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There is a security-bug in the first versions HomeKit on iOS 11.2
In version 11.2 of iOS, it is possible to take control of your all you HomeKit devices like smart lights, smart locks and smart thermostats.
Upgrade your iPhone, iPad or iPod Touch to iOS 11.2.x or iOS 11.3 or newer when they are released !
For now Apple has fixed this by altering the settings on the HomeKit servers, but a solution for iOS itself is in the works, so upgrade to a newer version of iOS when there is one available !