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In the information era, more and more data are produced, which imposes a heavy burden on local storage.Fortunately, the cloud computing paradigm provides users with a feasible and inexpensive way to upload their large data files on the cloud storage server to reduce the local storage requirements.However, the cloud servers are not fully trusted, which may bring about security issues to our data.So it becomes an important problem how to ensure the security of the outsourced data.To address this issue, Sookhak et al.proposed a protocol that can verify the data integrity and support dynamic data operations without retrieving the data from the cloud server.In this paper, we illustrate that Sookhak et al.s protocol suffers from a man-in-middle attack.One can easily tamper the data stored in the cloud server and then modify the challenge response returned by the cloud server.The modified response can pass through the verification process of the user without being detected.Furthermore, we adapt Sookhak et al.s protocol to resist the proposed attack.It is proved that the improvement obtains a provable security goal under the integer factorization intractability assumption.We also show that the improvement is still efficient.