The disclosed embodiments provide a system that authenticates a user of an unattended device. In response to sensing a presence of the user in proximity to the unattended device, the system makes a call from the unattended device to an authentication service to authenticate the user. In response to the call, the authentication service authenticates the user based on recently collected sensor data, which was obtained from one or more sensors in a portable electronic device belonging to the user. If authentication succeeds, the system allows the user to proceed with an interaction with the unattended device.