A method by a content server includes maintaining a historical repository of terminal signature tuples received from computer terminals. Each of the terminal signature tuples contains a terminal identifier for a computer terminal and a terminal signature characterizing a measured operation by the computer terminal. An access request message is received from a source computer terminal and contains a terminal signature tuple. The terminal signature tuple in the access request message contains a terminal identifier for the source computer terminal and a terminal signature characterizing a measured operation by the source computer terminal. A posterior probability value is generated based on processing a combination of the terminal signature tuple contained in the access request message and the terminal signature tuples contained in the historical repository. The content server controls access for the access request message to a resource controlled by the content server based on the posterior probability value.