

Syed Rafiul Hussain (Purdue University), Mitziu Echeverria (University of Iowa), Omar Chowdhury (University of Iowa), Ninghui Li (Purdue University), Elisa Bertino (Purdue University)

The cellular paging (broadcast) protocol strives to
the balance between a cellular device's energy consumption and quality-of-service by allowing the device to *only* periodically poll for pending services in its idle, low-power state. For a given cellular device and serving network, the exact time periods when the device polls for services (called the *paging occasion*) are
fixed by design in the 4G/5G cellular protocol. In this paper, we show that the fixed nature of paging occasions can be exploited by an adversary in the vicinity of a victim to associate the victim's soft-identity (e.g., phone number, Twitter handle) with its paging occasion, with only a modest cost, through an attack dubbed $mathsf{ToRPEDO}$. Consequently, $mathsf{ToRPEDO}$ can enable an adversary to verify a victim's coarse-grained location information, inject fabricated paging messages, and mount denial-of-service attacks. We also demonstrate that, in 4G and 5G, it is plausible for an adversary to retrieve a victim device's persistent identity (i.e., IMSI) with a brute-force $mathsf{IMSI-Cracking}$ attack while using $mathsf{ToRPEDO}$ as an attack sub-step. Our further investigation on 4G paging protocol deployments also identified an *implementation oversight* of several network providers which enables the adversary to launch an attack, named $mathsf{PIERCER}$, for associating a victim's phone number with its IMSI; subsequently allowing targeted user location tracking. All of our attacks have been validated and evaluated in the wild using commodity hardware and software. We finally discuss potential countermeasures against the presented attacks.