paying for a service.  

Also: Your home Wi-Fi isn't nearly as private as it should be - 6 free ways to lock it down

If someone gains access to your Wi-Fi, they can do more than just steal the bandwidth you're paying for; they can also endanger your privacy and security, steal your data, and potentially even tie you to someone else's illegal online activities.

Fortunately, it can be quite straightforward to identify strangers on your network and boot them out. Find out how to detect them, escort them out -- and keep them out. Look for these three warning signs

1. Your internet feels slow 

There are many reasons your internet connection suddenly goes from fast to sluggish, or you experience unexpected drops and lags. 

Legitimate reasons for internet interruptions include faulty router hardware, damage to external cables, weak signal strength, or issues with your internet service provider (ISP), such as regional blackouts. When it comes to software, using the wrong VPN or VPN server to protect your connection can also lead to severe speed drops. 

Also: Best VPN services: Expert tested and recommended

However, a slow internet connection can also be caused by unwanted visitors on your network who are stealing your bandwidth. The more devices that are connected to a Wi-Fi point, the higher the traffic demand. This is especially true if someone is streaming in high resolution, gaming online, or torrenting large files. 

It can be difficult to tell the difference between legitimate reasons for your internet slowing down and latency issues caused by a freeloader. Start by checking your ISP's service status for any problems, visit downforeveryoneorjustme.com to check disruptions to online services, and check your hardware, including cables and connectors -- as loose cabling can also cause broadband issues.

2. Unknown or unrecognized devices on your network

If someone is using your Wi-Fi without permission, they will need to connect a device such as a smartphone, PC, speaker, or smart home device.

You may also expect low speeds and high traffic loads, video buffering, and your own devices might even disconnect without warning as your network creaks under the strain if too many devices -- authorized or otherwise -- are fighting for the same bandwidth.

Also: Should you switch to a Wi-Fi 7 mesh router? My home testing reveals what got better (and what didn't)

Another symptom may be the active traffic and connections your router shows you. For example, flashing lights on your hardware may indicate network activity when you don't expect it. However, you should check your router's manual for other reasons before assuming it is due to unknown devices on your network.

3. Router stops working, or you lose Wi-Fi access

If your Wi-Fi suddenly stops working, it could be a sign that someone is tampering with your setup without your permission. It may be even more suspicious if your network turns off and on at specific times, for example, when you are playing music. 

To maintain access to your Wi-Fi hotspot, someone needs to be close by -- and if you have a less-than-cordial relationship with your neighbors or someone else in your home, there's always the risk that someone is deliberately messing with your Wi-Fi. For example, if someone has accessed your Wi-Fi and its gateway, they can disconnect your connection at any time. 

Also: How I turned my old Android phone into a Wi-Fi extender - and fixed dead spots at home

Furthermore, if the Wi-Fi password has changed and you have been kicked out, someone has probably seized control and changed it without your permission.

However, you should keep in mind that hardware failures may be at fault rather than a miscreant. Routers fail over time, ISPs provide poor service, and even weather conditions may impact your connectivity. If there's a pattern in interrupted services, though -- for example, at a particular hour every day -- it may be deliberate.

How can someone use my Wi-Fi without permission?

The simplest answer is often the right one, and with Wi-Fi, it usually comes down to your password.

A weak, easy-to-guess password might be at fault, or one that has been shared without your permission. It is also possible for people to perform brute-force or dictionary attacks on some Wi-Fi networks, running a program that tests combinations of letters, numbers, phrases, and special characters to guess the password and break in.

There's also the issue of Wi-Fi security itself. Wi-Fi standards include WPA, WPA2, and WPA3. You can find out your Wi-Fi's protocol by diving into the wireless settings of your router: on MacOS, hold down the option key and click your Wi-Fi icon; on Windows, go to settings, Wi-Fi, and then the security tab.

The higher the standard, the more difficult it is for attackers to intrude -- but if there are poor passwords or network misconfigurations, it is far from impossible, no matter the protocol. 

Outdated router firmware may also put your Wi-Fi at risk, as it may contain vulnerabilities that can be exploited. You can check your hardware's firmware version on the router gateway to see whether it updates automatically or if you need to update it manually via the web. 

Also: How to update your router's firmware (and why you should be doing it regularly)

How can I find suspicious devices on my network?

The easiest way to confirm your suspicion that someone is on your Wi-Fi network without permission is to run a network scan.

The first step is to access your router's gateway via a browser. Your router manufacturer or ISP should provide you with the address you need to visit; if you are unsure, try 192.168.0.1 or 192.168.1.1. Another option is to open your smartphone's Wi-Fi settings, tap your home Wi-Fi hotspot, go to Settings, and choose "Manage router" if available -- it should take you straight to the gateway.

Also: I used this free tool to see what data the internet has on me - and the results were disturbing

When you access your gateway via a browser, you may be warned that the connection isn't private, and you will need to sign in with your hub or router password. Once you're in, depending on the ISP, you may be able to see all devices listed on your network.

Alternatively, use a dedicated network scanner to find and display all devices and IP addresses, then check for anything unfamiliar. Network scanner options include Fing, Nmap, and WiFi Analyzer.

What else can I do to confirm Wi-Fi interlopers?

• Run a speed check: If you can, run it at different times of day to see if you spot any patterns. For example, if you have a low starting speed and someone is streaming Netflix in 4K after they've come home from work, you will probably notice an immediate drop in service -- or an inability to load anything entirely. 
• Check connection logs: Those comfortable with technology can generate logs using tools such as Nmap and Fing, dedicated network and Wi-Fi mapping tools. These could give you an idea of what is connecting, for how long, and when.
• Consider your environment: For example, if a neighbor is blaring their Spotify playlist, and the music suddenly stops when you disconnect your router, you may have found the cause of your intermittent Wi-Fi service.

Also: Your smart home may be at risk - 6 ways experts protect your devices from attacks

How do I stop network intruders?

• Change your password: If someone has been quietly using your Wi-Fi by guessing your password, changing your password is the quickest and easiest way to kick them out. You can change your password by accessing your router's gateway, as described above.
• Choose a better password: Don't rely on default passwords to protect your Wi-Fi, or something that's easy to remember or share with your guests. Create something complex and, while it is a pain, take the time to reconnect all your devices to your Wi-Fi, as changing your password means devices logged in with your old credentials will be removed from the network. While we don't recommend writing down the passwords for your important online accounts, for residential Wi-Fi, there's little risk in writing them down and keeping them in a safe place at home. 
• Check your admin password: You should also check your administrator password in your router/ISP gateway. If they are default options, such as admin/admin, change them to something stronger and store them somewhere safe.
• Check your Wi-Fi security and upgrade if necessary: If you are running on an obsolete or legacy protocol, such as WPA, it simply won't be secure, and it might be easy to break into, no matter what you do. In this case, you'll need to upgrade the protocol. You can usually do this yourself by visiting your router's gateway and changing its encryption standard under security or wireless settings. You may need to set a new password and reconnect your devices afterward. If you can, WPA3 is best. 
• Consider setting up a guest network: If you sometimes need to share your Wi-Fi, set up a guest network. This creates a separate access point away from your main network and devices, thereby giving you far more control over who is connected, for how long, and how much bandwidth they can use.