Robust and Precise Application Fingerprinting on 5G Physical Uplink Channel
For security researchers and network operators, this work exposes a new vulnerability in 5G that undermines encryption, but the attack is domain-specific and requires physical proximity.
The paper reveals a physical-layer side channel in 5G that bypasses encryption, enabling application fingerprinting by observing uplink spectrum. Crosshair achieves high accuracy in application recognition, outperforming SOTA methods and maintaining performance across varying MCS scenarios.
Air fingerprinting infers application activity by sniffing metadata from cellular control channels. 5G encrypts these channels, breaking the attack chain that prior attacks depend on. This paper reveals a physical-layer side channel that bypasses encryption: under the link adaptation mandated by the cellular communication standard, the uplink Modulation and Coding Scheme (MCS) remains stable, so the number of Physical Resource Blocks (PRBs) occupied by a transmission accurately reflects the IP packet length. Combined with the uplink control channel that carries downlink information, an attacker can reconstruct a bidirectional traffic profile. This bidirectional information recovery can be achieved simply by observing the uplink spectrum, without decoding any channel. Building on this side channel, we design Crosshair, a passive three-step attack. First, a blind extraction stage recovers the uplink physical channel occupancy from raw IQ samples via energy detection, reconstructing bidirectional traffic from uplink spectrum. Second, we design a data augmentation method that synthesizes spectral profiles across diverse channel conditions, eliminating the need for prior knowledge of the communication environment. Third, cross-modal alignment embeds the spectral and IP domains into a shared space, enabling new applications to be enrolled from a collected IP trace alone. Extensive experiments on a 5G NR testbed demonstrate the robustness and precision of Crosshair: it outperforms the State-of-the-Art (SOTA) physical layer fingerprinting method in application recognition accuracy, and maintains high accuracy in cross-MCS scenarios.