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In TAMC 06, Bao et al. proposed a new identity-based threshold proxy signature with known signers from the bilinear pairings for the first time. In this paper, however, an attack is proposed to show that their scheme have serious security flaws. By this attack, based on the proxy signature generated by proxy signers for a message on behalf of an original signer, an adversary can forge an illegal proxy signature for any message seemed to be generated by proxy signers on behalf of this adversary himself. After producing a forged proxy signature, the adversary has the same authority with the original signer to the proxy signer, and the verifier cannot distinguish that which one is the real original signer. Especially, this attack can be modified forge a valid threshold proxy signature for any message with knowing a previously valid threshold proxy signature generated by proxy signers on behalf of an illegal original signer, and can also be modified to change a proxy signature into a valid threshold signature belonging to the group that actually generates the proxy signatures for any message.