With the advancements in network technology and artificial intelligence, the interaction between network nodes is getting more frequent. The malicious data tampering can be extremely damaging to system stability, therefore the network security is of great importance. In this paper, we address the problem of system identification with binary‐valued observations under data tampering attacks and an information entropy‐based approach to optimal attack strategy from the viewpoint of attacker. Under the constrained attack energy, a method is proposed to obtain the optimal attack strategy using the information entropy as the optimization index. It is interestingly demonstrated that such optimal attack strategy is equivalent to the one by using the estimation error of the identification algorithm as the optimization index. The rationality of the proposed method is verified through numerical simulations.