Telegram lets users create local groups within a geographical area. Hassan said that scammers often spoof their location to crash such groups and then peddle fake bitcoin investments, hacking tools, stolen social security numbers, and other scams. A proof-of-concept video the researcher sent to Telegram showed how he could discern the address of a People Nearby user when he used a free GPS spoofing app to make his phone report just three different locations. He then drew a circle around each of the three locations with a radius of the distance reported by Telegram. The user’s precise location was where all three intersected.
In a blog post, Hassan included an email from Telegram in response to the report he had sent them. It noted that People Nearby isn’t enabled by default and that “it’s expected that determining the exact location is possible under certain conditions.” People Nearby poses the biggest threat to people using Android devices, since they report a user’s location with enough granularity to make Hassan’s attack work. The recently released iOS 14, by contrast, allows users to divulge only a rough approximation of their location. People who use this feature aren’t as exposed. Fixing the problem — or at least making it much harder to exploit it — wouldn’t be hard from a technical perspective. Rounding locations to the nearest mile and adding some random bits generally suffices. When the Tinder app had a similar disclosure vulnerability, developers used this kind of technique to fix it.