Inaudible voice command injection is one of the most threatening attacks towards voice assistants. Existing attacks aim at injecting the attack signals over the air, but they require the access to the authorized user’s voice for activating the voice assistants. Moreover, the effectiveness of the attacks can be greatly deteriorated in a noisy environment. In this paper, we explore a new type of channel, the power line side-channel, to launch the inaudible voice command injection. By injecting the audio signals over the power line through a modified charging cable, the attack becomes more resilient against various environmental factors and liveness detection models. Meanwhile, the smartphone audio output can be eavesdropped through the modified cable, enabling a highly-interactive attack.

To exploit the power line side-channel, we present GhostTalk , a new hidden voice attack that is capable of injecting and eavesdropping simultaneously. Via a quick modification of the power bank cables, the attackers could launch interactive attacks by remotely making a phone call or capturing private information from the voice assistants. GhostTalk overcomes the challenge of bypassing the speaker verification system by stealthily triggering a switch component to simulate the press button on the headphone. In case when the smartphones are charged by an unaltered standard cable, we discover that it is possible to recover the audio signal from smartphone loudspeakers by monitoring the charging current on the power line. To demonstrate the feasibility, we design GhostTalk-SC , an adaptive eavesdropper system targeting smartphones charged in the public USB ports. To correctly recognize the private information in the audio, GhostTalk-SC carefully extracts audio spectra and integrates a neural network model to classify spoken digits in the speech.

We launch GhostTalk and GhostTalk-SC attacks towards 9 main-stream commodity smartphones. The experimental results prove that GhostTalk can inject unauthorized voice commands to different smartphones with 100% success rate, and the injected audios can fool human ears and multiple liveness detection models. Moreover, GhostTalk-SC achieves 92% accuracy on average for recognizing spoken digits on different smartphones, which makes it an easily-deployable but highly-effective attack that could infiltrate sensitive information such as passwords and verification codes. For defense, we provide countermeasure recommendations to defend against this new threat.

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