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A type confusion attack is one in which a principal accepts data of one type as data of another. Although it has been shown by Heather (et al., 2000) that there are simple formatting conventions that will guarantee that protocols are free from simple type confusions in which fields of one type are substituted for fields of another, it is not clear how well they defend against more complex attacks, or against attacks arising from interaction with protocols that are formatted according to different conventions. In this paper we show how type confusion attacks can arise in realistic situations even when the types are explicitly defined in at least some of the messages, using examples from our recent analysis of the Group Domain of Interpretation Protocol. We then develop a formal model of types that can capture potential ambiguity of type notation, and outline a procedure for determining whether or not the types of two messages can be confused. This work extends our earlier work on the subject in that it includes an explicit model of attacker and defender and extends the informal model of the type confusion attacks in terms of a game between an intruder and a set of honest principals in or earlier work to a more formal model in which actions of intruder and honest principals are described explicitly. This gives us a simpler, more intuitive approach that allows us to calculate probabilities in a more systematic manner, and to compare different intruder strategies and different assumptions about the way in which the protocol is implemented in terms of their effects on type confusion.