Mobile devices often distribute measurements from a single physical sensor to
multiple applications using software-based multiplexing. On Android devices,
the highest requested sampling frequency is returned to all applications even
if other applications request measurements at lower frequencies. In this paper,
we demonstrate that this design choice exposes practically exploitable
side-channels based on frequency-key shifting. By carefully modulating sensor
sampling frequencies in software, we show that unprivileged malicious
applications can construct reliable spectral covert channels that bypass
existing security mechanisms, e.g. Android’s permissions framework. Moreover,
we present a variant of this technique that allows an unprivileged malicious
observer app to fingerprint other device applications and user interactions at
a coarse-grained level. Both techniques do not impose any assumptions beyond
application installation and accessing standard mobile services via the Android
Sensors SDK. As such, they open a powerful attack vector that exploits subtle
yet insecure design choices in mobile sensor stacks.

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