Clusters built from commodity computers are popular because of their cost efficiency. There are two basic types of such clusters. A prevalent type is a dedicated cluster where all machines participating in the cluster belong to a single administrative domain. Then there are non-dedicated clus- ters, where the cluster itself forms its own administrative domain, but the participating computers retain their own administrative domains. Non-dedicated clusters have a bet- ter potential for attracting computer owners to participate in a cluster, as they do not have to give up control over their computers. On the other hand, these clusters have much higher security demands than the dedicated ones. In this paper, we focus on the security issues of distributed file sys- tems in the environment of the non-dedicated clusters. We will identify the issues implied by the environment, propose our solution, and demonstrate the feasibility by the experi- mental implementation.