  Security of the three-party quantum secret sharing (QSS) schemes based on
entanglement and a collective eavesdropping check is analyzed in the case of
considerable quantum channel losses. An opaque attack scheme is presented for
the dishonest agent to eavesdrop the message obtained by the other agent
freely, which reveals that these QSS schemes are insecure for transmission
efficiencies lower than 50%, especially when they are used to share an unknown
quantum state. Finally, we present a general way to improve the security of QSS
schemes for sharing not only a private key but also an unknown quantum state.
