Active Attacks An enemy E can initiate two separate protocols with A and B: E -> m1 -> B E <- m2, S_B(m2, m1, A) <- B A <- m2 <- E A -> m3, S_A(m3, m2, B) -> E E -> m3, S_A(m3, m2, B) -> B Insecurity due to symmetry of messages Could vary structure or require m1 to be included in final message