Wireless Sensor Networks (WSNs) are exposed to many security attacks, and it can be easily compromised. One of the main reasons for these vulnerabilities is the deployment nature, where sensor nodes are deployed without physical guarding duty. That makes the network susceptible to physical attacks. The communication nature between sensor nodes is another reason, where intruders can easily send/receive information if they are located in the network communication range. In this paper, most of the possible WSN attacks are discussed, different security services expected in WSN are explained, and trust-based solutions proposed in the literature are listed. Moreover, the state-of-the-art of the attacks’ mitigation and avoidance techniques are presented. Besides, this paper is enriched with a new classification of the WSNs attacks regarding attacks’ characteristics. It will be beneficial to researchers in the field of WSNs security if they can distinguish between different attacks that have common characteristics.