An information processing device of the present invention includes: a degree-of-possibility calculation unit that calculates, based on a history of communication between a client and a server, the history including at least an identifier of the server, an identifier of the client, and a user agent character string included in a request header sent from the client, a degree of possibility that indicates a degree of certainty of a practical user agent permitted to operate as a portion of the client, with respect to each user agent that relates to the user agent character string; and a disguise information notification unit that outputs, based on the basis of the degree of possibility, disguise information that is information regarding communication performed by a fake user agent impersonating the practical user agent.