A MANET (Mobile Ad Hoc NETwork) can be considered as a collection of self-organizing mobile devices that can communicate without the aid of an integrated infrastructure or a back-end administration. Defense, crisis management, telemedicine, tele-geoprocessing, virtual navigation, commercial and civil environment, etc. are multiple applications of this type of network. However, MANETs present several significant constraints, including restricted processing capabilities, limited bandwidth, short battery life and vulnerability to multiple attacks. This paper introduces the negative effect of the Multipoint Relay (MPR) attack against an effective routing MANET protocol, OLSR (Optimized Link State Routing). In this attack, a malicious node can broadcast altered control messages over the network to cause loss of messages and connectivity. After a study of existing solutions, a security extension, named COLSR (CMAC for OLSR) and based on CMAC (Cipher-based Message Authentication Code) is proposed and validated by simulations under the OMNeT++/INETMANET environment with the integration of the Crypto ++ library. Our solution offers a good compromise between robustness in terms of security and protocol performance. It ensures the integrity of the OLSR control messages, nodes authentication and has the advantage of a modest impact on the network performances.