Mobile Ad hoc Network is an emerging research area with practical applications. Due to its fundamental nature, however, a malicious attack may disrupt the route discovery process and significantly affect the Quality of Service delivered by the network In this paper, we introduce a new secure routing protocol, Secure On-demand Source Routing (SOSR), which follows a number of basic techniques of ad hoc on-demand routing protocols. Our protocol is based on promiscuous receive mode, successfully defeats different types of identified attacks and detects misbehaviors of abnormal nodes. The only requirement of the protocol is a shared key between the communication end nodes. In particular, the scheme offers low processing overhead for intermediate nodes of the network.