We analyze the “sinkhole” problem in the context of the Dynamic Source Routing (DSR) protocol for wireless mobile ad hoc networks (MANETs). The sinkhole effect is caused by attempts to draw all network traffic to malicious nodes that broadcast fake shortest path routing information. Two reliable indicators of sinkhole intrusion are proposed and analyzed. We will study how these sinkholes may be detected and term such as sinkhole intrusion detection. One indicator is based on the sequence number in the routing message and the other one is related to the proportion of the routes that travel to a suspected node. Threshold values that imply possible sinkhole intrusion are derived for these two indicators. The simulation results show that the indicators are consistent and reliable for detecting sinkhole intrusion.