The Qubes U2F Proxy

The Qubes U2F Proxy is a secure proxy intended to make use of U2F two-factor authentication devices with web browsers without exposing the browser to the full USB stack, not unlike the USB keyboard and mouse proxies implemented in Qubes.

What is U2F?

U2F, which stands for “Universal 2nd Factor”, is a framework for authentication using hardware devices (U2F tokens) as “second factors”, i.e. what you have as opposed to what you know, like a passphrase. This additional control provides good protection in cases in which the passphrase is stolen (e.g. by phishing or keylogging). While passphrase compromise may not be obvious to the user, a physical device that cannot be duplicated must be stolen to be used outside of the owner’s control. Nonetheless, it is important to note at the outset that U2F cannot guarantee security when the host system is compromised (e.g. a malware-infected operating system under an adversary’s control).

The U2F specification defines protocols for multiple layers from USB to the browser API, and the whole stack is intended to be used with web applications (most commonly websites) in browsers. In most cases, tokens are USB dongles. The protocol is very simple, allowing the devices to store very little state inside (so the tokens may be reasonably cheap) while simultaneously authenticating a virtually unlimited number of services (so each person needs only one token, not one token per application). The user interface is usually limited to a single LED and a button that is pressed to confirm each transaction, so the devices themselves are also easy to use.

Currently, the most common form of two-step authentication consists of a numeric code that the user manually types into a web application. These codes are typically generated by an app on the user’s smartphone or sent via SMS. By now, it is well-known that this form of two-step authentication is vulnerable to phishing and man-in-the-middle attacks due to the fact that the application requesting the two-step authentication code is typically not itself authenticated by the user. (In other words, users can accidentally give their codes to attackers because they do not always know who is really requesting the code.) In the U2F model, by contrast, the browser ensures that the token receives valid information about the web application requesting authentication, so the token knows which application it is authenticating (for details, see here). Nonetheless, some attacks are still possible even with U2F (more on this below).

The Qubes approach to U2F

In a conventional setup, web browsers and the USB stack (to which the U2F token is connected) are all running in the same monolithic OS. Since the U2F model assumes that the browser is trustworthy, any browser in the OS is able to access any key stored on the U2F token. The user has no way to know which keys have been accessed by which browsers for which services. If any of the browsers are compromised, it should be assumed that all of the token’s keys have been compromised. (This problem can be mitigated, however, if the U2F device has a special display to show the user what’s being authenticated.) Moreover, since the USB stack is in the same monolithic OS, the system is vulnerable to attacks like BadUSB.

In Qubes OS, by contrast, it is possible to securely compartmentalise the browser in one qube and the USB stack in another so that they are always kept separate from each other. The Qubes U2F Proxy then allows the token connected to the USB stack in one qube to communicate with the browser in a separate qube. We operate under the assumption that the USB stack is untrusted from the point of view of the browser and also that the browser is not to be trusted blindly by the token. Therefore, the token is never in the same qube as the browser. Our proxy forwards only the data necessary to actually perform the authentication, leaving all unnecessary data out, so it won’t become a vector of attack. This is depicted in the diagram below (click for full size).

Qubes U2F Proxy diagram

The Qubes U2F Proxy has two parts: the frontend and the backend. The frontend runs in the same qube as the browser and presents a fake USB-like HID device using uhid. The backend runs in sys-usb and behaves like a browser. This is done using the u2flib_host reference library. All of our code was written in Python. The standard qrexec policy is responsible for directing calls to the appropriate domains.

The vault qube with a dashed line in the bottom portion of the diagram depicts future work in which we plan to implement the Qubes U2F Proxy with a software token in an isolated qube rather than a physical hardware token. This is similar to the manner in which Split GPG allows us to emulate the smart card model without physical smart cards.

