8. A system comprising one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, by a hotword detector of a mobile computing device that includes (a) the hotword detector, (b) an ultrasonic audio subsystem, and (c) a replay attack engine and that is (i) operating in a mode in which access to one or more resources is disabled, and (ii) configured to exit the mode in which access to the one or more resources is disabled upon detecting an utterance of a hotword, an initial audio input corresponding to an utterance of an initial portion of a hotword; while a remaining portion of the hotword is being uttered and before the hotword is fully uttered, providing, by the ultrasonic audio subsystem of the mobile computing device, verification audio for output through a speaker of the mobile computing device; and selectively exiting, by the replay attack engine of the mobile computing device, the mode in which access to the one or more resources is disabled after the hotword is fully uttered.
9. The system of claim 8 , wherein the verification audio comprises one of a plurality of unique ultrasonic audio samples.