In 2009, R. Tso et al. proposed an efficient pairing-based short signature scheme which is provably secure in the Random Oracle Model. In this paper, we propose a new key substitution attack on Raylin Tso et al.’s short signature scheme. For a given message and the corresponding valid signature, the malicious attacker can generate a substituted public key. Everyone verifies the signature successfully with the malicious attacker’s substituted public key. Therefore, Raylin Tso et al.’s short signature scheme has a security flaw in the multi-user setting.