The Permissions-Policy directive (formerly known as Feature-Policy) is a critical security mechanism that allows web developers and website owners to control which browser features and device APIs their websites can access.

One of the more advanced directives within this policy is "xr-spatial-tracking", which governs access to the WebXR API. This API enables immersive experiences like augmented reality (AR) and virtual reality (VR) by allowing websites to access spatial tracking capabilities on user devices. These features are used to track the position and orientation of devices in real-time, offering applications ranging from AR games to VR experiences. However, as with any technology that interacts with real-world data, this introduces significant privacy, security, and safety concerns.

History and Origin of Permissions-Policy

The Permissions-Policy header was introduced by Google in 2018 under the name Feature-Policy. It was designed to give developers granular control over which powerful browser features their websites could access. With the increasing complexity of web applications, many gained access to sensitive data or hardware components, such as sensors, cameras, and location services, making it necessary to limit or control these capabilities to protect users from potential misuse.

The WebXR Device API, developed by Google in collaboration with the W3C Immersive Web Working Group, was created to enable web-based AR and VR experiences. The API provides access to spatial tracking information, which allows websites to determine a device's position, orientation, and motion in real-time. This capability is essential for immersive applications, but it also opens up risks related to privacy, data collection, and security. The xr-spatial-tracking directive was added to Permissions-Policy to mitigate these risks by allowing developers to control when and how this API can be accessed.

What Does the XR-Spatial-Tracking Directive Do?

The "xr-spatial-tracking" directive in the Permissions-Policy header controls whether a website can access the WebXR API to enable AR and VR experiences that rely on spatial tracking. Spatial tracking allows websites to detect the position, movement, and orientation of a user's device in 3D space, which is crucial for immersive AR/VR content.

For example:

• Setting xr-spatial-tracking=() disallows all access to the WebXR API, ensuring that the website and any embedded third-party content cannot access spatial tracking features.
• Setting xr-spatial-tracking=* allows full access to the WebXR API, enabling the website and all its embedded content to request spatial tracking data.

This control is crucial because access to spatial tracking information can expose users to privacy violations and security risks if misused or accessed without proper consent.

Why Was It Added?

The xr-spatial-tracking directive was introduced to address several significant privacy, security, and ethical concerns that arise when websites access spatial tracking data through the WebXR API:

1. Preventing Unauthorised Spatial Tracking: Spatial tracking data allows websites to gather highly detailed information about a user’s physical environment, movements, and even location. Without proper controls, malicious websites or embedded third-party scripts could use this information to track users' movements without their consent. The xr-spatial-tracking directive helps mitigate this risk by ensuring that only trusted and necessary parts of a website can access spatial tracking data.

2. Protecting User Privacy: The ability to track a user's physical movement in space, including their body orientation and device location, introduces substantial privacy concerns. For example, an AR app could potentially map out a user’s environment, exposing personal details about their surroundings. The xr-spatial-tracking directive was added to ensure that users’ spatial data is not accessed by untrusted or unnecessary websites, protecting them from potential privacy violations.

3. Minimising the Risk of Data Collection and Surveillance: Spatial tracking data, when combined with other sources, could be used to create detailed behavioural profiles of users. This data could be exploited for surveillance or profiling purposes, potentially exposing users to privacy breaches or targeted attacks. By giving developers control over which content can access spatial tracking, the xr-spatial-tracking directive helps minimise the risk of this type of data collection.

4. Improving Security for Immersive Experiences: Immersive AR and VR experiences are growing in popularity, but they also bring new security risks. For instance, malicious websites could manipulate spatial tracking data to mislead or deceive users in a virtual environment, leading to physical safety concerns. The xr-spatial-tracking directive ensures that spatial tracking capabilities are only granted to trusted websites, reducing the risk of misuse in immersive applications.