One very important assumption of U2F is that the browser verifies every request sent to the U2F token — in particular, that the web application sending an authentication request matches the application that would be authenticated by answering that request (in order to prevent, e.g., a phishing site from sending an authentication request for your bank’s site). With the WebUSB feature in Chrome, however, a malicious website can bypass this safeguard by connecting directly to the token instead of using the browser’s U2F API.

The Qubes U2F Proxy also prevents this class of attacks by implementing an additional verification layer. This verification layer allows you to enforce, for example, that the web browser in your twitter qube can only access the U2F key associated with https://twitter.com. This means that if anything in your twitter qube were compromised — the browser or even the OS itself — it would still not be able to access the U2F keys on your token for any other websites or services, like your email and bank accounts. This is another significant security advantage over monolithic systems. (For details and instructions, see the Advanced usage section below.)

For even more protection, you can combine this with the Qubes firewall to ensure, for example, that the browser in your banking qube accesses only one website (your bank’s website). By configuring the Qubes firewall to prevent your banking qube from accessing any other websites, you reduce the risk of another website compromising the browser in an attempt to bypass U2F authentication.

Installation

The Qubes U2F Proxy tool can be installed in Qubes 3.2 and 4.0. (However, the Advanced usage features are only available in 4.0.) These instructions assume that there is a sys-usb qube that holds the USB stack, which is the default configuration in most Qubes OS installations.

In dom0:

$ sudo qubes-dom0-update qubes-u2f-dom0
$ qvm-service --enable work qubes-u2f-proxy

In Fedora TemplateVMs:

$ sudo dnf install qubes-u2f

In Debian TemplateVMs:

$ sudo apt install qubes-u2f

Repeat qvm-service --enable (or do this in VM settings -> Services in the Qube Manager) for all qubes that should have the proxy enabled. As usual with software updates, shut down the templates after installation, then restart sys-usb and all qubes that use the proxy. After that, you may use your U2F token (but see Browser support below).

Advanced usage: per-qube key access

If you are using Qubes 4.0, you can further compartmentalise your U2F keys by restricting each qube’s access to specific keys. For example, you could make it so that your twitter qube (and, therefore, all web browsers in your twitter qube) can access only the key on your U2F token for https://twitter.com, regardless of whether any of the web browsers in your twitter qube or the twitter qube itself are compromised. If your twitter qube makes an authentication request for your bank website, it will be denied at the Qubes policy level.

To enable this, create a file in dom0 named /etc/qubes-rpc/policy/policy.RegisterArgument+u2f.Authenticate with the following content:

sys-usb @anyvm allow,target=dom0

Next, empty the contents of /etc/qubes-rpc/policy/u2f.Authenticate so that it is a blank file. Do not delete the file itself. (If you do, the default file will be recreated the next time you update, so it will no longer be empty.) Finally, follow your web application’s instructions to enroll your token and use it as usual. (This enrollment process depends on the web application and is in no way specific to Qubes U2F.)

The default model is to allow a qube to access all and only the keys that were enrolled by that qube. For example, if your banking qube enrolls your banking key, and your twitter qube enrolls your Twitter key, then your banking qube will have access to your banking key but not your Twitter key, and your twitter qube will have access to your Twitter key but not your banking key.

TemplateVM and browser support

The large number of possible combinations of Qubes version (3.2, 4.0), TemplateVM (Fedora 27, 28; Debian 8, 9), and browser (multiple Google Chrome versions, multiple Chromium versions, multiple Firefox versions) made it impractical for us to test every combination that users are likely to attempt with the Qubes U2F Proxy. In some cases, you may be the first person to try a particular combination. Consequently (and as with any new feature), users will inevitably encounter bugs. We ask for your patience and understanding in this regard. As always, please report any bugs you encounter.

Please note that, in Firefox before Quantum (e.g. Firefox 52 in Debian 9), you have to install the U2F Support Add-on. In Firefox post-Quantum you may have to enable the security.webauth.u2f flag in about:config. Chrome and Chromium do not require any special browser extensions.