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Proxy signature allows the original signer to delegate his/her signing capability to a proxy signer such that the proxy signer can sign messages on behalf of the original signer. With an elliptic curve based scheme, one can obtain better efficiency compared to conventional proxy signatures. However, taking security into account, a good signature scheme should be designed to meet its basic security requirements. We present an original signer forgery attack on an elliptic curve based proxy signature proposed recently (Chen, T.S. et al., Proc. IEEE TENCOM'02, p.184-7, 2002). By first constructing a forged proxy signature key pair, the attack is successful in forging signatures on any message. An improvement is presented to he secure against the proposed attack and the efficiency is comparable to the original scheme.