This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS 1 A Survey on Cyber-Security of Connected and Autonomous Vehicles (CAVs) Xiaoqiang Sun, F. Richard Yu , Fellow, IEEE, and Peng Zhang Abstract— As the general development trend of the automotive industry, connected and autonomous vehicles (CAVs) can be used to increase transportation safety, promote mobility choices, reduce user costs, and create new job opportunities. However, with the increasing level of connectivity and automation, malicious users are able to easily implement different kinds of attacks, which threaten the security of CAVs. Hence, this paper provides a comprehensive survey on the cyber-security in the environment of CAVs with the aim of highlighting security problems and challenges. Firstly, based on the types of communication networks and attack objects, it classifies various cyber-security risks and vulnerabilities in the environment of CAVs into in-vehicle network attacks, vehicle to everything network attacks, and other attacks. Next, it regards cyber-risk as another type of attacks in the environment of CAVs. Then, it describes and analyzes up-to-date corresponding defense strategies for securing CAVs. In addition, it concludes several available cyber-security and safety standards of CAVs, which is helpful for the practical application of CAVs. Finally, several challenges and open problems are discussed for the future research. Index Terms— Connected and autonomous vehicles, cybersecurity, attacks, defense strategies, standards. I. I NTRODUCTION B ASED on incorporating many different technologies, connected and autonomous vehicles (CAVs) [1] have been envisioned to enhance transportation efficiency, reduce accidents, improve safety, offer great mobility service options, and alleviate environmental damage [2]. The CAVs paradigm will continue to unfold steadily and progressively throughout Manuscript received June 16, 2020; revised December 27, 2020; accepted February 12, 2021. This work was supported in part by the National Natural Science Foundation of China under Grant 61802118, in part by the Science and Technology Innovation Projects of Shenzhen under Grant JCYJ20190809152003992, in part by the China Postdoctoral Science Foundation under Grant 2019M653042, in part by the Government of Canada’s National Crime Prevention Strategy, and in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Training Experience (CREATE) Program for Building Trust in Connected and Autonomous Vehicles (TrustCAV). The Associate Editor for this article was H. L. Vu. (Corresponding author: F. Richard Yu.) Xiaoqiang Sun is with the Guangdong Key Laboratory of Intelligent Information Processing, College of Electronics and Information Engineering, Shenzhen University, Shenzhen 518060, China, and also with the Key Laboratory of Optoelectronic Devices and Systems, Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China (e-mail: xqsun@szu.edu.cn). F. Richard Yu is with the School of Information Technology, Carleton University, Ottawa, ON K1S 5B6, Canada (e-mail: richard.yu@carleton.ca). Peng Zhang is with the Guangdong Key Laboratory of Intelligent Information Processing, College of Electronics and Information Engineering, Shenzhen University, Shenzhen 518060, China (e-mail: zhangp@szu.edu.cn). Digital Object Identifier 10.1109/TITS.2021.3085297 the 2020s and 2030s with the improvements of infrastructure and communication technologies. The global CAVs market is becoming one of biggest markets in the world. It is expected to reach $7 trillion by 2050. Furthermore, many big automotive manufacturers and high-tech giants are racing against the clock to bring practical CAVs to the market. Considering broad market prospects of CAVs, several ambitious research programs have been established around the globe. In the near future, CAVs may be used widely by the public. Although CAVs have potential benefits, there are various security and privacy challenges [3]. In particular, the security risk is escalating with the increasing level of connectivity and automation. There exists a risk that an adversary can control a vehicle or its surrounding devices. For example, based on a vulnerability in the vehicle’s entertainment system, Charlie Miller and Chris Valasek demonstrated that they had hacked into a Jeep Cherokee successfully using a laptop via the Internet in 2015 [4]. Then, they can remotely control steering wheel, accelerator, air-conditioning, radio, windshield wipers, and even brakes, which may put the driver’s human life in the risk. Hence, Chrysler had to recall for 1.4 million vehicles to manually install the patch for this leak. This attack is a watershed for the automotive industry. Before this attack, many automotive manufacturers argued that it was impossible to implement remote attacks on the vehicles. In addition, the driver often listens to the downloaded music on the car stereo. However, the downloaded music may include the malware codes, for example buffer overflow codes, which can get into the vehicle’s entertainment system and make its way into other systems, including those that control the engine or brakes. For the widespread deployment of CAVs in intelligent transportation systems, the potential cyber-security risks and vulnerabilities need to be addressed. Hence, there is a need to research how to defend and mitigate these security issues. In this paper, we endeavor to survey the cyber-security in the environment of CAVs. Eventually, we make following four contributions. • In the environment of CAVs, based on the types of communication networks and attack objects, we classify the existing cyber-security risks and vulnerabilities into in-vehicle network attacks, vehicle to everything network attacks, and other attacks. Because the research of cyber-risk is helpful for ranking potential risks and forecasting future vulnerabilities, we regard cyber-risk as another type of attacks in the environment 1558-0016 © 2021 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 2 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS Fig. 2. The status of the papers related to the cyber-security in CAVs based on the countries. Fig. 1. The status of the papers related to the cyber-security in CAVs based on the research directions. of CAVs, which is different from the previous work. • In order to resist and mitigate potential cyber-security risks and vulnerabilities, we analyze the corresponding defense solutions, which are helpful for securing CAVs. • Most of the research of CAVs remains at the theoretical level. In order to realize the leap from theory to application, we conclude existing cyber-security and safety standards in the environment of CAVs. • We present several challenges and open problems that can be considered as future research directions. The remainder of this paper is organized as follows. In Section II, we provide a status of the papers related to the cyber-security in the environment of CAVs. In Section III, we present an overview of CAVs. Existing cyber-security risks and vulnerabilities are surveyed and classified in Section IV. In addition, the cyber-risk assessment in the environment of CAVs is summarized. Then, up-to-date corresponding defense strategies are presented in Section V. Section VI describes several related security and safety standards for CAVs. In Section VII, we present challenges and open problems. Finally, we conclude this paper in Section VIII. II. R ELATED W ORK Scholars over the world have carried out the research on the cyber-security in the environment of CAVs. By using the statistics related to the cyber-security in the environment of CAVs from January 1, 2013 to November 30, 2020 on Web of Science,1 we have drawn Figs. 1, 2, and 3 based on the research directions, countries, and publication years respectively. As can be seen from Fig. 1, most of papers focused on the aspects of computer science, engineering, telecommunications, and transportation for the cyber-security in the environment of CAVs. From Fig. 2, it can be observed that USA, UK, and China are the top three countries that contribute to the papers related to the cyber-security in the environment of CAVs. We can observe from Fig. 3 that there is an increasing number of papers related to the cyber-security in the environment of CAVs since 2013. 1 https://apps.webofknowledge.com/UA_GeneralSearch_input.do?product= UA&search_mode=GeneralSearch&SID=7AXLTD1C7ABZjZg6YU3& preferencesSaved= Fig. 3. The status of the papers related to the cyber-security in CAVs based on the publication years. There are several surveys [5]–[13] related to the cyber-security of CAVs. But, most of them focused on the partial cyber-security risks and vulnerabilities in the environment of CAVs. As shown in Table I, the attacks in the environment of CAVs were classified in [5]–[11], [13]. Defense strategies for potential attacks were pointed in [5]–[9], [11]–[13]. However, the impacts of the potential attacks were only discussed in [7]–[9], [12]–[14]. The features of potential attacks were only discussed in [8], [9]. Unfortunately, other attacks, which are important cyber-security attacks in the environment of CAVs, are not mentioned in the above surveys. In addition, the above surveys do not summarize the cyber-risk, which is vital for CAVs. Furthermore, cyber-security and safety standards can subsequently be used to motivate a roadmap for addressing the potential cyber-risks and vulnerabilities in the environment of CAVs. They should be discussed in the above surveys. There is an absence of a survey, which can utilise available literature to identify other important attacks, cyber-risk, and cyber-security and safety standards in the environment of CAVs. III. A N OVERVIEW OF C ONNECTED AND AUTONOMOUS V EHICLES In this section, we describe wireless access technologies, framework of CAVs, and vehicles automation, which are shown as follows. A. Wireless Access Technologies Nowadays, there exist lots of usable wireless access technologies, which can be used for vehicle-to-vehicle and vehicle-to-infrastructure communications. In the environment of CAVs, common wireless access technologies include dedicated short range communications, cellular networks, WLAN, worldwide interoperability for microwave access (WiMAX), Bluetooth, and satellite communication, which are described as follows. Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs 3 TABLE I C OMPARISON OF E XISTING S URVEY A RTICLES • Fig. 4. The 75 MHz spectrum of dedicated short range communications. Fig. 5. The protocol stack of dedicated short range communications. 