Ever noticed how almost every other message you receive on Facebook Messenger has a location attached to it? Well, creepier and creepier: a Harvard computer science and mathematics undergraduate, Aran Khanna, has written an app that exploits this to allow people to pinpoint and track the locations of other Facebook Messenger users; the tracked users don't even have to be friends of the app user - or stalker! - in question.

The app, which Khanna called Marauder's Map (after the magical map in the Harry Potter books that revealed the identities, locations and movements of all), exploits Facebook's default location settings to grab GPS co-ordinates (accurate to five decimal places!) from communications, and thereby to identify the location of users to an accuracy of less than a meter. It’s accurate enough not just to identify the building in which a user lives or works, but indeed which part of a room they’re in. And because time stamps are included in the data, the app can be used to track movements of an individual minute by minute, over the course of months or even years.

It might be of little interest to most of your friends to learn the minute details of your movements between your home, office, the grocery store, and so on. But, as I’ve noted in a previous post, it’s also easy to see what a godsend such information could be to stalkers, paranoid spouses, criminals, overzealous government agencies (let’s remember we live in an age where the US government has carried out extrajudicial killings of its own citizens merely because of apparent links to suspected terrorists), and more.

The Marauder’s Map app was apparently developed to demonstrate, in a tangible way, just how much meta-data we unwittingly transmit with each message we send, especially if we are not careful about manually changing default privacy and location settings. The moral here is clear: double check your privacy settings!