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An attack on the authentication protocol run in which the Attacker positions himself in between the Claimant and Verifier so that he can intercept and alter data traveling between them.
SOURCE: SP 800-63
A form of active wiretapping attack in which the attacker intercepts and selectively modifies communicated data to masquerade as one or more of the entities involved in a communication association.
SOURCE: CNSSI-4009