Virtual Organizations (VOs) allow the permanent or temporary establishment of collaborative networks with specific rules and policies to provide VO resources access to researchers. VOs can be formed by distinct domains (organizations) that go beyond the barriers of an Academic Federation; this requires the establishment of trust relationship among domains in different federations. Identity management (IdM) systems are not designed to be used in open and dynamic environments such as VOs. This work aims to describe a security management mechanism for VOs (SM4VO), which is responsible for providing the trust relationship establishment among administrative domains that are in different and heterogeneous federations, and for assisting the authentication and authorization of VO users. Furthermore, SM4VO assures the interoperability among SAML and non-SAML VO domains. As a proof of concept, we developed a prototype and IdM experts evaluated its usability and feasibility.