Particular embodiments of a verification authority associated with a web service may receive a request to access the web service. The request may comprise data uniquely identifying a client device. The request may have been received from a shared device, wherein the shared device is configured for use by a plurality of users. The verification authority may access a social graph of a user associated with the client device to determine whether one or more social-networking users have previously accessed the shared device. The verification authority may then transmit to the client device information indicating which of the social-networking users have previously accessed the shared device.