1) Dedicated Short Range Communications: Most vehicular communications are based on dedicated short range communications (DSRC) [15]. It is particularly designed for achieving low latency and high reliability in vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications. In different countries and regions, spectrums and standards are different. In this paper, we only focus on those that are adopted in North America. DSRC is initially allocated 75 megahertz (MHz) of spectrum at 5.9 gigahertz (GHz) band by United States Federal Communication Commission in 1999. As shown in Fig. 4, the spectrum is divided into one control channel and six service channels. The volume of every channel is 10 MHz. The channel number begins from 172 to 184. The control channel is the channel 178, which can be used specially for secure communications. In addition, channels of 172 and 184 are retained for some secure applications. Other service channels can be used for secure and non-secure applications. The range of DSRC is at most one kilometre (km). The data rate of DSRC is up to 27 megabits per second (Mbps) for the 10 MHz channel. For the 20 MHz combined channel, its maximum data rate is 54 Mbps. As shown in Fig. 5, DSRC protocol stack mainly consists of the IEEE 802.11p family, which is called wireless access in vehicular environments (WAVE) and IEEE 1609 family. Detailed protocols are listed and described as follows. • IEEE 802.11p physical layer protocol: In this protocol, each channel is 10 MHz. It is based on orthogonal frequency division multiplexing. The alternate frequency between control channel and service channel is 10 Hz. IEEE 802.11p media access control layer protocol: According to the type of traffic, this protocol adopts the enhanced distributed channel access with the quality of service management. • IEEE 1609.0 protocol: This protocol defines the structure of IEEE 1609 standards family. • IEEE 1609.1 protocol: This protocol specifies data flows, data resources, fundamental components, device types which are supported by on board unit (OBU), command messages, and storage formats. • IEEE 1609.2 protocol: This protocol depicts the security and privacy of the vehicle. • IEEE 1609.3 protocol: This protocol describes routing service, addressing service, WAVE short messages, and management information base. • IEEE 1609.4 protocol [16]: This protocol defines wireless multi-channel radio operations, which are relied on the IEEE 802.11p protocol. • IEEE 1609.11 protocol: This protocol is utilized to support electronic payment formats. 2) 4G/5G Cellular Networks: In 4G cellular networks [17], they can provide service of mobile ultra-broadband internet access. Users have the access to various networks without changing one network to another network manually. In order to realize the fast transmission in some specified regions, there exist some available technologies, which include long term evolution-advanced mobile communication systems and microcell base stations. As supported by internet protocol version 6, its data transmission speed can be one gigabit per second in the communication with low mobility. In addition, the transmission cost per bit of multimedia service is low. Due to users’ various requirements, the service should be more differentiated. Users can enjoy the benefits of different kinds of services simultaneously. In 2018, the organization of third generation partnership project defined 5G cellular networks [18], which are the extensions of 4G cellular networks. In 5G cellular networks, a large amount of multi-input multi-output antennas are integrated into base stations. For wireless transmission, the technique of millimeter wave communication is used to offer the bandwidth, which frequency is hundreds of MHz. There are several advantages in 5G cellular networks. The main advantage is that its data transmission rate can be up to 10 gigabits per second, which is 100 times faster than that of 4G cellular networks. Another advantage is that its network latency is less than one millisecond, which is less than that of 4G cellular Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 4 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS networks. Above 4G/5G cellular networks are helpful for the construction of CAVs. 3) WLAN: WLAN [19] is a wireless communication technology, which can provide a link to the wider internet by access points with high flexibility. In WLAN, the coverage of each access point is about 100 meters. The range of a WLAN can easily be extended by adding one or more repeaters. WLAN is not limited by the number of physical ports on the router. Therefore, it can support dozens or even hundreds of devices. The data transmission rate of WLAN is tens of million bits per second. However, WLAN is naturally not secure. Any wireless device can attempt to connect to a WLAN, so it is important to authenticate its access to the network by using the WiFi protected access or other techniques. 4) WiMAX: WiMAX [20] is a wireless broadband communication technology, which is designed based on the IEEE 802.16 standard. In WiMAX, the coverage range of wireless broadband access is at most 50 km for fixed stations. For mobile stations, the coverage range of wireless broadband access is between 5 km and 15 km. The frequency of WiMAX is between 2 GHz and 66 GHz. It can offer the wireless broadband access for two fixed mobile networks. 5) Bluetooth: Bluetooth is a widely used standard for short range communications. It was first invented by Ericsson for replacing RS-232 cable connections. Bluetooth works conventionally between 2402 MHz and 2480 MHz. The proper operating range for Bluetooth is 10 meters. It can extend up to 100 meters at the price of higher complexity or lower quality of service. However, there are some security challenges in Bluetooth. For example, the pairing pin can be eavesdropped in Bluetooth. 6) Satellite Communication: Satellite communication [21] can transmit and magnify telecommunication signals. It can establish communication channels between signal senders and receivers, which are distributed at different places on earth. In satellite communication, the data rate of downlink is at most 1000 gigabits per second. And the maximum data rate of uplink is 1000 mbps. Its coverage range is between 100 km and 6000 km. In the environment of CAVs, satellite communication is usually connected with 4G/5G cellular network, WLAN, etc. It may result in some major security issues. For example, the broadcast nature of satellite communication is a security threat. In addition, satellite messages are prone to burst errors. Hence, the security patch needs to be robust enough for coping with burst errors. B. Framework As shown in the framework of CAVs (Fig. 6), existing entities include OBU, application unit, road side unit (RSU), and sensors. They are described as follows. 1) On-Board Unit: Based on the IEEE 802.11p radio protocol [22], OBU is used to exchange messages with road side units or other OBUs. It is often equipped in a vehicle. In an OBU, there exists a resource command processor, a user interface, a special interface that can be used to communicate with other OBUs, and a network facility that can be used for short distance wireless communication and resources, which Fig. 6. The framework of connected and autonomous vehicles. include a random access memory for storing and retrieving data. OBU can provide services, including wireless radio access, ad hoc and geographic routing, regulation of network congestion, dependable message transmission, data safety, and internet protocol mobility. 2) Application Unit: Application unit is usually embedded in a vehicle. It can be a special device, which is used for some secure applications. It can also be a general device, for example, personal digital assistant. Application unit can communicate with the network only by the road side unit, which is in charge of the whole mobility and network functions. The communication between the application unit and road side unit is based on a wired or wireless channel. It may co-exist with the road side unit in a physical cell. 3) Road Side Unit: As a wave device, the road side unit is often equipped along two sides of the road or in some special places, such as road junctions and neighbouring parking lots. On the one hand, it can be used to expand the communication scope of CAVs by redistributing messages to other on-board units and transmitting messages to other road side units. On the other hand, it can be used in some secure applications, for example, early warning of traffic accidents or low bridges. Furthermore, it also can be used to connect the internet. 4) Sensor: Common sensors include ultrasonic sensor, radar, and lidar, which are described as follows. The ultrasonic sensor uses high-frequency sound waves to measure the vehicle distance. The sensor head first releases sound waves, which frequencies are usually 50 kHz. Then, it listens for reflected sound waves from the target vehicle. If it receives the reflected sound waves, the distance can be computed by using the transmission time between the ultrasonic sensor and target vehicle. Radar is a sensor, which consists of a transmitter, a transmitting antenna, a receiving antenna, a receiver, and a processor. The radar is usually set around the vehicle. In the process of detecting the target vehicle, the transmitter first generates electromagnetic waves in the domain of radio or microwaves. Next, they are emitted and spread in all directions. Then, they reflect off the target vehicle and return to the receiver. Finally, according to the received electromagnetic waves, the processor analyzes the range, angle, position, and velocity of the target vehicle. In the lidar, in order to compute the distance of the target vehicle, its brightness is measured by the pulsed laser light. Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs Fig. 7. 