Wireless Sensor Networks (WSNs) are deployed in adversarial environments and used for critical applications such as battle field surveillance and medical monitoring, then security weaknesses become a big concern. The severe resource constraints of WSNs give rise to the need for resource bound security solutions. The Implicit Geographic Forwarding Protocol (IGF) is considered stateless, which means that it does not contain any routing tables and does not depend on the knowledge of the network topology, or on the presence or absence of the node in WSN. This protocol is developed to provide a range of mechanisms that increase security in IGF. Thus it keeps the dynamic connectivity features and provides effective defenses against potential attacks. These mechanisms supported the security against several attacks as Black hole, Sybil and Retransmission attacks, but the problem was the inability of mechanisms to deal with physical attack. This research deals with a detailed study of the SIGF-2 protocol and proposes an improvement for it, in which we use the concept of deployment knowledge from random key pool algorithm of keys management to defend against physical attack . The evaluation of simulation results, with different parameters, proved that our proposal had improved the studied protocol.