Universal Second Factor (U2F) USB Key Device

U2F is an open authentication standard that enables relying parties exposed to the internet to offer a strong second factor option for end user authentication.

The second factor is provided by a device implementing the U2F protocol. In case of a USB U2F security key, it is a USB HID device that implements the U2F protocol.

QEMU supports both pass-through of a host U2F key device to a VM, and software emulation of a U2F key.


The u2f-passthru device allows you to connect a real hardware U2F key on your host to a guest VM. All requests made from the guest are passed through to the physical security key connected to the host machine and vice versa.

In addition, the dedicated pass-through allows you to share a single U2F security key with several guest VMs, which is not possible with a simple host device assignment pass-through.

You can specify the host U2F key to use with the hidraw option, which takes the host path to a Linux /dev/hidrawN device:

qemu-system-x86_64 -usb -device u2f-passthru,hidraw=/dev/hidraw0

If you don’t specify the device, the u2f-passthru device will autoscan to take the first U2F device it finds on the host (this requires a working libudev):

qemu-system-x86_64 -usb -device u2f-passthru


u2f-emulated is a completely software emulated U2F device. It uses libu2f-emu for the U2F key emulation. libu2f-emu provides a complete implementation of the U2F protocol device part for all specified transports given by the FIDO Alliance.

To work, an emulated U2F device must have four elements:

  • ec x509 certificate

  • ec private key

  • counter (four bytes value)

  • 48 bytes of entropy (random bits)

To use this type of device, these have to be configured, and these four elements must be passed one way or another.

Assuming that you have a working libu2f-emu installed on the host, there are three possible ways to configure the u2f-emulated device:

  • ephemeral

  • setup directory

  • manual

Ephemeral is the simplest way to configure; it lets the device generate all the elements it needs for a single use of the lifetime of the device. It is the default if you do not pass any other options to the device.

qemu-system-x86_64 -usb -device u2f-emulated

You can pass the device the path of a setup directory on the host using the dir option; the directory must contain these four files:

  • certificate.pem: ec x509 certificate

  • private-key.pem: ec private key

  • counter: counter value

  • entropy: 48 bytes of entropy

qemu-system-x86_64 -usb -device u2f-emulated,dir=$dir

You can also manually pass the device the paths to each of these files, if you don’t want them all to be in the same directory, using the options

  • cert

  • priv

  • counter

  • entropy

qemu-system-x86_64 -usb -device u2f-emulated,cert=$DIR1/$FILE1,priv=$DIR2/$FILE2,counter=$DIR3/$FILE3,entropy=$DIR4/$FILE4