Secure authentication using multiple authentication factors in a non-secure domain

In general, the techniques of this disclosure describe a computing device that is configured to verify an identity of a user based on authentication factors received from multiple authentication devices. The computing device, which may be configured to operate as a server device, may receive an authentication factor from at least three authentication devices in a group of three or more authentication devices via a guard device. The computing device may determine a probability that the respective user of each respective authentication device is a particular trusted user based on the received authentication factors. If the probability exceeds a threshold authentication probability, the computing device may send an authentication confirmation to a client device.