Security researcher Ahmed Hassan has shown that spoofing the Android’s “People Nearby” feature allows him to pinpoint the physical location of Telegram users:
Using readily available software and a rooted Android device, he’s able to spoof the location his device reports to Telegram servers. By using just three different locations and measuring the corresponding distance reported by People Nearby, he is able to pinpoint a user’s precise location.
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.
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.