In resent years

mobile ad hoc networks are a new emerging field with its potential applications in extremely unpredictable and dynamic environments.However

it is particularly vulnerable due to its features of open medium

dynamic changing topology

cooperative routing algorithms.The article surveys the state of the art in security for mobile ad hoc networks.Firstly

we analyze various possible threats to security in mobile ad hoc networks.Secondly

we illustrate representative solutions to deal with those threats

including key management

intrusion detection and cooperation enforcement.We also make a comparison and discussion of their respective merits and faults

and propose some ways to fix these problems.Finally

we also present the challenges which are still worth to further research in the area.