# Introduction Wireless Sensor Network is a special type of network that consist of distributed, low-power, small-size devices using sensors to cooperatively collect information through infrastructure less ad-hoc wireless network [1]. They are envisioned to play an important role in a wide variety of areas ranging from critical military surveillance applications to building security monitoring in the near future [2]. It shares some commonalities with a typical computer network, but also exhibits many characteristics which are unique to it. The security services in a Wireless Sensor Network should protect the information communicated over the network and the resources from attacks and misbehavior of nodes. The most important security requirements in Wireless Sensor Network are listed below: Data confidentiality: The security mechanism should ensure that no message in the network is understood by anyone except intended recipient. A sensor node should not allow its readings to be accessed by its neighbors unless they are authorized to do so. Data integrity: The mechanism should ensure that no message can be altered by an entity as it traverses from the sender to the recipient. Data freshness: It implies that the data is recent and ensures that no adversary can replay old messages. This requirement is especially important when the WSN nodes use shared keys for message communication, where a potential adversary can launch a replay attack using the old key as the new key is being refreshed and propagated to all the nodes in the WSN. Self-organization: Each node in a WSN should be self organizing and self-healing. The dynamic nature of a WSN makes it sometimes impossible to deploy any preinstalled shared key mechanism among the nodes and the base station [3]. Secure localization: In many situations, it becomes necessary to accurately and automatically locate each sensor node in a WSN. For example, a WSN designed to locate faults would require accurate locations of sensor nodes identifying the faults. A potential adversary can easily manipulate and provide false location information by reporting false signal strength, replaying messages etc. if the location information is not secured properly. Authentication: It ensures that the communicating node is the one that it claims to be. An adversary can not only modify data packets but also can change a packet stream by injecting fabricated packets. It is, therefore, essential for a receiver to have a mechanism to verify that the received packets have indeed come from the actual sender node. # II. # Charesterstics & applications of wsns There # Security attacks in wsn Wireless Sensor Networks are vulnerable to various types of attacks. These attacks are mainly of three types (denial of service attack, distributed attack and phishing attack.), Attacks on secrecy and authentication: standard cryptographic techniques can protect the secrecy and authenticity of communication channels from outsider attacks such as eavesdropping, packet replay attacks. Attacks on network availability: attacks on availability of WSN are often referred to as denial-of-service (DoS) attacks. Stealthy attack against service integrity: in a stealthy attack, the goal of the attacker is to make the network accept a false data value. In these attacks, keeping the sensor network available for its intended use is essential. The DoS attack usually refers to an adversary's attempt to disrupt, subvert, or destroy a network. However, a DoS attack can be any event that diminishes or eliminates a network's capacity to perform its expected functions. IV. # Dos attacks Wood and Stankovic have defined a DoS attack as an event that diminishes or attempts to reduce a network's capacity to perform its expected function. Some of the important types of DoS attacks in Wireless Sensor Networks are discussed below. # a) Physical Layer Attacks The physical layer is responsible for frequency selection, modulation, and data encryption [4]. As with any radio-based medium, the possibility of jamming is there. In addition, nodes in Wireless Sensor Networks may be deployed in hostile or insecure environments where an attacker has the physical access. Two types of attacks in physical layer are (i) jamming and (ii) tampering. # b) Link Layer Attacks The link layer is responsible for multiplexing of data streams, data frame detection, medium access control, and error control [4]. Attacks at this layer include purposefully created collisions, resource exhaustion, and unfairness in allocation. # c) Network Layer Attacks The network layer of Wireless Sensor Networks is vulnerable to the different types of attacks such as: spoofed routing information, selective packet forwarding, sinkhole, Sybil, wormhole, hello flood etc. # i. Spoofed routing information The most direct attack against a routing protocol is to target the routing information in the network. An attacker may spoof, alter, or replay routing information to disrupt traffic in the network [5]. ii. Selective forwarding Thn a multi-hop network like a Wireless Sensor Network, for message communication all the nodes need to forward messages accurately. An attacker may compromise a node in such a way that it selectively forwards some messages and drops others [5]. # iii. Sinkhole In this attack, a malicious node acts as a blackhole [6] to attract all the traffic in the sensor network. Especially in a flooding based protocol, the attacker listens to requests for routes then replies to the target nodes that it contains the high quality or shortest path to the base station. Once the malicious device has been able to insert itself between the communicating nodes (for example, sink and sensor node), it is able to do anything with the packets passing between them. In fact, this attack can affect even the nodes those are considerably far from the base stations. In many cases, the sensors in a wireless sensor network might need to work together to accomplish a task, hence they can use distribution of subtasks and redundancy of information. In such situation, a node can pretend to be more than one node using the identities of other legitimate nodes. This type of attack where a node forges the identities of more than one node is the Sybil attack [7]. Sybil attack tries to degrade the integrity of data, security and resource utilization that the distributed algorithm attempts to achieve. Sybil attack can be performed for attacking the distributed storage, routing mechanism, data aggregation, voting, fair resource allocation and misbehavior detection [7]. Basically, any peer-to-peer network (especially wireless ad hoc networks) is vulnerable to Sybil attack. However, as WSNs can have some sort of base stations or gateways, this attack could be prevented using efficient protocols. Douceur [8] showed that, without a logically centralized authority, Sybil attacks are always possible except under extreme and unrealistic assumptions of resource parity Wormhole attack [9] is a critical attack in which the attacker records