Mobile ad hoc network (MANET) is a wireless network formed by the cooperation of arbitrary nodes moving randomly in arbitrary directions, the communication between nodes is established by using routing protocols. In the scenario of a dense network with a low-mobility, we use proactive routing protocols. The optimised link state routing protocol is one of the most proactive routing protocols commonly used, despite the fact that OLSR has many advantages, it is still vulnerable to attacks such as the black hole. In this paper, we present a kind of black hole attack in which the malicious node announces a fake link with the three hop neighbours, this may lead to the selection of this node as an MPR. Then it can divert all traffic. We demonstrate the feasibility of this attack through a detailed simulation using NS2 simulator. After that, we present a preferment scheme to detect and isolate malicious nodes.