The recent advancements in the wireless arena and their wide-spread utilization have introduced new security vulnerabilities. The wireless media being shared is exposed to outside world, so it is susceptible to various attacks at different layers of OSI network stack. For example, jamming and device tampering at the physical layer; disruption of the medium access control (MAC) layer; routing attacks like Blackhole, rushing, wormhole; targeted attacks on the transport protocol like session hijacking, SYN flooding or even attacks intended to disrupt specific applications through viruses, worms and Trojan Horses. Wormhole attack is one of the serious routing attacks amongst all the network layer attacks launched on MANET. Wormhole attack is launched by creation of tunnels and it leads to total disruption of the routing paths on MANET. In this paper, Wormhole detection algorithm (WDA) is proposed based on modifying the forwarding packet process that detects and isolates wormhole nodes in ad hoc on demand distance vector (AODV) routing protocol.