V2X communication facilitates information sharing between a vehicle and the infrastructure, pedestrians, devices, or any other entity that may affect the vehicle, which is known as a critical component in 5G that promises to realize the vision of connected and autonomous vehicles. Crowd sensing, a.k.a. collective perception, is one of the essential concepts of V2X networks, where vehicles share their information collected by local perception sensors about the environment for improving safety, saving energy, optimizing traffic, and so on. Although the operational aspects of V2X networks are being studied actively, its security aspect has received little attention. In this article, we discuss security issues that may pose serious threats to crowd sensing in V2X networks, and we focus on V2X-specific threats that are unique in V2X networks, e.g. platoon disruption and perception data falsification. We also discuss countermeasures against these threats and the technical challenges that must be overcome to implement such methods.