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Stalk Facebook Users with 'Marauder's Map' App

Time and time again, we find that yet another popular institution is not secure enough for our trust. One day it's Target and credit cards, the next it's the IRS and tax-return data. Now it's Facebook Messenger, which records every step you take and every move you make. Thanks to Marauder's Map, a Chrome plugin made by Harvard undergraduate Aran Khanna, your daily routines can now be followed by all your Facebook friends.

Named after a magical chart in the Harry Potter novels, Marauder's Map is extremely accurate, able to pinpoint location to less than a meter. It compiles the data stored in Messenger transcripts, and plots the Messenger user's commute, including dots for locations where he or she stopped for a period of time. So even if users don't have dense geotagged Instagram accounts that show their favorite coffeeshops, burger houses and gyms, it's now possible to have Marauder's Map sketch that for you.

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Geolocation data is retained by Messenger if a user clicked "Yes" when Messenger first requested permission to access his or her location. This mass of data was accessed by Khanna, who will himself be begin an internship at Facebook in June. Khanna explained in a blog post that he made Marauders Map to teach users about privacy, not to allow them to be stalked:

I decided to write this extension because we are constantly being told how we are losing privacy with the increasing digitization of our lives. However, the consequences never seem tangible. With this code you can see for yourself the potentially invasive usage of the information you share, and decide for yourself if this is something you should worry about.

At Facebook's request, Khanna disabled the Chrome extension, but Marauder's Map can still be installed using the map's GitHub repository. While the reference to J.K. Rowling's novels was meant as a cute reference, Facebook had to be concerned with the potential for nefarious or malicious use.

If you're just now realizing you've shared your location on Facebook Messenger this whole time, and want to turn that feature off, it's relatively simple.

On iOS, open Settings -> Privacy -> Location Services and set Messenger's permissions to Never.

On Android, you can disable location in the Facebook Messenger app itself. Tap on the menu icon -> Settings -> Location Services and uncheck the box for location services.

Henry T. Casey is a Staff Writer at Tom's Guide. Follow him on Twitter @henrytcasey. Follow us@tomsguide, on Facebook and on Google+.