  In Yang et al.'s literatures (J. Phys. A: Math. 42, 055305, 2009; J. Phys.
A:Math. 43, 209801, 2010), a quantum private comparison protocol based on Bell
states and hash function is proposed, which aims to securely compare the
equality of two participants' information with the help of a dishonest third
party (TP). However, this study will point out their protocol cannot resist a
special kind of attack, TP's same initial states attack, which is presented in
this paper. That is, the dishonest TP can disturb the comparison result without
being detected through preparing the same initial states. Finally, a simple
improvement is given to avoid the attack.
