The thermostat in question has a large LCD display, runs the operating system Linux, and has an SD card that allows users to load custom settings or wallpapers. The researchers found that the thermostat didn’t really check what kind of files it was running and executing. In theory, this would allow a malicious hacker to hide malware into an application or what looks like a picture and trick users to transfer it on the thermostat, making it run automatically.

Source: Hackers Make the First-Ever Ransomware for Smart Thermostats