1. A method comprising: by a verification authority associated with a social-networking system, receiving 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, determining that a social-networking account for each of one or more second users of the social-networking system was 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, sending, to the shared device, a message comprising the information indicating that the social-networking accounts for the second users were compromised.