  A two-party quantum private comparison scheme using GHZ states and
error-correcting code (ECC) was introduced in Li et al.'s paper [Int. J. Theor.
Phys. 52: 2818-2815, 2013], which holds the capability of fault-tolerate and
could be performed in a none-ideal scenario. However, this study points out
there exists a fatal loophole under a special attack, namely the
twice-Hadamard-CNOT attack. A malicious party may intercept the other's
particles, firstly executes the Hadamard operations on these intercepted
particles and his (her) own ones respectively, and then sequentially performs
twice CNOT operations on them and the auxiliary particles prepared in advance.
As a result, the secret input will be revealed without being detected through
measuring the auxiliary particles. For resisting this special attack, an
improvement is proposed by applying a permutation operator before TP sends the
particle sequences to all the participants.
