Not even your refrigerator is safe from hackers.

Between Dec. 23 and Jan. 6, Internet security firm Proofpoint discovered a cyber attack that delivered about 750,000 malicious emails. There's nothing unusual about that. What surprised the experts was what the hackers used—the messages were coming from inside the kitchen.

"In the course of tracking down the source of these attacks we discovered many of the messages were not emanating from traditional PCs or servers," says David Knight, Proofpoint general manager of information security. "These were from embedded devices—home entertainment systems, home security, smart TVs, and a refrigerator."

This is probably the first time the "Internet of Things" (all the smart devices that now connect to you, whether your fridge or your fitness wristband) has been used in a major hack. And it certainly won't be the last. Technology research firm Gartner estimates that there will be about 30 billion Internet of Things devices used worldwide by 2020, versus 2.5 billion in 2009.

How do you hack a refrigerator? Currently, most smart home devices run on BusyBox, which is known as the Swiss Army Knife of embedded Linux because it is so versatile and easy to use. Attackers can search the Internet using complex automated scripts for vulnerable BusyBox software configurations. From there they can easily crack the password, since most people don't change the administrative credentials. (You might be on top of your computer's security settings, but have you thought about your refrigerator's?). Then they can reconfigure the embedded email server on that device to relay malicious emails. It's simple and stealthy. When low volumes of messages come from many devices, it's harder to track down the source.

While the types of attacks that Proofpoint uncovered are unsettling, and finding out that your connected toilet was used to send malware may leave you feeling as dirty and used as a plumber's favorite hanky, these attacks are not necessarily harmful to you or your devices. However, the same methods can be used to target you and your home. Burglars could turn off your security system and listen in on your baby monitor. Pranksters could turn off all your lights and blare Rick Astley on your TV. And you might never know you've even been hacked. While PCs usually run slower after being hijacked, embedded devices might not show any signs of foul play. That means something in your home could have already been hacked.

Here are a four ways to make sure your appliances don't become targets for cyber criminals:

Change the administrative settings.

Most devices have insecure default passwords or pin numbers, and when you get a new coffeemaker, your last thought is what the login should be. Come up with a password that is just as complex as what you would use for your email or bank account.

Turn off the smart features.

If you just want to use your new refrigerator as, you know, a refrigerator, then don't hook it up to the Internet. It's becoming more difficult to buy new appliances that don't have some level of connectivity, though sometimes those features are just unnecessary extras intended to justify an inflated price. If you don't want your gadget on the grid, make sure it does not automatically connect.

Install a firewall.

Even the Internet of Things needs firewalls, though having security is not as big a priority for many home automation products as it is for computers. You might have to add a firewall to the point of connection to the router. Most routers designed for home use have firewalls, so if your device is hooked up to a wireless router, you're probably already behind one. But you should confirm that your router provides sufficient protection, and be sure to change a router's administration credentials when you buy it as well.

Be aware.

None of these solutions is all that surprising. It's really just a matter of treating your appliances and home systems like the computers that they have become.

Smart devices provide shortcuts to installation and configuration, meaning you don't always know exactly what you're putting in your home. Our associate home editor, David Agrell, just installed a Nest Thermostat, and said he was amazed at how easy it was to install: "It literally asked me one question: What's the coldest I want it to get?" he said. He set it at 68 degrees Fahrenheit (after a short debate with his wife) and left it at that. Even a PopMech editor didn't think about security when installing a smart home device (at least, not until Google's purchase of Nest put these concerns front and center.)

Take the time to get to know these products. Find out if your new connected device uses a password, if it connects through Wi-Fi or Bluetooth. Take notes on everything—maybe even in an old-fashioned notebook.

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