His application, called Marauder's Map — a clever name that Harry Potter fans will appreciate — was a Chrome extension that used data from Facebook Messenger to map where users were when they sent messages. The app also showed the locations, which were accurate to within three feet, in a group chat with people he barely knew. That meant complete strangers could hypothetically see that he had messaged them from a Starbucks around the corner, while he could see that they had messaged from their dorms.

The app capitalized on a privacy flaw that Facebook had been aware of for about three years: the Facebook Messenger app automatically shared users' locations with anyone who they messaged.

Khanna tweeted about the app on May 26 and posted about it on Reddit and Medium. Marauder's Map began to go viral. Facebook, never one to miss a trend, quickly caught on.