A collaborative cloud, formed by private enterprise clouds, is usually task-oriented and tightly correlated. It brings new ways to build and manage computing systems in terms of software development, resource sharing, and maintenance. However, there is little research on the security of collaborative clouds. This paper presents a virtual private cloud for collaborative clouds based on security-enhanced gateways. It enables users in each private cloud to access other private clouds in the collaboration transparently, dynamically and anonymously.