Use Cases It Guards Against

The xr-spatial-tracking directive helps prevent several problematic use cases:

1. Unauthorised Tracking of User Movements: Without proper restrictions, websites could misuse the WebXR API to track users' physical movements and positions without their consent. This could result in detailed records of users’ activities, locations, and behaviours being collected and potentially misused for surveillance or tracking purposes. The xr-spatial-tracking directive prevents this by blocking unauthorised access to spatial tracking data.

2. Privacy Violations in AR/VR Experiences: Augmented reality and virtual reality experiences often require access to users' physical spaces to function properly. However, if unregulated, websites could gather excessive or unnecessary spatial data about a user’s environment. This could reveal personal details about their home, workspace, or other private settings. The xr-spatial-tracking directive ensures that access to this sensitive data is limited to trusted applications.

3. Third-Party Misuse of Tracking Data: Many websites include third-party content, such as ads, analytics tools, or embedded widgets. If these third-party scripts gain access to spatial tracking data, they could misuse it for data collection or intrusive advertising. By setting the xr-spatial-tracking directive correctly, developers can ensure that only first-party, trusted content can access this feature, preventing third-party misuse.

4. Physical Safety Risks in Immersive Environments: If spatial tracking data is manipulated or used improperly, users could be misled in AR or VR environments, potentially leading to safety risks. For example, incorrect tracking could result in users physically bumping into objects or experiencing disorienting environments. By restricting access to trusted websites, the xr-spatial-tracking directive helps prevent misuse that could compromise user safety.

Why Should You Set Permissions-Policy XR-Spatial-Tracking Correctly?

There are several important reasons why website owners should ensure that the xr-spatial-tracking directive is configured properly:

1. Protecting User Privacy: The WebXR API allows access to highly sensitive spatial data, including information about a user’s movements and surroundings. If this data is misused, it could lead to serious privacy violations. By configuring the xr-spatial-tracking directive to limit access, website owners can protect users from unwanted tracking and ensure that their spatial data is not collected without consent.

2. Preventing Data Collection and Profiling: Spatial tracking data, when combined with other sources, could be used to build detailed profiles of user behaviour and activity. This profiling could be exploited for intrusive advertising, surveillance, or other malicious purposes. Setting the xr-spatial-tracking directive correctly prevents unauthorised websites from accessing this data, helping to safeguard user privacy.

3. Ensuring Safe AR/VR Experiences: Misuse of the WebXR API could lead to unsafe AR or VR environments. If spatial tracking data is manipulated or used inappropriately, users could be misled in virtual spaces, leading to physical safety concerns. By configuring the xr-spatial-tracking directive to restrict access to trusted content, website owners can ensure that immersive experiences remain safe and reliable.

4. Building User Trust: As AR and VR applications grow in popularity, users are becoming more aware of the potential risks associated with spatial tracking. By actively controlling which parts of a website can access this data through the xr-spatial-tracking directive, website owners demonstrate their commitment to privacy and security, helping to build trust with users.

5. Minimising the Attack Surface: Following the principle of least privilege, websites should only be granted access to features they absolutely need. Allowing unrestricted access to spatial tracking data increases the attack surface for malicious actors who could exploit this information. By setting the xr-spatial-tracking directive to restrict access, website owners can reduce the risk of misuse and protect their users from potential threats.

Conclusion: Securing Immersive Experiences with the XR-Spatial-Tracking Directive

The Permissions-Policy xr-spatial-tracking directive is a crucial tool for managing access to the WebXR API, which enables immersive AR and VR experiences by providing spatial tracking capabilities. While this API enhances the possibilities for web-based AR/VR applications, it also introduces significant privacy and security risks if not properly managed. These risks include unauthorised tracking, data collection, and physical safety concerns.

By configuring the xr-spatial-tracking directive correctly, website owners can prevent malicious or unauthorised access to sensitive spatial tracking data, protecting users from privacy violations, surveillance, and safety risks. For any website that offers AR or VR experiences, ensuring that the xr-spatial-tracking directive is properly set is essential for safeguarding user data and maintaining a secure, trustworthy web environment.