When applications require a union calculation of different distributed datasets, two requirements are often necessary to fulfill: anonymity and security. Anonymity means for scenarios with more than 2 participants that the owner of a certain data item cannot be identified. Security means that no participant can prevent that data items of other participants will be included in the union set. In this paper, we present a union protocol that guarantees both properties: anonymity and security even if participants act malicious, i.e. to modify messages or change the protocol. We prove the correctness of protocol and give experimental results that show the applicability of our protocol in a common environment.