: Denial of Service (DoS) attacks can be easily launched in Wireless Sensor Networks (WSNs). Due to their resource constraints, namely limited energy, memory and bandwidth, WSNs are especially vulnerable to DoS attacks. This paper addresses a particular class of DoS attacks that overwhelm resources along a multihop data delivery path. Since WSNs are typically tree-structured, then a DoS attack on a path will be especially effective in denying routing service to an entire branch of sensor nodes, not just the nodes along the path. This paper proposes a solution using one-way hash chains to protect end-to-end multihop communications in WSNs against such Path-based DoS (PDoS) attacks. The proposed solution is lightweight, tolerates bursty packet losses and can easily be implemented in modern WSNs. This paper reports on performance measured from a prototype implementation.