Passive attack to the controlled secure direct communication with seven-qubit entangled states protocol

It is found that the controlled quantum secure direct communication (CQSDC) based on a seven-qubit entangled state protocol [S. K. Wang, X. W. Zha and H. Wu, Int. J. Theor. Phys. 57, 48 (2018)] can be passively attacked. That is to say, this protocol has the security loophole of information leakage. To be specific, the first bit of the sender’s two-bit message can be deduced by an attacker Eve according to the first and the third bits of Alice’s announcement and the first bit of the controller Charlie’s announcement. Then, a way to fix this loophole is discussed.