5 The potential attack surfaces in connected and autonomous vehicles. In addition, the reflected pulses are measured for the creation of the 3D area map. Lidar is helpful for the detection of the boundaries of roads and identification of lane markings by using active pulses of light around the vehicle. C. Vehicles Automation The concept of autonomous vehicle is visualized and innovated by science fiction writers firstly. Then, Houdina Radio Control Company created the first autonomous vehicle American Wonder in 1925. Since then, vehicle manufactures, high-tech companies, and researchers began to study autonomous vehicle. Up to now, autonomous vehicle has arrived at Level 4, which means that the vehicle can perform all the driving functions under some certain conditions, even sometimes the driver does not reply to a request. In vehicles automation [23], necessary components include sensors, global positioning system (GPS), and camera, which are described as follows. GPS can provide an accurate location-based service which ranges from centimeter to meter. The navigation and localization of vehicles can be achieved by GPS and predefined road maps. There exist some factors, which may affect the quality of the received GPS signal. For example, the wideband radio frequency interference can reduce the effective GPS signal to noise ratio. Namely, the received GPS signal is attenuated. There are various approaches, including the advanced antenna design and signal processing techniques, such as beam steering, null forming, and adjusting radio frequency front-end gains, which can be used for GPS receivers to enhance the ability of tracking weak, attenuated, or corrupted GPS signals. The images, which are collected by the camera, include a large array of the values of individual pixels. The collected images are used to detect real-time obstacle, check lane departure, and track roadway information. In addition, in order to understand low-level images, they are converted into high-level images by using computer vision algorithms, which consist of segmentation, classification, and 3D reconstruction. IV. C YBER -ATTACKS IN C ONNECTED AND AUTONOMOUS V EHICLES In the environment of CAVs, we classify existing cyber-attacks into in-vehicle network attacks, vehicle to everything network (V2X) attacks, and other attacks. In addition, we summarize the cyber-security risks and vulnerabilities in the environment of CAVs. As shown in Fig 7, we first identify the attack surfaces of the vehicle for the implementation of these cyber-attacks. Attack surfaces of a system are the sum of different attack points, where the adversary can try to inject or extract data from the system for compromising the vehicle security. For example, remote sensor attacks can be implemented by using the compromised radar, which is considered as an attack surface. Then, these attacks are summarized in Table II, which is described as follows. A. In-Vehicle Network Attacks Existing in-vehicle network attacks include remote sensor attacks, GPS spoofing attacks, location trailing attacks, close proximity vulnerabilities, controller area network (CAN) and society of automotive engineers (SAE) J1939 buses vulnerabilities, electronic control units (ECUs) software flashing attacks, and integrated business services attacks, which are described as follows. 1) Remote Sensor Attacks: In the environment of CAVs, one of the central challenges is that various electrical components, which include ultrasonic radar, lidar, camera, and other sensors, are connected by an in-vehicle network. Each type of sensor has its own strength and weakness in terms of range, detection capabilities, and reliability. Furthermore, external entities can establish the connection with sensors by existed wireless access technologies. Hence, compared with the environment of autonomous vehicles, the adversary in CAVs is easier to gain access to a vulnerable and peripheral sensor. Then, the adversary will be able to take control of the sensor and implement remote sensor attacks. Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 6 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS TABLE II S UMMARY OF VARIOUS ATTACKS IN CAVs Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs Fig. 8. The model of GPS spoofing attacks. 2) GPS Spoofing Attacks: CAVs rely heavily on GPS, which provides location and time information. It is the basis of many key processes, such as pseudonym certificates, basic safety messages, and timestamps in messages. Truthfulness and accuracy are critical for CAVs. In GPS spoofing attacks [24], the adversary manipulates the received GPS signal arbitrarily in the attacked area. It provides fake information to receivers. In addition, the adversary broadcasts the fake GPS signal, which signal strength is higher than the true GPS signal. Hence, GPS receivers ignore the true GPS signal. These attacks usually happen in the physical layer. As shown in the model of GPS spoofing attacks (Fig. 8), the adversary is easy to interfere or replace the low-powered GPS signal by its stronger fake signal. Thereby, the adversary can provide any fake time or location to the vehicle in the attacked range. Then, the vehicle drifts out the desired trajectory. 3) Location Trailing Attacks: In location trailing attacks, the adversary can obtain drivers’ private information through locating and tracking their vehicles. With the help of location information, the adversary can discover behaviors and activities of vehicles, even obtain the profile of the driver and match to personal privacy in the real world. These attacks may damage to the transportation-based cyber-physical systems, for example, the increasing number of congested roads. In order to protect the privacy, messages are anonymously transmitted. In addition, pseudonyms are changed frequently. However, it has been proved that a single pseudonym is not sufficient to resist location trailing attacks [25]. Specifically, pseudonymous position samples can be collected and combined into pseudonymous location profiles. Then, an adversary can easily relate them to specific vehicles. For example, profiles, which include the same starting/ending location on weekday mornings, may reveal home and work addresses. 4) Close Proximity Vulnerabilities: Close proximity vulnerabilities are exposed by the short-range communication mechanisms. These vulnerabilities may happen by coincidence. They can be executed by Bluetooth, tire pressure monitoring system (TPMS), or keyless entry and ignition systems. Bluetooth: It has been proved that the Bluetooth control code contains a potential memory exploit, which allows the execution of code from any paired Bluetooth device. A compromised device, which has been paired with a vehicle, may 7 attack the vehicle’s engine control units. However, the driver does not aware of the attack. In addition, many secure Bluetooth protocols have been designed by cryptographic algorithms. However, due to the inefficiency of these secure protocols, most of commercial solutions are not implemented by them. Furthermore, these secure protocols cannot prevent the implementation of malicious code by memory exploitation. Fortunately, they can be used to prevent the implementation of open source tools, which are used for the analyzation of packets in the public domain. a) Tire Pressure Monitoring System: TPMS communications are usually relied on standard modulation schemes and simple protocols. Because these protocols do not rely on cryptographic mechanisms, TPMS communications may be reverse-engineered. In addition, spoofing attacks and battery drain attacks [26] may cause TPMS to malfunction. TPMS messages can be received correctly up to 10 meters from the vehicle with a cheap antenna and up to 40 meters by a basic low noise amplifier. This means the adversary can overhear or spoof transmissions from a nearby vehicle. Every in-tire sensor module includes a 32-bit fixed identifier [27] in each message. The length of the identifier field renders the identities of the tire sensor module sufficiently unique to track vehicles in a low-cost way. b) Key Fob and Keyless Entry: In order to entry the vehicle, there are two ways, namely key fob entry and keyless car entry. The adversary may block the signals of the key fob when the driver attempts to lock his vehicle by using a number of different devices, including garage door openers and house light controllers/dimmers. Once these devices are hidden in bushes and activated for long periods of time, the signals of the key fob will be blocked in the whole areas of car parks or streets. The adversary cannot block the signal for a vehicle with the keyless entry. However, this unchanging signal can be captured and replicated to gain access to a vehicle. And the alarm and immobilizer can be disabled for allowing thieves to start and steal vehicles. For example, it has been reported that the adversary can build an inexpensive and small electronic device, which can be concealed on or near a locked vehicle. This device can capture a single keyless entry code for unlocking the vehicle at a later time. It transmits a jamming signal to block the vehicle’s reception of rolling code signals from the owner’s fob, while recording these signals from both of his two attempts which are needed to unlock the vehicle. In addition, some alarms and immobilizers are relied on very weak secret keys. They can be cracked in just a few minutes by using a laptop. 5) CAN and SAE J1939 Buses Vulnerabilities: Automotive buses include CAN and SAE J1939 buses, which have following vulnerabilities. The main problem of CAN bus is the lack of authentication and encryption [28]. The lack of authentication means that any unauthorized nodes can join the network and participate in the communication. The unauthorized nodes can listen to any messages in the CAN bus. Because these messages are not encrypted, the adversary can sniff the CAN bus and illegally collect the driver’s personal information, including mobile Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 8 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS phone, location, and address book. In addition, the adversary can insert the unauthorized CAN frame to the network. CAN bus is also vulnerable to the DoS attacks [29]. In CAN bus, the arbitration mechanism allows the node with the higher priority to speak first. If a malicious node that has the highest priority is active all the time, the other nodes cannot communicate with each other. In addition, DoS attacks will misuse the error confinement mechanism. If the adversary generates an error during the communication, it will increase the error counters and eventually cause the elimination of the node. Because SAE J1939 bus is designed based on the CAN bus, it inherits all the specific vulnerabilities of CAN bus. One important vulnerability is a consequence of the arbitration mechanism [30] that allows to implement DoS attacks by continuously transmitting the frames with the identifier field set to the value that is as low as possible. In addition, in SAE J1939 bus, when a message is sent to a specific destination in the connection mode, the connection can be broken by both the sender and the receiver. The adversary can interrupt the connection by sending an abort message, which is directed to either the sender or the receiver. 6) ECUs Software Flashing Attacks: ECUs are the embedded systems, which can control gear shift, servosteering, ignition system, electronic window lift, climate controls, etc. In addition, ECUs are reprogrammable, which is helpful for correcting bugs and integrating new functionalities without replacing ECUs. ECU software flashing includes the development, delivery, and installation of software. However, possible attacks on ECU software flashing [31] include reverse engineering, code modification, fuzzing attacks [32], and phlashing attacks [33]. Reverse engineering may disclose information. Code modification may corrupt information and degrade hardware performance. Fuzzing attacks can find vulnerabilities in the embedded systems. Phlashing attacks can trick a remote device into allowing you to flash its firmware. Hence, the machine can never be rebooted. It must be pulled out and replaced. Phlashing attacks may use unpatched vulnerabilities in the embedded systems to gain access. 