Metaverse devices, such as virtual reality (VR), have seen substantial development and widespread applications in numerous areas. Although recent studies have revealed privacy leakages in VR, these vulnerabilities were limited in the scope of observable behaviors in virtual scenes (e.g., what a user is seeing). In this work, we uncover the feasibility of going beyond the scope of observable user behaviors to unobservable brain EEG-correlated representations (e.g., what a user is perceiving) by leveraging unrestricted motion sensors in VR headsets to reconstruct brain EEG signals, a seemingly neglected but promising vector. The insight is that the inbuilt motion sensors (e.g., accelerometers) in the VR headset can capture subtle vibrations induced by pupillary responses, which are highly correlated with users' visual stimuli and in-brain perceptions. Therefore, we design and implement BraVeSpy to systematically investigate and demonstrate the feasibility of this severe privacy leakage originating from brain EEG-correlated representations reconstructed from variations of inbuilt motion sensors. Our extensive evaluation results from different VR devices show that BraVeSpy, for the first time in the Metaverse, can reveal unobservable privacy, where we successfully unveiled perceptive images in the brain with 52.0%-67.2% accuracy. In particular, we also find that BraVeSpy outperforms the current approaches that are limited to coarse-grained inference of observable behaviors and achieves over 85.0% accuracy in inferring user activity-related sensitive information, such as fingerprinting websites, apps, and streaming videos, and over 96.0% accuracy in user de-anonymization, gaze movement tracking, and virtual keystroke inference.

翻译：元宇宙设备（如虚拟现实VR）在众多领域经历了实质性发展并得到广泛应用。尽管近期研究揭示了VR中的隐私泄露问题，但这些脆弱性仅限于虚拟场景中可观察行为的范畴（例如用户正在观看的内容）。在本工作中，我们揭示了突破用户可观察行为范畴的可能性——通过利用VR头显中无限制的运动传感器重建脑电信号，从而实现不可观察的脑电图关联表征（例如用户正在感知的内容），这是一条看似被忽视但极具前景的路径。其核心洞察在于：VR头显内置的运动传感器（如加速度计）能够捕捉由瞳孔反应引发的细微振动，而这种振动与用户的视觉刺激及脑内感知高度相关。为此，我们设计并实现了BraVeSpy系统，以系统性地研究并论证源自内置运动传感器变化重建的脑电图关联表征所引发的严重隐私泄露的可行性。基于不同VR设备的广泛评估结果表明，BraVeSpy首次在元宇宙中揭示了不可观察的隐私，我们成功以52.0%-67.2%的准确率重建了大脑中的感知图像。值得注意的是，我们还发现BraVeSpy优于当前局限于粗粒度可观察行为推断的方法，在推断用户活动相关敏感信息（如对网站、应用程序和流媒体视频进行指纹识别）时准确率超过85.0%，在用户去匿名化、凝视运动追踪及虚拟击键推断中准确率超过96.0%。