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In many medium access control (MAC) schemes for wireless networks, an Acknowledgment (ACK) packet is transmitted from the data receiver to the data sender to announce the successful reception of the data packet. Such a protocol requirement may become a system weakness when malicious nodes attack these wireless networks. In this paper, we demonstrate the effects of such a Jamming ACK (JACK) attack to networks employing the popular carrier sense multiple access with collision avoidance (CSMA/CA) scheme in IEEE 802.11 DCF. Our study shows that a JACK attacker can easily disrupt the traffic flow between two wireless nodes when it sends out JACK packets at the right time. The benefits of such a JACK attack include low energy consumption by the attacker, attack stealthiness, and great damage to the victim nodes. To mitigate the effects of JACK attacks, we propose in this paper an extended network allocation vector (ENAV) scheme. Our analysis and simulations show that the ENAV scheme recovers a significant portion of the lost throughput and reduces the energy drainage of the attacked nodes to 40%.