Voice assistants (VAs) have become integral to daily life, yet their always-on microphones make them attractive targets for attacks that threaten user privacy and safety. We present MetaGuardian, the first system to leverage acoustic metamaterials to defend against three major classes of attacks for VAs - inaudible, adversarial, and laser-based - within a single, portable design. Unlike prior defenses, MetaGuardian can be seamlessly integrated into the enclosures of commercial smart devices, providing strong protection without requiring software modification, hardware redesign, or costly machine learning models. MetaGuardian leverages mutual impedance effects between metamaterial units to extend the protection range to 16–40 kHz, effectively blocking wideband inaudible attacks. It also employs a carefully designed coiled space structure to disrupt adversarial signals while preserving normal VA operations. Its universal design allows flexible adaptation to different devices, striking a balance between portability and protection effectiveness. In controlled evaluations, MetaGuardian achieves a high defense success rate across all attack types, offering a practical and reliable foundation for securing VAs on smart devices.

We present METAATTACK, the first approach to leverage acoustic metamaterials for inaudible attacks for voice control systems. Compared to the state-of-the-art inaudible attacks requiring complex and large speaker setups, METAATTACK achieves a longer attacking range and higher accuracy using a compact, portable device small enough to be put into a carry bag. These improvements in portability and stealth have led to the practical applicability of inaudible attacks and their adaptation to a wider range of scenarios. We demonstrate how the recent advancement in metamaterials can be utilized to design a voice attack system with carefully selected implementation parameters and commercial off-the-shelf components. We showcase that METAATTACK can be used to launch inaudible attacks for representative voice-controlled personal assistants, including Siri, Alexa, Google Assistant, XiaoAI, and Xiaoyi. The average success rate of all assistants is 76%, with a range of 8.85 m.