7) Integrated Business Services Attacks: In many vehicles, the embedded systems can be connected with the back-end network of the manufacturers. Then, the driver can use the services that are provided by the manufacturers, such as remote telediagnosis, entertainment, and remote software update. However, there exist several attacks in the integrated business services, namely gaining the client-level access to the business platform in the vehicle, gaining access to the vehicle system, and gaining access to the user data. The adversary can gain the client-level access to the business platform by launching phish attacks [34] or using malicious codes. In addition, the adversary can use the client-level access to implement another attack, which can obtain the system-level access to the business platform. If the adversary can run the arbitrary software on the business platform in the vehicle, he will control the embedded system and may gain access to the other vehicle systems. The user may have personal data risk [35], which is related to a third-party application, for example online banking. B. Vehicle to Everything Network Attacks Vehicle to everything network enables data exchange among a connected vehicle, other vehicles and the infrastructure of CAVs. The adversary exploits this type of communication to expose network access points. Communicating channels between a vehicle and external devices, for example smartphones, are established through WiFi, Bluetooth, and global system for mobile communication protocols. Once the vehicles are connected by the communicating channels, they are vulnerable to the network attacks, which are described as follows. 1) DoS Attacks: DoS attacks [36] happen when the adversary blocks the whole communication channel with interference signals. The adversary inserts useless messages or creates some issues on the network nodes. Hence, authentic users cannot access network services. Correct messages cannot reach their destinations. DoS attacks can cause delay and interfere the receiver’s response. In the environment of CAVs, some delay may affect the driving security of the vehicle. Even one second can cause or avoid an accident. In addition, the request for response time is comparatively high [37]. DoS attacks are dangerous and fatal for CAVs. 2) Impersonation Attacks: Every vehicle has an unique identification, which can help to recognize the vehicle and the messages. Impersonation attacks [38] are implemented by using another identity or a fake identity. There are two kinds of impersonation attacks, namely node impersonation attacks and sybil attacks. In the node impersonation attacks, a single identity is spoofed at a time. Different identities are spoofed simultaneously in the sybil attacks. In addition, the sybil adversary can carry out several malicious operations, such as sending fake messages, spreading modified received messages, and dropping critical messages. 3) Replay Attacks: In replay attacks [39], the adversary basically records and retransmits early valid packets at a later time. It often occurs in the network or transport layer. It may confuse the authorities, mislead the entire traffic, or even damage the transportation safety. In order to impersonate a legitimate vehicle or RSU, replay attacks usually happen in some authorization and key agreement protocols. 4) Routing Attacks: Routing attacks exploit the drawback and vulnerability of routing protocols. In these attacks, the adversary can disturb the normal routing process or drop passing packets. Routing attacks include black hole attacks [40], grey hole attacks [41], and wormhole attacks [42]. Black hole attacks can be launched by a single compromised node or some cooperative nodes. In the grey hole attacks, the adversary drops packets in a selective manner. In addition, it can switch from correct behaviors to the behaviors that are performed as if by a black hole. Due to its occasional correct behaviors, grey hole attacks are difficult to be detected. In the wormhole attacks, there are at least two cooperative nodes, which can form a private high speed tunnel. The adversary captures packets at a location. Then, they are tunneled to another location. 5) Data Falsification Attacks: For guaranteeing road safety and avoiding accidents, one essential requirement of vehicular communication is truthfulness. The fake information will Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs mislead receivers’ reaction and result in fatalities. In the data falsification attacks [43], the adversary can send or broadcast false information and safety alerts. Message tampering, suppression, fabrication, and alteration may produce the fake message. For example, Jie et al. [44] pointed that data falsification attacks can effectively disrupt route guidance mechanisms, cause significant traffic congestion, increase traveling time, and result in the imbalanced usage of transportation resources in transportation-based cyber-physical systems. 6) Eavesdropping Attacks: Eavesdropping attacks [45] are implemented by overhearing the wireless medium. These attacks are also known as stealth attacks. In these attacks, the adversary monitors the traffic of network and the particular victim’s current positions and activities silently. Then, the adversary can gather private information of CAVs. The difficulty of detecting these attacks is that the victim is not conscious of it. There is no direct impact in the network. 7) Password and Key Attacks: Password and key attacks are quite complicated to be performed. They usually need some special hardwares and softwares. These attacks are possible to be executed for some financial reasons. In password and key attacks [7], the mechanism is tested by different values until it can be compromised. Finally, the password or key may be cracked. These attacks can be classified into three categories, namely dictionary attacks, rainbow table attacks, and brute force attacks. In the dictionary attacks, the adversary uses a list of words to crack the password repeatedly. Other two kinds of attacks are similar with dictionary attacks. However, in the rainbow table attacks, the adversary uses a list of precomputed hashes, which are computed from all the possible passwords and the given algorithm. The brute force attacks can identify non-dictionary words by working through all the alpha-numeric combinations. C. Other Attacks Other attacks include infrastructure attacks, slight attacks, and attacks on machine learning systems, which are described as follows. 1) Infrastructure Attacks: CAVs are likely to require the development of new transportation infrastructure. In addition, vehicle manufacturers should cope with the resulting compatibility issues between the infrastructure and autonomous vehicles. In the infrastructure of CAVs, existed entities include road side unit, on board unit, cloud server, intelligent traffic light and signal, traffic camera, traffic management center, etc. It is worthy to consider the interactions among these entities for studying the attack surface of autonomous vehicles. In [46], it has been pointed that there exist multiple connections among autonomous vehicles and the infrastructure in the environment of CAVs. Then, there are many entry points, which can be exploited for the adversary. These connections can increase the risk of intrusion. Because an issue in a vehicle can involve accidents with other vehicles that are communicating with this vehicle, it will also increase the risk that a coding bug can cause widespread damage. Based on the compromised traffic lights and on board units, Liu et al. [47] simulated several attacks on the vehicle traffic in the smart transportation 9 system. The authors argued that many vehicles were locked in a specific area for a long time. The advanced density-based attacks can achieve better performance with fewer compromised vehicles to disrupt the whole network. In the environment of CAVs, the vehicle is connected to the infrastructure, it can send a signal phase and timing message to the vehicle for knowing the status of the light and when it is going to change. If the adversary takes control of the infrastructure, false messages will be sent to the vehicle for threatening its security. Furthermore, CAVs applications require interactions with the cloud server. Cloud service will introduce cyber-security concerns, including data theft, cyber attacks, compliance with data protection, and privacy regulations. For example, cloud service will add some vulnerabilities to the infrastructure of CAVs [48]. The adversary can use the vulnerability, which is related to message transfer, to cut off the communication in the infrastructure of CAVs. Hence, more detailed reference architectures and threat models should be used to analyze cloud in depth. Juliadotter and Choo [49] pointed that the top level of the cloud attack taxonomy is comprised of five dimensions. Alhebaishi et al. [50] conducted comprehensive threat modeling exercises based on two representative cloud infrastructures by using several popular threat modeling methods, including attack surfaces, attack trees, attack graphs, security metrics based on attack trees, and attack graphs, respectively. 2) Slight Attacks: Ye et al. [51] first proposed the concept of slight attacks for CAVs. In these attacks, the communicated data are randomly deviated from the actual ones. In addition, deviations do not exceed the threshold. If the error between expected and measured behaviors exceeds a predetermined threshold, a security scheme will be activated. In this case, the predetermined security mechanism may be useless. Even slight attacks may cause hazardous conditions and result in serious safety problems for all the vehicles in a CAVs fleet. The authors pointed that when CAVs are under slight attacks, it is more dangerous if communicated positions are attacked than speeds. It is possible that a situation with more vehicles under attack at a low severity may be more dangerous than that with fewer vehicles but under attack at a high severity. 