Multicast routing in Wireless Sensor Networks (WSN) offers less protection against routing attacks. Wireless sensor nodes are normally deployed in an open, hostile environment, and are prone to active and passive attacks. Therefore, a secure routing protocol incurring low computation cost, energy and memory is essential to secure communication in WSN. This paper proposes a secure location based routing protocol, using TESLA based certification. The GMR routing protocol, a reactive and location based protocol, is implemented using NS-2, and secured using TESLA based certificates. Performance metrics, such as authentication delay, energy consumption and dropping probability are analyzed. From the results, it is evident that, a TESLA based certificate secures GMR against wormhole, rushing, Sybil and black hole attacks.