Ad hoc routing protocols are vulnerable due to the absence of security mechanisms, allowing attacks such as forged routing advertisements to disrupt the routing scheme. Research work has been proposed for securing the routing protocol in ad hoc networks; however, solutions that utilize asymmetric cryptographic primitives are often infeasible in a constrained mobile environment. In this paper, we discover a strict, cooperative disruption attack on the route path and identify the deficiency of present security mechanisms for protecting routing information. We then propose a path authentication scheme which relies on efficient symmetric cryptographic primitives. The Random Assignment Path Authentication (RAPA) scheme guarantees the integrity of a complete request route path in the route discovery procedure and helps the current on-demand routing protocol in resisting routing disruption attacks.