  A broadcasting multiple blind signature scheme based on quantum GHZ
entanglement has been presented recently by Tian et al. It is said that the
scheme's unconditional security is guaranteed by adopting quantum key
preparation, quantum encryption algorithm and quantum entanglement. In this
paper, we prove that each signatory can get the signed message just by an
intercept-resend attack. Then, we show there still exists some participant
attacks and external attacks. Specifically, we verify the message sender Alice
can impersonate each signatory to sign the message at will, and so is the
signature collector Charlie. Also, we demonstrate that the receiver Bob can
forge the signature successfully, and with respect to the external attacks, the
eavesdropper Eve can modify the signature at random. Besides, we discover Eve
can change the signed message at will, and Eve can impersonate Alice as the
message sender without being discovered. In particular, we propose an improved
scheme based on the original one and show that it is secure against not only
the attacks mentioned above but also some collusion attacks.
