  Short Message Service (SMS) remains one of the most popular communication
channels since its introduction in 2G cellular networks. In this paper, we
demonstrate that merely receiving silent SMS messages regularly opens a
stealthy side-channel that allows other regular network users to infer the
whereabouts of the SMS recipient. The core idea is that receiving an SMS
inevitably generates Delivery Reports whose reception bestows a timing attack
vector at the sender. We conducted experiments across various countries,
operators, and devices to show that an attacker can deduce the location of an
SMS recipient by analyzing timing measurements from typical receiver locations.
Our results show that, after training an ML model, the SMS sender can
accurately determine multiple locations of the recipient. For example, our
model achieves up to 96% accuracy for locations across different countries, and
86% for two locations within Belgium. Due to the way cellular networks are
designed, it is difficult to prevent Delivery Reports from being returned to
the originator making it challenging to thwart this covert attack without
making fundamental changes to the network architecture.
