The security vulnerabilities of a number of provable secure proxy signature schemes are examined,with the assumption that users can register their public keys without having corresponding private keys. This assumption is different from that of a standard proxy signature in which the public keys of users are authentic. Under this assumption, both the Triple Schnorr scheme and Kang et al's scheme are shown to be vulnerable to a rogue public key registration attack. This attack gives an adversary the ability to generate a proxy signature without the valid agreement of the original signer. Moreover, it is shown that an adversary can manipulate the description of a delegated signing right at will. This work can be considered as an awakening to the importance of Proof of Possession (PoP) in the PKI environment, as in many cases certificate authorities do not require the PoP protocol, as has been stated .