Automotive industry is about to make a cutting-edge step in terms of vehicular technologies by letting vehicles communicate with each other and create an Internet of Things composed by vehicles, i.e., an Internet of Vehicles (IoV). In this context, information dissemination is very useful in order to support safe critical tasks and to ensure reliability of the vehicular system. However, the industrial community focused more on safe driving and left security as an afterthought, leading to the design of insecure vehicular and transportation systems. In this paper, we address potential security threats for vehicular safety applications. In particular, we focus on a representative vehicular alert messaging system, and we point out two security threats. The first threat concerns relay broadcast message attack that forces the honest nodes to not collaborate in forwarding the message. The second threat focuses on interrupting the message relaying to degrade the network performance. Finally, we run a thorough set of simulations to assess the impact of the proposed attacks to vehicular alert messaging systems.