Mobile ad hoc networks have inherently different properties than traditional wired networks. These new characteristics present different security vulnerabilities and this paper provides a detailed classification of these threats. Threats exist to a mobile ad hoc network both from external nodes unauthorized to participate in the mobile ad hoc networks, and from internal nodes, which have the authorization credentials to participate in the mobile ad hoc network. Internal nodes giving rise to threats can be further divided according to their behavior -failed, badly failed, selfish and malicious nodes. All categories of node behavior should be considered when designing protocols for mobile ad hoc networks.