## 2 Security Issues in WSN
### Vulnerabilities and Attacks  
Wireless sensor networks are vulnerable to several types of attacks, which may compromise one or more of the security requirements previously described. A vulnerability is a weakness in a system that can be exploited by attackers. In general, attacks may be centered on exploiting vulnerabilities in some communication layer, eavesdropping on transmitted data, altering confidential data or prejudicing the network operation with artificial malicious information. Besides the resource-constrained nature of sensor networks, wireless _ad hoc_ communications are inherently vulnerable to many attacks that are common in conventional wireless networks. The authors in [12] classify security threats for WSN into different groups. First, an attack may be external or internal, depending on its origin. An external attack comes from outside the network and can be performed through passive eavesdropping or injecting malicious packets to consume processing and energy resources, while internal attacks will be executed by legitimate nodes that will behave in unintended ways. An attack may also be passive, with no modification of the network, and active, where data streams are modified or created. At last, they also classify attacks into a more class or laptop class. In more class attacks, sensors with similar capabilities of the nodes at the target network are employed, while laptop class attacks are based on devices with better computational and transmission capabilities. In a different perspective, the authors in [13] classify security threats according to their context, which can be node-centric, data-centric, user-centric or network-centric. Each of these perspectives poses particular challenges when addressing security. Attacks in wireless sensor networks can also be classified according to their nature, as described in [12, 14] and summarized as follows:  
* Interruption: when network availability is compromised, usually resulting from DoS attacks. * Interception: when network confidentiality is compromised, allowing unauthorized access to sensor nodes and sensed data. * Modification: when network integrity is compromised, with modified packets potentially leading to an unexpected and misled operation of the network. * Fabrication: when network authentication is compromised, the trustworthiness of network elements and transmitted data may be affected by false information. A lightweight protocol stack is usually defined for wireless sensor networks, where protocols for different tasks may operate in a cross-layer design. Each protocol has particularities and vulnerabilities that can be exploited by attackers. In the physical layer, jamming is a denial of service (DoS) attack that may deplete the energy resources of sensors. Actually, DoS is a popular attack [9] in which malicious information is injected into the network, which can rapidly deplete the processing, memory and energy resources of nodes, compromising their operation. In jamming attacks, one or more malicious nodes interfere with the radio frequencies being used by valid nodes. It may then interrupt packet transmissionsor incur excessive retransmissions. On the other hand, nodes deployed in outdoor environments are especially vulnerable to tampering attacks. This kind of attack is defined by physical access to captured nodes, which can be damaged or have their configuration and circuitry modified. The link layer is also vulnerable to jamming attacks, which can be designed to interfere with the access control operation of MAC protocols to produce packets collisions. For protocols with higher abstraction, attacks may be even more elaborate [15; 16]. Routing protocols may be attacked to create loops or redirect packets to malicious nodes. Relaying nodes may provide wrong information to neighbor nodes as multiple false identities, defining what is known as a Sybil attack. Transport-layer protocols may be attacked when having to deal with excessive connection requests.