Wormhole attack is one of the severe threats to wireless sensor and ad hoc networks. Most of the existing countermeasures either demand high network overheads or require specialized hardware to capture the specific symptoms induced by the wormholes, which in result, limits their applicability. This paper exploits an inevitable symptoms of wormhole attack and proposes Pworm, a real-time and passive wormhole detection and localization scheme based on the key observation that a large amount of network traffic are attracted by the wormholes. The proposed scheme can silently observe the variations in network topology to infer the existence of wormholes. Besides the scheme solely depends on network routing information and does not demand any specialized hardware. System performance of the scheme is evaluated through extensive simulations of 100 to 500 nodes for various network scales and the results show that Pworm is well suited for false alarms with good scalability and low time delay.