Recent advances in wireless networking technologies have made it possible to construct a Mobile Ad hoc Network (MANET) which can be applied in infrastructureless situations. However, due to their inherent characteristics, they are much more vulnerable to malicious attacks than a conventinal wired network. In MANET, routing plays an important role in providing connectivity for mobile nodes who are not within the same radio range. Existing routing protocols in MANET assume a trusted and reliable environment. However, in hostile environment mobile nodes are susceptible to various types of routing attacks. This paper identifies a new routing attack, called Node Isolation attack, against Optimized Link State Routing (OLSR) protocol, one of the four standard routing protocols for MANETs. We analyze in detail and demonstrate the impact of this attack in order to show the necessity for a countermeasure to guard against the attack. As a first step to defend against the attack, we present a simple technique to detect the attack and identify the source of the attack.