Augmented/Mixed Reality (AR/MR) devices are unique from other mobile systems because of their capability to offer an immersive multi-user collaborative experience. While previous studies have explored privacy and security aspects of multiple user interactions in AR/MR, a less-explored area is the vulnerability of gait privacy. Gait is considered a private state because it is a highly individualistic and a distinctive biometric trait. Thus, preserving gait privacy in emerging AR/MR systems is crucial to safeguard individuals from potential identity tracking and unauthorized profiling. This paper first introduces GaitExtract, a framework designed to automatically detect gait information in humans, shedding light on the nuances of gait privacy in AR/MR. In this paper, we designed GaitExtract, a framework that can automatically detect the outside gait information of a human and investigate the vulnerability of gait privacy in AR. In a user study with $20$ participants, our findings reveal that participants were uniquely identifiable with an accuracy of up to $78\%$ using GaitExtract. Consequently, we propose GaitGuard, a system that safeguards gait information of people appearing in the camera view of the AR/MR device. Furthermore, we tested GaitGuard in an MR collaborative application, achieving $22$ fps while streaming mitigated frames to the collaborative server. Our user-study survey indicated that users are more comfortable with releasing videos of them walking when GaitGuard is applied to the frames. These results underscore the efficacy and practicality of GaitGuard in mitigating gait privacy concerns in MR contexts.

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