Ideally, there would be an event emitted by the browser that could detect that a U2F key was inserted, but do to the design limitations of the key this is unlik

Ideally, there would be an event emitted by the browser that could detect that a U2F key was inserted, but do to the design limitations of the key this is unlikely to happen. The U2F key is essentially a keyboard that has been plugged into your USB port. Some people keep "nano"[0] ones plugged in at all times for convenience. To address the common use cases of real users, you would need the browser to be able to check if an external keyboard is plugged in to a USB port on DOM ready, and when inserted into the computer. I will let your imagination run wild with what a nefarious person could do if every browser has the ability to detect if you are using a specific type of USB keyboard. I could point you in direction of device fingerprinting to start, but that's the bare minimum of what you expose. [0]: https://www.yubico.com/product/yubikey-4-series/#yubikey-4-nano https://www.yubico.com/product/yubikey-4-series/#yubikey-4-n...