Existing filtering schemes in wireless sensor networks can only filter out false reports but not the replayed reports during forwarding. Furthermore, they can not resist cooperative attacks. In this article, a one-way hash chain based filtering scheme (HFS) is presented. In HFS, each node distributes its key and initial hash value to some other nodes after deployment. When a report is generated for an observed event, it carries the MACs and fresh hash values from t detecting nodes. Each forwarding node validates the legitimacy of the relative position of the detecting nodes carried in the report, the correctness of the MACs and hash values, and the freshness of these hash values. Analysis and simulation results show that HFS can not only filter out false reports and replayed reports simultaneously, but also resist collaborative attacks efficiently.