In the last few years, Vehicular ad hoc Networks (VANETs) have attracted a lot of attention and have triggered the development of many new attractive applications. They have become a fundamental component of many intelligent transportation systems and VANETs are being used to improve road safety and enable a wide variety of value-added services. Nevertheless, VANET applications have stringent security requirements because they affect road traffic safety. Several security threats and different types of attacks against VANETs have emerged recently that attempt to compromise the security of such networks. These attacks can cause catastrophic results such as the loss of lives or the loss of revenue for those value-added services. Therefore, making VANETs secure has become a key objective for VANET designers. Since traditional security mechanisms are not always suitable to some unique features in VANETs (e.g., high mobility of nodes), industry and academia have been focusing on improving VANET security. We present threats and attacks that can be launched on VANETs and we identify the security solutions to mitigate them. We also highlight VANET security challenges that still need to be addressed to enable robust, scalable, cost-effective secure solutions for VANETs.