3) Attacks on Machine Learning Systems: In the environment of CAVs, machine learning systems can be used for some security-sensitive tasks, including security monitoring and vulnerability detection. However, machine learning systems have limitations [52], and may be attacked in the phases of data collection, training, and prediction. There are some attacks on the machine learning systems, including data poisoning attacks [53], attacks on the learning algorithm and its libs [54], escape attacks [55], model steal attacks [56], and model inference attacks [57]. In data poisoning attacks, the adversary can modify the original training data or insert some malicious data, which will affect the decision of the machine learning algorithm. In the attacks on the learning algorithm and its libs, the adversary can modify the training algorithm and leak the user’s private information without affecting the accuracy and generalization of the machine learning model. The main idea of escape attacks is to craft inputs that will be misclassified by a machine Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 10 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS learning model. In the model steal attacks, the adversary can obtain the internal parameters of the machine learning model, some pre-knowledge of training data, or a machine learning model that is very close to the targeted model. Based on the combination of the output result and black-box access, model inference attacks can extract some private information about the training data. D. Cyber-Risk Cyber-risk is defined as the risk of financial loss, disruption or damage to the reputation of an organization from some sort of failure of its information technology systems [58]. Due to persistent digital innovations, intensifying global connectivity, and increasing sophistication of hackers, cyber-risk is actually dynamic. Because of the fast pace of technological innovations, potential risk exposure, and the lack of historical claims data, cyber-risk becomes a complex phenomenon for CAVs. Hence, a comprehensive cyber-risk analysis and assessment method should be designed to discover, understand, and address any vulnerability in each component [59]. In addition, Sheehan et al. [60] pointed that traditional risk assessment techniques are not effective. Maple et al. [61] found that existing approaches are either too simple for sufficiently detailed modelling or require too many details to be specified for analyzing the attack surface of CAVs. V. D EFENSE S TRATEGIES In this section, in order to resist above cyber-security risks and vulnerabilities in the environment of CAVs, we analyze corresponding defense strategies. These strategies are summarized in Table III, which is described as follows. A. Against In-Vehicle Network Attacks In order to defend in-vehicle network attacks, based on the types of these attacks, there are following strategies. 1) Against Remote Sensor Attacks: In order to defend remote sensor attacks, there are several efficient solutions, including authentication [62], [63], consistency check [62], sensor fusion [64], [65], and spatio-temporal challengeresponse [66]. In order to improve the security of ultrasonic sensors, Xu et al. [62] proposed two defense strategies, namely single sensor-based physical shift authentication which is used for verifying signals on the physical level, and multiple sensor consistency check that multiple sensors are employed to verify signals on the system level. In order to defend sensor spoofing attacks, Matsumura et al. [63] proposed a solution that authentication fingerprint is superimposed onto light wave itself. In order to counter sensor interference [116], sensor fusion and backup cameras [64] are used to verify the legality and precision of ultrasonic sensor measurements. Kapoor et al. [66] used the technique of spatio-temporal challenge-response to detect and stop sensor spoofing attacks by verifying physical signals in the analog domain. In 2019, Miura et al. [117] used random-chirp modulation to defend a low-cost distance-spoofing attack on a mmWave Frequency Modulated Continuous Wave radar. In [65], the sensor fusion technique, which intelligently combines data from various sensors to improve performance, is used to defend lidar spoofing attacks. The influence of remote sensor attacks can be reduced by modifying the internal sensing structure of the lidar. 2) Against GPS Spoofing Attacks: In order to defend GPS spoofing attacks, there exist several strategies, including bias estimation range check [67], velocities consistency check [68], statistical test [69], least absolute shrinkage and selection operator [70], and global navigation satellite system augmentation [71]. In [67], bias estimation range check and the backtracking mechanism are used for the detection and mitigation of GPS spoofing attacks. Based on the consistency check of velocities, Tao et al. [68] proposed an efficient GPS spoofing detection mechanism, which does not need any other aids. In [69], a statistical test strategy is used to defend GPS spoofing attacks. Based on the least absolute shrinkage and selection operator, Schmidt et al. [70] proposed a detection and classification mechanism for single antenna receivers. Based on the global navigation satellite system augmentation, Jeong et al. [71] proposed a GPS spoofing detection method, which has the advantage that it can be used for a receiver directly. 3) Against Location Trailing Attacks: In order to defend location trailing attacks, there are several methods, including k-anonymity [72], mix-zone [73], software defined networks [74], location perturbation [75], and perturbation-hidden [76]. In [72], the k-anonymity technique is used to obfuscate the user’s physical locations, which can enhance the user’s location privacy and can also enjoy the accurate location services. Zhou and Zhang [73] proposed a mix-zone framework for protecting the actual locations of mobile users. Boualouache et al. [74] proposed a cost-efficient vehicular location privacy zones placement method, which is based on the concept of software defined networks. In order to protect the location privacy of location-based services in vehicular networks, Luo et al. [75] designed an improved location perturbation mechanism. A new privacy definition, namely perturbation-hidden, is proposed [76] to provide a stricter privacy guarantee for location-based services than geo-indistinguishability [118]. 4) Against Close Proximity Vulnerabilities: In order to defend close proximity vulnerabilities, there are several available strategies, including encryption and cryptographic checksum, which are described as follows. a) Bluetooth: Several secure Bluetooth protocols have been developed by cryptographic techniques. However, most of commercial products do not adopt these secure protocols, which have adverse effects on the usability. These secure protocols use the cryptographic algorithm to establish trust. They will defend the attack of using some open source tools, for example, Bluesniff [119]. Unfortunately, they do not take measures to prevent the execution of malicious nodes by memory exploitation. b) Tire Pressure Monitoring System: There are several methods that can improve the dependability and security of TPMS. Firstly, the software that runs on TPMS should Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs 11 TABLE III S UMMARY OF D EFENSE S TRATEGIES IN CAVs obey basic reliable design principles [120]. Secondly, TPMS packets should be encrypted by a light-weight cryptographic algorithm. In addition, an extra sequence number field should be added to the packet for insuring the freshness of the packet. Furthermore, in order to prevent message forgery, an extra cryptographic checksum should be added prior to the Cyclic Redundancy Check checksum. c) Key Fob and Keyless Entry: Once the key fob has been activated, the driver needs to ensure the lights have flashed, which indicates the vehicle has received the signal. Then, the driver checks whether the vehicle is locked by lifting a door handle. In premium Audi RS4 vehicles, if the adversary gains the physical access to vehicles, it is able to add a new key into the system [121]. Although this is a premeditated Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 12 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS sophisticated attack, Audi has dismissed any liability. Scholars from University of Birmingham [122] developed techniques of cracking mechanisms, which can be used for keyless entry. However, they were prevented from publishing their research result by united kingdom government. 5) Against Can and SAE J1939 Buses Vulnerabilities: In order to defend the vulnerabilities in the CAN and SAE J1939 buses, there are some solutions, including network segmentation, encryption [77], and authentication [78], [79]. The network segmentation is an easy way to provide security for the CAN bus. Then, critical and non-critical ECUs are separated. The adversary cannot access critical ECUs easily. The connection between networks is based on a gateway ECU. However, the gateway ECU may be manipulated. If the gateway ECU is programmed to transmit related identities to the subnetwork, it can be fooled by passing a malicious CAN frame with an identity of a node that belongs to the subnetwork. Besides, the network segmentation may increase the difficulty of maintenance. There are various software-based encryption mechanisms, for example, Trillium [77]. However, some mechanisms are not secure. Because ECUs do not have the strong computational capability, software-based encryption mechanism may cause latency. It will not be acceptable for the safety-critical vehicle. In addition, it may be restricted by the limited bandwidth. Software-based encryption mechanism may be suitable for a limited CAN traffic. Some scholars have studied on authentication mechanisms. Based on an ordered Cypher-based Message Authentication Code buffer, Groza et al. [78] authenticated the identifiers of CAN frames and checked whether the sender is a legal node while remaining unchanged arbitration on the bus. In order to mitigate the weaknesses in the existing frame-level authentication mechanism, Palaniswamy et al. [79] proposed a new protocol suite, which provides the security of session key. In order to defend the shortcomings in the SAE J1939 bus, Murvay and Groza [30] proposed an authentication scheme and evaluated its effects by using the CAN with flexible data-rate frames to transmit the additional authentication data. There are some available intrusion detection methods. Desta et al. [123] proposed an intrusion detection system for CAN bus by analyzing arbitration identity sequences. Based on the usage of a trained Long Short Term Memory, this system can predict an arbitration identity, which will appear in the future by reviewing the last twenty packet arbitration identities. Olufowobi et al. [124] proposed a specification-based intrusion detection system by the usage of anomaly-based supervised learning with the input of the real-time model. 6) Against ECUs Software Flashing Attacks: In order to defend ECUs software flashing attacks, there are some strategies, including encryption [80], authentication [81], and integrity check. Reverse engineering may destroy the confidentiality of ECU data, ECU data should be encrypted [80]. Therefore, the adversary cannot read them. Authentication [81] can identify the origin of the software correctly. Integrity [125] is important for preventing software modification by unauthorized users. Hence, every ECU should check the integrity. Nonrepudiation is helpful for tracking the software version that has been installed, installation time, and who installed the software. Then, the adversary is difficult to implement malicious software flashing. 