the packets at one location in the network and tunnels those to another location. The tunneling or retransmitting of bits could be done selectively. Wormhole attack is a significant threat to wireless sensor networks, because; this sort of attack does not require compromising a sensor in the network rather, it could be performed even at the initial phase when the sensors start to discover the neighboring information. When a node B broadcasts the routing request packet, the attacker receives this packet and replays it in its neighborhood. Each neighboring node receiving this replayed packet will consider itself to be in the range of Node B, and will mark this node as its parent. Hence, even if the victim nodes are multi-hop apart from B, attacker in this case convinces them that B is only a single hop away from them, thus creates a wormhole. # vi. Hello flood Most of the protocols that use Hello packets make the naive assumption that receiving such a packet implies that the sender is within the radio range of the receiver. An that receiving such a packet implies that the sender is within the radio range of the receiver. An attacker may use a high-powered transmitter to fool a large number of nodes and make them believe that they are within its neighborhood [5]. Subsequently, the attacker node falsely broadcasts a shorter route to the base station, and all the nodes which received the Hello packets, attempt to transmit to the attacker node. vii. Acknowledgment spoofing Some routing algorithms for Wireless Sensor Networks require transmission of acknowledgment packets. An attacking node may overhear packet transmissions from its neighboring nodes and spoof the acknowledgments thereby providing false information to the nodes [5]. # d) Transport layer attacks The attacks that can be launched on the transport layer in a Wireless Sensor Network are flooding attack and de-synchronization attack. # i. Flooding Whenever a protocol is required to maintain state at either end of a connection, it becomes vulnerable to memory exhaustion through flooding. An attacker may repeatedly make new connection request until the resources required by each connection are exhausted or reach a maximum limit. In either case, further legitimate requests will be ignored. ii. De-synchronization De-synchronization refers to the disruption of an existing connection. An attacker may, for example, repeatedly spoof messages to an end host causing the host to request the retransmission of missed frames. If timed correctly, an attacker may degrade or even prevent the ability of the end hosts to successfully exchange data causing them instead to waste energy attempting to recover from errors which never really exist. # e) Attacks on secrecy and authentication There are different types of attacks under this category as discussed below: i. Node replication attack In a node replication attack, an attacker attempts to add a node to an existing WSN by replication the node identifier of an already existing node in the network. A node replicated and joined in the network in this manner can potentially cause severe disruption in message communication in the Wireless Sensor Network by corrupting and forwarding the packets in wrong routes. ii. Attacks on privacy Since Wireless Sensor Networks are capable of automatic data collection through efficient and strategic deployment of sensors, these networks are also vulnerable to potential abuse of these vast data sources. Privacy preservation of sensitive data in a Wireless Sensor Network is particularly difficult challenge [10]. Moreover, an adversary may gather seemingly innocuous data to derive sensitive information if he knows how to aggregate data collected from multiple sensor nodes. Following are some of the common attacks on sensor data privacy [10]. iii. Eavesdropping and passive monitoring This is most common and easiest form of attack on data privacy. If the messages are not protected by cryptographic mechanisms, the adversary could easily understand the contents. Packets containing control information in a WSN convey more information than accessible through the location server, Eavesdropping on these messages prove more effective for an adversary. # iv. Traffic analysis In order to make an effective attack on privacy, eavesdropping should be combined with a traffic analysis. Through an effective analysis of traffic, an adversary can identify some sensor nodes with special roles and activities in a WSN. # v. Camouflage An adversary may compromise a sensor node in a WSN and later on use that node to masquerade a normal node in the network. This camouflaged node then may advertise false routing information and attract packets from other nodes for further forwarding. After the packets start arriving at the compromised node, it starts forwarding them to strategic nodes where privacy analysis on the packets may be carried out systematically. # V. # Distributed attack A distributed attacks occurs when multiple systems flood the bandwidth or resources of a targeted system, usually one or more web servers. Distributed attacks are traditionally viewed to be fundamentally more difficult to detect than single -source attacks. One reason why distributed attacks are dif ficult to contain is because defenses against these attacks are typically deployed at edge networks, near the victim. Deploying defenses at the edge makesdetecting attacks easier, VI. # Phishing attack In phishing attack the hacker creates a fake web site that looks exactly like a popular site such as the SBI bank or PayPal. The phishing part of the attack is that the hacker then sends an e-mail message trying to trick the user into clicking a link that leads to the fake site. When the user attempts to log on with their account information, the hacker records the username and password and then tries that information on the real site. # VII. # Challenges There are following challenges occurring in the wireless technology which are given in the following two categories-Challenges Vs Internet: # Conclusion Wireless Sensor networks have become promising future to many applications. In the absence of enough security, deployment of sensor networks is vulnerable to variety of attacks. Overall security for wireless sensor networks is very hard to develop due to the limited resources of the sensors. Sensor network security will always be a field in which much work needs to be done. Current research in sensor network security is mostly built on a trusted environment [11]; however there are several research challenges remain unanswered before we can trust on sensor networks. In this paper we have discussed threat models and unique security issues faced by wireless sensor networks. In WSNs, there are still some challenges that are to be addressed. ![Fig 1 shows the conceptual view of a sinkhole attack.](image-2.png "") 1![Figure 1 : Sinkhole Attack iv. Sybil attack](image-3.png "Figure 1 :") ![and coordination among entities. 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