One of the growing problems faced by network administrators is the abuse of computing resources by authorized and unauthorized personnel. The nature of abuse may vary from using unauthorized applications to serving unauthorized content. Proliferation of peer-to-peer networks and wide use of tunnels makes it difficult to detect such abuses and easy to circumvent security policies. This paper presents the design and implementation of a system, called Nabs, that characterizes content types of network flows based solely on the payload which can then be used to identify abuses of computing resources. The proposed method does not depend on packet headers or other simple packet characteristics hence is more robust to circumvention.