7) Against Integrated Business Services Attacks: In order to defend integrated business services attacks, there exist several strategies, including content filtering [82], data separation and trusted path, and software or hardware virtualization [83]. Based on the usage of a program, content filtering can block the access to some items, which may be harmful if they are opened or accessed. It can be used to defend malicious codes and phish attacks. Data separation provides a boundary among applications. Then, the implementation of malicious codes is limited to a single domain. Data separation involves either high reliability software development or machine virtualization. The security level is based on the quality of data separation. The trusted path should be implemented in a manner, which is outside of the applications control. Software or hardware virtualization is an useful tool for maintaining secure separation. Software virtualization’s reliability level is less than that of hardware virtualization. However, software virtualization may be subjected to safety risks and performance degradation. One benefit of software virtualization is that it does not need a special hardware. B. Against Vehicle to Everything Network Attacks In order to defend vehicle to everything network attacks, based on the types of these attacks, there are following strategies. 1) Against DoS Attacks: DoS attacks are difficult to be corrected even though they can be detected. Early detection will be helpful to thwart the attacks or alert the driver to take some efficient measures. To resist DoS attacks, there are some strategies, including sliding mode and adaptive estimation [84], bandwidth and entropy [85], and similarity of sliding windows [86]. A real-time mechanism [84], which includes a set of observers that are designed by the usage of sliding mode and adaptive estimation theory, can be used to detect the occurrence of DoS attacks, and estimate the effect of these attacks on the connected vehicle system. Inspired by the port-hopping mechanism, Jie et al. [126] designed a simple but effective defense mechanism, which has the advantage that the detection and filtering out of malicious packets can be implemented without any change in the existing protocol. Based on the usage of bandwidth and entropy, Kumar and Mann [85] proposed an algorithm for the detection of DoS attacks in vehicular networks. Then, the authors proposed a packet detection algorithm that can be used for preventing DoS attacks. Based on the similarity of sliding windows, an improved DoS attacks detection strategy [86] is designed to detect each type of DoS attacks. 2) Against Impersonation Attacks: In order to defend impersonation attacks, there are some available strategies, including secure transmission [87], [88], integrity verification [89], authentication [90], [91], and secure key agreement [92]. Limbasiya and Das [87] designed a secure geo-data transmission mechanism for vehicular ad-hoc network, in which Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs vehicles can transmit data with the higher probability. This mechanism is protected against impersonation attacks. In [89], the proposed hash-based integrity verification mechanism can defend impersonation attacks effectively in vehicular cloud computing. Li et al. [90] proposed a certificate-less conditional privacy-preserving authentication protocol, which supports both privacy and security requirements in the internet of vehicles system. It has been proved that this protocol is secure against impersonation attacks. Park et al. [92] designed a dynamic secure and lightweight key agreement protocol for vehicle-to-grid in social Internet of Things. Limbasiya’s secure V2V data transfer protocol [88] uses a one-way hash function to transmit valuable data at the receiver side quickly. Sutrala et al. [91] designed a novel conditional secure authentication mechanism by using Elliptic Curve Cryptography in the internet of vehicles. 3) Against Replay Attacks: In order to thwart replay attacks, available strategies include noisy control signal method and cross correlator [93], reset controller [94], and techniques of timestamping and XOR encoding [95]. Based on the usage of noisy control signal method and cross correlator, Merco et al. [93] used a decentralized diagnosis algorithm to detect the replay attacks for a cooperative adaptive cruise control connected vehicles system. In order to defend replay attacks in the unmanned aerial vehicles, Sànchez’s [127] frequency-based detection mechanism utilizes a sine wave with the time-varying frequency as the authentication signal. In order to preserve the oscillation damping ability and improve the speed tracking ability of a connected car under a replay attack, combined with a robust reset controller, Xu et al. [94] proposed a dynamic output-feedback robust controller. In Oza’s alternative threshold-based detection method [128], the detector observes the occupancy sensor data over time. Greene et al. [95] proposed an improved remote keyless entry system by using timestamping and XOR encoding. This system can countermeasure the replay attacks. 4) Against Routing Attacks: In order to defend the routing attacks, there are some strategies, including ant colony optimization [96], swarm algorithms of artificial intelligence [97], variable control chart [98], and trust calculation [99]. Based on the ant colony optimization method, Panda and Kumar Pattanayak [96] proposed a multi-path intelligent routing protocol, which can find an optimal path from the sender to the receiver along with the increasing lifetime of the network. Based on the usage of swarm algorithms of artificial intelligence, Krundyshev’s new method [97] is used for providing security for vehicular ad hoc networks and other kinds of networks that are related to transportation. In order to defend the routing attacks in CAVs, Hassan et al. [129] proposed an intelligent detection scheme. Based on a variable control chart, a new detection mechanism [98] is designed for vehicular ad hoc networks. Bhawsar et al. [99] proposed a mechanism by using the ad hoc on-demand distance vector routing protocol that is relied on trust calculation. 5) Against Data Falsification Attacks: For the detection of data falsification attacks, there are various strategies, including location detection [100], dynamic time window [43], forged 13 data filtering [44], reputation threshold [101], and information sharing [102]. Boeira et al. [100] studied that how a location detection mechanism can be used to alleviate the location falsification attacks before it can result in a crash among an array of coordinating vehicles. Based on the usage of dynamic time window, Shukla and Sengupta [43] proposed an anomaly detection strategy. In [44], the forged data filtering mechanism, in which the falsified traffic state data can be removed during data transmission in vehicular networks, is used to mitigate the data falsification attacks. Based on the obtained reputation threshold, a novel cooperative spectrum sensing mechanism [101] is designed to be robust against the data falsification attacks under the imperfect common control channel. In order to improve the reliability and support the detection of anomalous behaviors, Alotibi and Abdelhakim [102] applied the information sharing technique in the cooperative adaptive cruise control model, which allows vehicles and fixed infrastructure to obtain and share information about platoon leaders. 6) Against Eavesdropping Attacks: In order to prevent the eavesdropping attacks, there are some strategies, including anonymization [103], resource management [104], trust-based recommendation [105], and scheduling mechanism. In order to defend the eavesdropping attacks on the vehicles’ queries, a fog server with the fog anonymizer [103] is used to anonymize the messages from the fog node. In order to defend the eavesdropping attacks on vehicular users’ computation offloading over radio frequency channels, based on the measurement of the physical layer security, Wu et al. [104] researched the resource management that has secrecy provisioning. Li et al. [130] designed a Nash equilibrium method of the game under the imperfect channel estimation for maximizing the efficiency of the transmitter and suppressing the eavesdropping attacks from the unmanned aerial vehicle. To ensure real-time data transmission and security in a vehicular cyber physical systems network, a new trust-based recommendation mechanism [105] is proposed to defend the eavesdropping attacks. 7) Against Password and Key Attacks: In order to defend password and key attacks, there are some available solutions, including secure cryptographic mechanisms and multifactor authentication [106]. One solution is relied on the secure cryptographic mechanisms, such as keys with large sizes, secure algorithms, and secure passwords. Although some cryptographic algorithms are secure now, they may be cracked easily by the powerful computational ability in the future. Instead of relying on the secure cryptographic mechanisms, another solution is the multifactor authentication, which puts different layers of identity security on every account. Multifactor authentication monitors diverse factors, including time of access request and geolocation. In addition, it can incorporate biometric authentication and hard tokens. Multifactor authentication can mitigate the effectiveness of password and key attacks. It may not prevent all password and key attacks. Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 14 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS C. Against Other Attacks In order to defend other attacks, based on the types of these attacks, there are following strategies. 1) Against Infrastructure Attacks: In order to defend the infrastructure attacks, there are several efficient strategies, including cyber-security architecture [107], secure aggregation [108], software defined security [109], and authentication [110]. In [48], the cloud-based method is used to detect and mitigate cyber-attacks in the infrastructure of CAVs. Islam et al. [107] designed a new vehicle-to-infrastructure cyber-security architecture, which can detect and prevent cyber-attacks on the vehicle-to-infrastructure applications. However, there are some key challenges of designing this architectures, such as scalability, resiliency, and future usability. In [108], a light-weight and secure aggregation protocol is designed for the fog computing-based vehicle-to-infrastructure communication. By using the proposed protocol, the road side unit can securely obtain the data that are collected by the vehicles. In order to provide sufficient levels of protection for the infrastructure of CAVs, software defined security [109] can be used by abstracting security mechanisms from the hardware layer to a software layer. Based on the bilinear map, Ali and Li [110] designed an efficient and secure identity-based conditional authentication signature mechanism for the vehicleto-infrastructure communication. 