In one embodiment, a method includes, by one or more computing devices of an online social network, receiving, from a client system of a first user and from a second user, a biometric input used to identify the second user, sending, to the client system, a personal identifier for presentation to the second user, receiving, from the client system in response to the presentation of the personal identifier to the second user, an audio input from the second user, determining, based on a comparison of the audio input to a voiceprint of the second user, wherein the voiceprint comprises audio data for auditory identification of the second user, whether the audio input comprises the personal identifier spoken by the second user, and authenticating the second user to access an online account associated with the second user via the client system if the audio input is determined to be spoken by the second user and comprise the personal identifier spoken by the second user.