# Recent Trends of Authentication Methods in Extended Reality: A Survey
## 1 Introduction  
Engagement in various online activities, such as education and meetings, has heightened during the COVID-19 pandemic. However, it has limitations in terms of real-world experiences, physical expression, and collaboration since online environments only provide simple interaction services using microphones, cameras, and screens. In the recent past, Metaverse environments--interconnected virtual spaces where users could interact using Extended Reality (XR), a general term for immersive technologies namely Augmented Reality (AR), Virtual Reality (VR), and Mixed Reality (MR)--have attracted interest to unravel these limitations. However, the realization of a fully functional metaverse is still complicated by the fact that it requires the integration of different technologies. Many advanced technologies, such as 5G, Web 3.0, Blockchain, and Artificial Intelligence (AI), are becoming more promising and feasible to make the metaverse a reality, which marks a fundamental change in terms of human-computer interactions (HCI), ubiquity, and decentralization. This has caught the attention of numerous large tech corporations, which are investing billions in developing various metaverse platforms and XR technologies. For instance, Microsoft offers Microsoft Mesh for live virtual collaborations, Meta offers Horizon Workrooms for immersive virtual workspaces, Nvidia offers Omniverse for the development of advanced 3D applications, and Decentraland which is owned by its users and developers, among many others. As a result, XR innovative solutions naming AR developing platforms (e.g., Google ARCore, Apple ARKit), VR headsets (e.g., HTC Vive Pro 2), and MR glasses (e.g., Microsoft Hololens) have progressed into marketable and affordable technologies. Furthermore, the numbers of XR users are anticipated to increase, and XR experiences are expected to become more immersive. Given the future potential of the Metaverse, there are significant cybersecurity threats that need to be resolved before the metaverse can be put to practical use. Sensitive user data such as identity, passwords, and biometric information is likely to become vulnerable to privacy and security breaches. Extended reality devices collect massive amounts of sensitive biometric information about their users. Various privacy and security risks stemming from this considerable data collection, inputs, outputs, and user interactions have been identified by researchers. Another difficulty that applies to XR wearables is that of user authentication, which is used as a first barrier for technology devices and systems. Efficient authentication techniques and strong access control models aim to identify authorized and unauthorized access. Less reliable authentication systems lead to users risking their security. The absence of a physical keyboard in XR makes it challenging to authenticate users and secure user profiles and the data generated by XR devices. Therefore, additional safety measures will be needed to ensure users' privacy in the future. In essence, to further understand authentication and its related privacy risks in the XR ecosystem, it became crucial to study XR authentication as a whole instead of segmented AR, MR, and VR technologies as it has been typically practiced. The evolution of authentication in XR has been marked by a shift from traditional methods to more advanced and context-aware approaches. At first, XR authentication relied heavily on conventional methods like passwords and PINs, leading to usability challenges within immersive environments. As XR technologies developed, biometric authentication methods gained attention due to their convenience and added layers of security. Behavioral biometrics such as eye tracking and gait recognition seem promising considering users' unique interactions in XR environments. This allows for smoother and more robust authentication solutions suitable for the immersive nature of XR, enhancing both security and user experience across various applications and sectors.