2) Against Slight Attacks: In order to resist slight attacks, Li et al. [51] focused on the influence of slight attacks on the longitudinal safety of CAVs. More specifically, the authors considered the communicated position and speed data from preceding CAVs. An empirical model is first utilized to describe dynamics of CAVs. Then, a rear-end collision risk index is introduced to establish the relation between longitudinal safety and vehicles’ dynamic data. Parameters are also investigated via sensitivity analysis. Furthermore, the authors pointed that slight attacks on communicated position have more serious effects on longitudinal safety than on speed. The defenses may focus on how to detect the imprecise position data. Although vehicle equipped sensors can also detect preceding vehicles’ position, the time delay of current sensors is much larger than that of wireless communication. The defense scheme should be considered over the whole CAV fleet instead of on individuals. A coordinated defending method, which regards all vehicles, is more robust than isolated defenses. The defending scheme should not be the same in the whole driving period. It is evident that the deceleration period has more risks. Hence, more rigorous schemes should be utilized in this period. A multi-period defending method is more suitable in that case. 3) Against Attacks on Machine Learning Systems: In order to defend attacks on machine learning systems, there are several efficient solutions, including data sanitization [111], algorithm robustness improvement, homomorphic encryption [112], and differential privacy [113]. Data poisoning attacks can be resisted by using the data sanitization technique [111] or improving the robustness of the machine learning algorithm against malicious training data. Hunt et al. [131] used a system, which is called Chiron, to conceal the training data from the service operator. The passive defense or active defense strategy can be used to defend the escape attacks. The passive defense strategy is to detect the adversarial example after the construction of the machine learning system. The active defense strategy is to construct a robust machine learning system before the generation of the adversarial example. In order to defend the model steal attacks, one method is to restrict the model user that only has the black-box access. The user who provides the training data can inquiry the model and obtain the prediction result. In order to defend the model inference attacks, available techniques include homomorphic encryption [112] and differential privacy [113]. Homomorphic encryption can guarantee that data are encrypted in the training phase. Based on the usage of differential privacy, the machine learning algorithm carefully adds noise to protect the security of data. D. Cyber-Risk Assessment In order to assess cyber-risk in the environment of CAVs, there are several efficient solutions, including vulnerabilities assessment [114], dynamic risk management [115], and cyber-attacks classification [60]. An available mechanism, which can be used for assessing the vulnerabilities in the environment of CAVs, is called common vulnerability scoring system (CVSS) [114]. In 2011, CVSS version 2.0 is regarded as an international standard for assessing vulnerabilities by International Telecommunications Union. The U.S. National Institute of Standards and Technology provided National Vulnerability Database (NVD), which includes a large repository of known vulnerabilities. In [115], a profile-driven approach is proposed to manage internet of things data in the context of CAVs by a dynamic risk management framework. Unlike the current inflexible risk assessment strategies, this framework encourages more flexible investigation of risks by different risk profiles. Based on a Bayesian network model, Sheehan et al. [60] designed a proactive cyber-attacks classification method for CAVs by incorporating existed software vulnerabilities, which are contained within NVD, into model building and testing phases. Maple et al. [61] proposed a reference architecture by using a hybrid functional-communication viewpoint for cyber-risk surface analysis of CAVs. VI. C YBER -S ECURITY AND S AFETY S TANDARDS OF CAV S Except for the above defense strategies, several cyber-security and safety standards have been designed for CAVs, including International Organization for Standardization (ISO) 26262 [132]–[134], SAE J3061 [135], and British Standards Institute’s standard [136], which are described as follows. A. ISO 26262 ISO 26262 is derived from International Electrotechnical Commission 61508, which is used for electrical safety-related Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs systems. This standard provides a safety lifecycle in the phases of management, development, production, operation, service, and decommissioning. In addition, it supports tailoring necessary activities in these phases. It includes ten parts, including vocabulary, management of functional safety, product development, supporting processes, automotive safety integrity level (ASIL), etc. ASIL is determined by severity classification, exposure classification, and controllability classification. It can quantify the severity of an injury, probability of occurrence, and controllability of the situation. To assess ASIL, available techniques contain hazard analysis, risk assessment, fault tree analysis, failure mode, and effects analysis. In 2018, ISO 26262 was revised by adding rules for trucks, buses, trailers, semitrailers, and motorcycles [137]. In addition, the revised standard includes the guidance on model-based development, software safety analysis, dependent failure analysis, semiconductors, fault tolerance, safety-related special features, and software tools. Furthermore, the revised standard also contains extended vocabulary, detailed objectives, objective-orientated confirmation measures, information on managing safety anomalies, references to cyber-security, updated values for hardware architecture metrics, and methods of evaluating hardware elements. B. SAE J3061 Based on ISO 26262, SAE J3061 identifies the growing threat landscape. This standard establishes the terminology of threat, vulnerability, and risk, which provides an overview and distinction between system safety and system cyber-security. In addition, it presents some general guiding cyber-security principles, which are applicable to any organization within a company. As with system safety, cyber-security must be built into features. Hence, it requires an appropriate lifecycle process from concept phase through production, operation, service, and decommissioning. Furthermore, it provides information on some common existing tools and methods for designing, verifying, and validating cyber-physical vehicle systems. Then, the foundation is presented for further standards development activities in vehicle cyber-security. Last but not the least, appendices provide additional information which may be helpful for improving cyber-security of feature designs. In SAE J3061, for maintaining consistency between safety and cyber-security, there are some suggestions, which include building appropriate checkpoints into the product lifecycle of both processes, using a risk analysis method to address threats, and identifying various paths between safety and cybersecurity. In addition, it points out that a cyber-security-critical system may be not safety-critical. C. British Standards Institute’s Standard In 2018, British Standards Institute published a new cyber security standard [136]. In this standard, principles of system security include three aspects, which are shown as follows. Firstly, personal accountability is held at the board level for product and system security. In addition, it is delegated 15 appropriately and clearly throughout the organization. Awareness and training is embedded into a “culture of security” for ensuring individuals’ understanding of their role and responsibility in security of CAVs. All aspects of security (physical, personnel, and cyber) should be integrated into the product and service development process. Secondly, security risk assessment and management procedures should be in place within the organization. Appropriate processes for identification, categorization, prioritization, and treatment of security risks, including those from cyber, should be developed. Thirdly, incident response plans are in place. Organizations plan for how to respond to potential compromise of safety critical assets, non-safety critical assets, and system malfunctions. It also should be considered that how to return affected systems to a safe and secure state. An active program should be able to identify critical vulnerabilities. Appropriate systems can mitigate them in a proportionate manner. Organizations should ensure their systems are able to support data forensics and the recovery of forensically robust and uniquely identifiable data. In this standard, principles of secure system design include five aspects, which are shown as follows. • The organizations, which include sub-contractor, suppliers, and potential third parties, should work together to enhance the security of the system. They must be able to provide assurance, such as independent validation or certification, of their security processes and products. In addition, they jointly plan for how systems will safely and securely interact with external devices, connections (including the ecosystem), services (including maintenance), operations, or control centers. This may include agreeing standards and data requirements. • The security of the system does not rely on single points of failure. The security architecture uses defencein-depth and segmented techniques to mitigate risks with complementary controls, such as monitoring, alerting, segregation, reducing attack surfaces (such as open internet ports), trust layers, and other security protocols. In addition, remote and back-end systems (for example, cloud server), which might provide access to a system, have appropriate levels of protection and monitoring mechanism for preventing the unauthorized access. • The security of all software should be managed throughout its lifetime. Organizations should adopt secure coding practices to manage risks from known and unknown vulnerabilities in the software proportionately. It must be possible to ascertain the status of all software, firmware, and their configuration, including the version, revision, and configuration data of all software components. The software should be updated safely and securely. If the software becomes corrupt, it should be returned to a known good state. In addition, the software should adopt open design practices. • Data must be sufficiently secure and can be controlled in storage and transmission. Only the intended recipient or system functions can receive and access them. Incoming communications are considered unsecure before validation. Users are able to delete sensitive data, for example Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 16 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS personal identifiable data. Furthermore, personally identifiable data must be managed appropriately. • The system should be able to resist receiving corrupt, invalid or malicious data and commands by its external and internal interfaces, while remaining available for primary use. If safety-critical functions