Internet worms have repeatedly revealed the susceptibility of network hosts to malicious intrusions. Recent studies have proposed to employ the underlying principles of worm propagation to disseminate security-critical information in a network. Wireless sensor networks can benefit from a thorough understanding of worm propagation over sensor networks to defend from worms and to efficiently disseminate security-critical information. In this paper, we develop a topologically-aware worm propagation model (TWPM) for wireless sensor networks. In addition to simultaneously capturing both time and space propagation dynamics, the TWPM also incorporates physical, MAC and network layer considerations of practical sensor networks. Simulation results show that the proposed model follows actual propagation dynamics quite closely.