Man-in-the-Middle (MitM) attacks pose severe threats to the Robust Security Network (RSN). Based on the state machine model of the authenticator and supplicant in 802.1X-2004, MitM attacks were analyzed systematically from the respect of the whole establishment of RSN associations. With the unilateral cognition of the MitM attacks in RSN clarified, a framework for the MitM attacks in RSN and its conditions of the effective launch of the attacks were brought forward, which were fully verified by an effective attack instance. The analytical results reveal that RSN can withstand MitM attacks if strong mutual authentication methods are adopted; otherwise it is vulnerable to this threat.