are compromised or ceased to work, systems are still resilient and fail-safe. The mechanism is proportionate to the risk. If non-safety critical functions fail, the system should respond appropriately. VII. C HALLENGES AND O PEN P ROBLEMS In this section, we describe some important and challenging problems. In addition, we outline several possible research directions. A. Dynamic Risks under development, related security attacks may start after a couple of years. The good news is that being-developed 5G security features are well defined and heavily rely on authentication and encryption. Nonetheless, its effectiveness needs to be tested thoroughly before and after CAVs are deployed in the field. In addition, security protocols may need to be updated later. E. Testing Standard For the cyber-security of vehicle systems, they are tested strictly before they are used by consumers. However, there is no unified standard for testing. In addition, manufacturers usually choose their own testers. Although there exist some guidelines for testing, they need to be revised for various complicated situations. Furthermore, testing can only find some easy vulnerabilities in vehicle systems. An adversary may intrude vehicle systems by trying all the possible methods. As a moving system, the environment of CAVs is changing frequently. As shown in Section V-D, extensive research has been carried out on cyber-risk assessment of CAVs. However, there is little study which can efficiently tackle dynamic risks that CAVs are facing with. Besides, given that cyber-risk assessment has the advantage of updating and viewing the risks from different angles, it is interesting to explore that whether it can be used to predict the unknown threats in the future. F. Data Collection and Storage B. Lightweight Security Model In order to hide trade contents and transaction relationships, it is interesting to use zero-knowledge proof for the verification of vehicle data. However, zero-knowledge proof confronts with the issue of inefficient performance. For example, the execution time of the prover algorithm is excessively long. Hence, some efforts are needed for exploring efficient implementation of zero-knowledge proof in the environment of CAVs. Because the road entities in the V2X communication have various capabilities and resources, the security model should always consider the efficient use of devices’ resources. An example is the power consumption, because they were implemented on vehicles which have a long life battery, all current security models did not consider the power consumption as a challenge. However, in V2X communication, some road entities, such as mobile phones, have a limited battery. Therefore, it is an essential task to achieve the trade-off between devices’ resources and security level in the environment of CAVs. As a result, a lightweight security model is necessary for V2X communication. C. Trust Levels In the environment of CAVs, if any single trusted component exists, single point of failure will manipulate the whole security of the system. Defenders should put redundant security resources in different components to cross-check each other. However, when attackers manipulate more than one component, they may also be able to compromise the crosscheck. Therefore, it is necessary to design proper trust levels in each component. When there are inconsistencies between them, designed trust-levels will decide which components are in favour for making security decisions. D. 5G Cellular Based V2X Security The security of 5G cellular based V2X (C-V2X) is another important research area. Since 5G C-V2X products are still Based on collected data from vehicles, judicial agencies and law enforcers can investigate evidences and guilty parties for some traffic accidents. For the effectiveness of investigation, data should be collected in a pattern, which can form the intact evidence-chain. Then, data should be stored in a format, which can be accessed securely and independently. G. Zero-Knowledge Proof H. Blockchain As a research hotspot, blockchain can guarantee that transaction data are impossible to be tampered, forged, or discarded by the adversary. Hence, in the environment of CAVs, the privacy of vehicle data can be protected by using the blockchain technique. However, most blockchains cannot support a large amount of users. In addition, there is no available standard for the seamless communication among various blockchains. Furthermore, the consensus mechanism requires tremendous computation power, which will affect the operating efficiency of the system. I. Spectrum Sharing US Department of Transportation used the spectrum sharing method [138] in the 5.9 GHz DSRC band. In this method, before operating the equipment in that spectrum, an operator does not need a license, which can avoid the expensive and time consuming licensing process. Available spectrum sharing mechanisms include the detect & vacate mechanism and detect & mitigate mechanism [139], which are proposed by the WiFi industry in the European Telecommunications Standards Institute. However, these mechanisms are both vulnerable to the Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on July 01,2021 at 04:29:53 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. SUN et al.: SURVEY ON CYBER-SECURITY OF CAVs delayed detection problem. That is to say, a WiFi device can usually detect the existence of DSRC transmissions only if it itself is not transmitting, which may affect the communications among vehicles. Although the detection performance can be improved by adding extra idle periods, due to the unilateral hidden terminal problem, the detect & mitigate mechanism can cause up to 30% additional packet loss to DSRC transmissions after detecting the existence of DSRC devices, which can further decrease the effectiveness of safety applications that are relied on V2V communications. VIII. C ONCLUSION CAVs are helpful to reduce traffic accidents, enhance quality-of-life, and promote the efficiency of transportation systems. However, there are various security and privacy challenges in the environment of CAVs. Hence, this paper has investigated related up-to-date literatures. Based on the types of communication networks and attack objects, this paper has firstly classified various cyber-security risks and vulnerabilities in the environment of CAVs into in-vehicle network attacks, vehicle to everything network attacks, and other attacks. Next, it has regarded cyber-risk as another type of attacks in the environment of CAVs. Then, it has described corresponding defending strategies to secure CAVs. Finally, several related cyber-security and safety standards have been summarized for improving the practical application of CAVs. In addition, several challenges and open problems have been presented. Classified potential cyber-security attacks and summarized corresponding countermeasures are helpful for the healthy development of CAVs and speeding up its practical deployment for intelligent transportation systems. Specifically, based on the lessons from the study of these cyber-security attacks, it may be useful for policymakers, engineers and researchers to cope with unknown security threats of CAVs in the future. In addition, they can obey the important design principles in the cyber-security and safety standards of CAVs. Besides, the policymakers and system operators can analyze the attacks and vulnerabilities in the environment of CAVs by using a comprehensive cyber-risk assessment strategy, which is helpful for understanding and prioritizing strategies to mitigate the security risk. In summary, the research on the cyber-security of CAVs is quite broad and many challenges lay ahead. 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He received the B.S. degree in mathematics and applied mathematics from Fuyang Normal University in 2011 and the Ph.D. degree in information and communication engineering from Shenzhen University in 2018. He was a Post-Doctoral Fellow with the F. Richard Yu’s Group, Carleton University, Ottawa, from 2019 to 2020. He is currently a Post-Doctoral Fellow with the Guangdong Key Laboratory of Intelligent Information Processing, College of Electronics and Information Engineering, Shenzhen University. He has published more than ten academic journal articles and conference papers. His research interests include cryptography, information security, and fully homomorphic encryption. F. Richard Yu (Fellow, IEEE) received the Ph.D. degree in electrical engineering from the University of British Columbia (UBC) in 2003. He is a registered Professional Engineer in ON, Canada, and a fellow of the Canadian Academy of Engineering (CAE), the Engineering Institute of Canada (EIC), and the Institution of Engineering and Technology (IET). From 2002 to 2006, he was with Ericsson, Lund, Sweden, and a start-up in California, USA. He joined Carleton University in 2007, where he is currently a Professor. His research interests include connected/autonomous vehicles, security, artificial intelligence, distributed ledger technology, and wireless cyber-physical systems. He received the IEEE TCGCC Best Journal Paper Award in 2019, the Distinguished Service Awards in 2019 and 2016, the Outstanding Leadership Award in 2013, the Carleton Research Achievement Awards in 2012 and 2020, the Ontario Early Researcher Award (formerly Premiers Research Excellence Award) in 2011, the Excellent Contribution Award at IEEE/IFIP TrustCom 2010, the Leadership Opportunity Fund Award from Canada Foundation of Innovation in 2009, and the Best Paper Awards at IEEE ICNC 2018, VTC 2017 Spring, ICC 2014, Globecom 2012, IEEE/IFIP TrustCom 2009, and Int’l Conference on Networking 2005. He has served as the Technical Program Committee (TPC) Co-Chair for numerous conferences. He has been named in the Clarivate Analytics list of Highly Cited Researchers in 2019 and 2020. He is an IEEE Distinguished Lecturer of the Vehicular Technology Society (VTS) and the Communication Society. He is an Elected Member of the Board of Governors of the IEEE VTS and Editor-in-Chief for IEEE VTS Mobile World newsletter. He serves on the editorial boards of several journals, including the Co-Editor-in-Chief for Ad Hoc and Sensor Wireless Networks, the Lead Series Editor for IEEE T RANSACTIONS ON V EHICULAR T ECHNOLOGY, IEEE C OMMUNICATIONS S URVEYS AND T UTORIALS , and IEEE T RANSACTIONS ON G REEN C OMMUNICATIONS AND N ETWORKING. Peng Zhang received the M.S. and Ph.D. degrees in signal and information processing from Shenzhen University in 2008 and 2011, respectively. She is currently an Associate Professor with the College of Electronics and Information Engineering, Shenzhen University. She has published more than 30 academic journal articles and conference papers. Her current research interests include cryptography technology and security in the blockchain, cloud computing, and the IoT. Authorized licensed use limited to: Univ of Calif Santa Barbara. 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