  Designing a quantum key agreement (QKA) protocol is always a challenging
task, because both the security and the fairness properties have to be
considered simultaneously. Recently, Zhu et al. (Quantum Inf Process 14(11):
4245-4254) pointed out that Shukla et al.'s QKA protocol (Quantum Inf Process
13(11): 2391-2405) has some security flaws (which lead to the Participant
Attack). Moreover, they proposed an improvement to avoid these weaknesses.
However this study points out that the improved protocol also suffers from a
colluding attack, i.e., two dishonest participants in the protocol can
collaborate to manipulate the final secret key without being detected.
