A student developer in Cambridge, Massachusetts, has created a Google Chrome browser extension that lets users find their Facebook friends’ locations with a scary amount of accuracy. The extension, named “Marauder’s Map” in reference to the magical tracking map in Harry Potter, uses location data embedded within the messages you send in Facebook Messenger.

The tracking depends on Facebook’s “Location Sharing” feature, which is enabled by default in the site’s iOS and Android apps. And that information doesn’t go away; a writer for The Next Web was able to track his locations dating back to 2013. When Facebook Messenger users install the app, they’re encouraged to enable Location Sharing. However, Facebook does not make it clear to these new users what that information means. Most people probably haven’t thought about their geo-location data at all, but this hack brings it life in scary detail. In fact, it can pin down your location to within one meter.

Fortunately, there are a few easy ways to prevent this tool from following your every move. You can either go to “Settings” inside of your Messenger app and uncheck “Location,” or you can go to your phone’s settings and disable location services for each of your apps individually.  Also, be careful who you friend on Facebook. You don’t want casual acquaintances using this tool to track you down! Make sure your family and friends understand the dangers of location based services!