Recently, Yu et al. (Quantum Inf Process 13(6): 1457-1465, 2014) proposed the first semi-quantum scheme without the need of a classical channel to generate a secret key, while employing a "master key" and the entanglement properties of Bell states. This study points out a vulnerability that allows a malicious person to recover a partialmaster key and to launch a successful Man-In-The-Middle attack. Accordingly, we present the most likely leakage information scenarios where an outside attacker affects the security of the proposed protocol.