The Permissions-Policy directive (formerly known as Feature-Policy) is a web security tool that gives developers control over which browser features and device APIs their websites can access.

One of the important directives within this policy is "usb", which governs access to the WebUSB API. This API allows websites to communicate directly with USB devices connected to the user’s computer, such as printers, scanners, cameras, or custom hardware. While this functionality opens up new possibilities for web-based applications, it also introduces significant security and privacy risks if misused.

History and Origin of Permissions-Policy

The Permissions-Policy header was introduced by Google in 2018, originally as Feature-Policy, to give developers more granular control over which powerful browser features their websites could use. This was in response to growing concerns about the security and privacy risks posed by modern web applications accessing device features, such as cameras, sensors, and storage. As these capabilities increased, so did the potential for misuse, particularly when websites embedded third-party content or relied on insecure APIs.

The WebUSB API was developed by Google and added to the web platform to enable websites to interact directly with USB devices. This feature allows developers to build web applications that can communicate with custom hardware or peripherals, opening up possibilities for industrial, medical, and consumer device applications. However, allowing websites to directly interact with users’ USB devices poses significant security risks, such as the potential for data theft, device manipulation, or malware distribution. The usb directive in Permissions-Policy was created to give developers control over when and how this API is used, ensuring that only trusted websites can access USB devices.

What Does the USB Directive Do?

The "usb" directive in the Permissions-Policy header allows website owners to control whether their site can access the WebUSB API. This API enables websites to interact with USB devices connected to the user’s computer, allowing for direct communication between the web application and the device.

For example:

• Setting usb=() will block all access to the WebUSB API, ensuring that the website or any embedded third-party content cannot interact with USB devices.
• Setting usb=* will allow the website and all its embedded content to request access to USB devices, although this should be used with caution due to the security risks involved.

This level of control allows developers to prevent unauthorised or malicious websites from accessing connected USB devices, ensuring that only trusted content can use this powerful feature.

Why Was It Added?

The usb directive was added to address several critical security and privacy concerns associated with the use of the WebUSB API:

1. Preventing Unauthorised Device Access: Without restrictions, malicious websites or compromised third-party scripts could use the WebUSB API to access connected devices without the user’s knowledge or consent. This could result in data theft, device manipulation, or even the installation of malware on USB devices. The usb directive allows developers to limit access to the WebUSB API, ensuring that only trusted websites can communicate with USB devices.

2. Mitigating the Risk of Data Theft: USB devices, such as external storage devices or printers, can contain sensitive information. If a website gains unauthorised access to these devices, it could potentially extract or manipulate data stored on them. The usb directive was introduced to help mitigate this risk by giving developers the ability to prevent unauthorised access to users’ USB devices.

3. Reducing the Potential for Malware Distribution: The ability to communicate directly with USB devices also raises concerns about the distribution of malware. Malicious websites could potentially use the WebUSB API to inject malicious code into connected devices or trick users into downloading compromised firmware updates. The usb directive ensures that websites cannot misuse the WebUSB API to spread malware or infect USB devices without proper authorisation.

4. Controlling Access to Critical Hardware: In industrial or medical applications, USB-connected hardware is often mission-critical. Allowing unrestricted access to these devices via the WebUSB API could lead to disastrous consequences if a malicious actor were to gain control of the device. The usb directive allows developers to enforce strict controls on which web applications can interact with critical hardware, safeguarding these devices from manipulation.

Use Cases It Guards Against

The usb directive is essential for protecting against several problematic scenarios:

1. Unauthorised Data Access from USB Devices: Without restrictions, websites could use the WebUSB API to access sensitive data stored on USB devices, such as external hard drives or flash drives. This could result in data theft or privacy violations. By restricting access through the usb directive, developers can prevent untrusted content from interacting with USB devices, ensuring that sensitive data remains secure.

2. Malware Injection into USB Devices: A compromised or malicious website could exploit the WebUSB API to inject malware into connected USB devices, such as storage drives or peripheral hardware. This malware could then be spread to other devices when the USB is plugged into different computers. The usb directive helps protect against this by preventing unauthorised websites from accessing USB devices.

3. Manipulation of Critical Hardware: In certain industries, USB devices are used to connect critical hardware, such as medical equipment, manufacturing systems, or industrial controllers. Unregulated access to these devices could allow malicious actors to interfere with their operation, potentially causing harm or disruption. The usb directive enables developers to restrict access to trusted web applications, preventing unauthorised manipulation of these devices.

4. Third-Party Content Misuse: Many websites include third-party content, such as advertisements, widgets, or analytics scripts, which could request access to the WebUSB API. If these third-party elements are not trusted, they could misuse the API to interact with connected USB devices, leading to security or privacy breaches. The usb directive allows developers to control which content can access USB devices, reducing the risk of third-party misuse.

Why Should You Set Permissions-Policy USB Correctly?

There are several compelling reasons why website owners should ensure the usb directive is configured properly:

1. Protecting User Privacy and Data Security: USB devices often store sensitive information, and unauthorised access to these devices could result in data theft or manipulation. By configuring the usb directive to restrict access to the WebUSB API, website owners can prevent untrusted content from interacting with users’ USB devices, protecting their privacy and data security.

2. Preventing Malware Distribution: The WebUSB API’s ability to communicate directly with USB devices makes it a potential vector for malware distribution. Malicious websites could exploit this API to inject malware into USB devices or trick users into downloading compromised firmware. Setting the usb directive correctly helps mitigate this risk by ensuring that only trusted websites can access USB devices.

3. Safeguarding Critical Hardware: In industries where USB-connected hardware plays a vital role in operations—such as healthcare, manufacturing, or scientific research—unauthorised access to these devices could cause severe disruption or even physical harm. Configuring the usb directive properly ensures that only authorised applications can communicate with these critical devices, preventing manipulation or damage.

4. Complying with Security Best Practices: Following the principle of least privilege, websites should only be granted access to the features they absolutely need. Allowing unrestricted access to the WebUSB API increases the attack surface and exposes users to unnecessary risks. By setting the usb directive to restrict access, website owners can adhere to best security practices, reducing the risk of misuse and enhancing overall security.

5. Building User Trust: Users are increasingly aware of the privacy and security risks associated with websites requesting access to their devices. By actively controlling access to sensitive features like the WebUSB API through the usb directive, website owners can demonstrate their commitment to protecting user data and device security. This helps build trust and encourages users to engage more confidently with the website.

Conclusion: Securing USB Device Access with the USB Directive

The Permissions-Policy usb directive is a vital security control for managing access to the WebUSB API, which allows websites to communicate directly with USB devices connected to users’ computers. While the WebUSB API opens up new possibilities for web-based applications that interact with hardware, it also introduces significant security and privacy risks if misused. These risks include data theft, malware injection, and manipulation of critical hardware.

By configuring the usb directive correctly, website owners can prevent unauthorised access to USB devices, protect sensitive data, and reduce the risk of malware distribution. Properly managing USB access is essential for creating a secure, trustworthy online environment, especially in industries that rely on connected hardware. For any website that interacts with USB devices, setting the usb directive appropriately is crucial for protecting users and maintaining a secure web experience.