Abstract: The proxy signature schemes allow proxy signers to sign messages on behalf of an original signer, a company or an organization. Proxy signature is an active cryptographic research area and a wide range of literature can be found nowadays, which suggest improvement and generalization of existing protocols in various direction. However, most of existing proxy signature schemes is based on discrete logarithm problem. Recently, Zhou, Cao and Lu proposed two simple efficient proxy-protected signature schemes based on factoring. They claimed that their schemes are provably secure in random oracle model based on the RSA problem and factoring. Moreover, they believed that their schemes are more efficient and easy to implement. In this study, the author finds that malicious signers can easily forge proxy signatures for arbitrary messages without the permission of the original signer. Hence, the claim of Zhou, Cao and Lu is wrong.