## 1 Introduction  
In vehicle-to-everything (V2X) communications, various threats exist. They range from physical types such as signal jamming and Global Positioning System (GPS) spoofing to more logical types such as higher level protocol attacks or cryptanalyses. Unless thoroughly addressed, they can pose grave safety problems for human lives, not to mention less serious losses. Unfortunately, there is not a single overarching solution for all, due to the aforementioned diversity of the potential attacks. Each threat may require a different set of countermeasures. The most notable security feature of V2X communication is the use of security credentials, a.k.a. certificates, commonly assumed by the Wireless Access in Vehicular Environment (WAVE) in IEEE 1609.2 [1] and the 3G Partnership Project (3 GPP) Cellular V2X framework in 3 GPP TR 36.885 [2]. Every safety message, the Society of Automotive Engineers (SAE) J2735 basic safety message (BSM) [3] broadcast from each vehicle in particular, is required to carry the certificate or its digest to provide the message source authentication and integrity check [1]. Also, it is used to provide privacy, by requiring frequent and continuous change [4]. However, one obvious threat that the certificate-based V2X security framework is not addressing is the message content fabrication. Unless appropriately filtered, vehicles can utilize received BSMswithout knowing whether the contents of the messages sent by the nearby vehicles are genuine or fake. In case an attacker has access to the device with a valid credential, she could forge an erroneous message and transmit it, which will not be filtered by the certificate check. Then, the attacker could cause the receiver vehicles to inappropriately react to the faulty message content and possibly face critical consequences such as collisions. Since tampering with the sensory input data such as GPS position information to be transferred by communication protocols is easier than breaking the protocols or the cryptographic system, this type of attack may be more beneficial for the attackers. Although the content fabrication attacks from a close distance can easily be refuted by the multitude of on-board sensors such as radars, cameras, and lidars on the vehicle, remote attacks out of the line of sight (LoS) of these sensors cannot be adequately filtered. A notable case would be roadside attacks, where the attacker can broadcast false information to the passing vehicles. For example, the attacker can fabricate false positional messages of a \"ghost vehicle\" to cause a multi-car pile-up in a long platoon [5]. Therefore, to obtain the users' confidence in this technology, the credibility of the V2X messages from remote locations out of the range of the LoS sensors should be checked. Below, we propose a solution where vehicles employ low-power beaconing messages (called \"whispers\") to judge whether the vehicles can trust the contents of the received vehicle messages or not. This paper particularly focuses on the data fabrication attacks on the position information, a crucial and endangered subpart of the system [6]. We demonstrate the value of the proposed approach by considering a promising use case of V2X, platooning, so that the platoons can maintain traffic efficiency in the situation that the roadside attacker transmits fake messages. Although studies applying vehicular communication to truck platooning have dealt with these issues theoretically and practically Ref [7; 8], the fact that platooning vehicles can receive vehicle messages containing false contents was not addressed in them. As the most effective attack target is the platoons, which is considered one of the promising applications of cooperative autonomous driving [9]. Finally, our proposal can be implemented only by utilizing the existing power control functionality that is already used for the congestion control in the V2X communication. The contributions of this paper can be summarized as follows.