In the cooperative vehicular positioning networks (CVPN), non-line-of-sight (NLOS) delay in the vehicle-to-vehicle (V2V) ranging and malicious attacks, such as location spoofing and the manipulation of V2V ranging, can be significant threats to vehicle safety. However, the resulting observation from a vehicle in the event of any of the threats is the same; difference observation between the measured range and the Euclidian distance based on vehicles' shared location coordinates. In this paper, we propose a decentralized malicious vehicle detection technique for cooperative positioning technique (CPT), in which each vehicle evaluates the reliability of neighboring vehicles based on the observed difference and share the reliability with each other, so that each vehicle in the CVPN can collect the reliability of neighboring vehicles evaluated by their own neighboring vehicles. However, it is found that a malicious vehicle can make another attack; distribution of distorted reliability of its neighboring vehicles. To cope with the malicious attacks and NLOS delay effectively, we propose the weighted reliability and demonstrate that the proposed technique can detect malicious attacks (location spoofing, ranging manipulation, message distortion) and distinguish malicious vehicles from vehicles with a NLOS link. With Monte Carlo simulations using V2V radio parameters determined empirically, we demonstrate that the proposed technique achieves higher enhancement and superior robustness to malicious attacks than the conventional CPTs.