8. One or more computer-readable non-transitory storage media embodying software that is operable when executed by one or more processors to: by a verification authority associated with a social-networking system, receive a request to access a shared device, wherein the request comprises data uniquely identifying a first user of the social-networking system, wherein the first user is represented by a first user node in a social graph associated with the social-networking system, and wherein the social graph comprises a plurality of user nodes and a plurality of edges connecting the user nodes; by the verification authority, determine that a social-networking account for each of one or more second users of the social-networking system has been compromised, wherein the second users are respectively associated with second user nodes in the social graph, and wherein the second users previously accessed the shared device; and by the verification authority, send, to the shared device, a message comprising the information indicating that the social-networking accounts for the second users are compromised.
8. One or more computer-readable non-transitory storage media embodying software that is operable when executed by one or more processors to: by a verification authority associated with a social-networking system, receive a request to access a shared device, wherein the request comprises data uniquely identifying a first user of the social-networking system, wherein the first user is represented by a first user node in a social graph associated with the social-networking system, and wherein the social graph comprises a plurality of user nodes and a plurality of edges connecting the user nodes; by the verification authority, determine that a social-networking account for each of one or more second users of the social-networking system has been compromised, wherein the second users are respectively associated with second user nodes in the social graph, and wherein the second users previously accessed the shared device; and by the verification authority, send, to the shared device, a message comprising the information indicating that the social-networking accounts for the second users are compromised. 9. The media of claim 8 , wherein each of the second user nodes are connected to the first node in the social graph.