In the last years, data outsourcing has received an increasing attention by the research community thanks to the benefits that it brings in terms of data management. A basic requirement in such a scenario is that outsourced data be made accessible only to authorized users, that is, no unauthorized party (including the storing server) should have access to the data. While existing proposals provide a sound basis for addressing such a need with respect to data dissemination (i.e., enforcement of read authorizations), they fall short on the support of write authorizations.In this paper we address such an open problem and present an approach to enforce write privileges over outsourced data. Our work nicely extends and complements existing solutions, and exploiting key derivation tokens, hashing, and HMAC functions provides efficient and effective controls.