When a transmitter node sends a data frame in an IEEE 802.11 distributed coordinated function (DCF) network, the receiver sends an acknowledgment (ACK) frame within a short inter-frame space (SIFS) interval if a data frame is successfully received. If the transmitter node fails to receive this ACK frame, it considers the transmission a failure and attempts to retransmit the data frame. We consider a data frame jamming scenario in which an attacker transmits a jamming signal while a transmitter is sending a data frame. In this scenario, the transmitter does not receive the ACK frame from its intended receiver and may suspect the presence of signal jammers if it experiences repeated failures. We propose a covert jamming method that deceives a transmitter into believing that the transmission was successful by injecting a fake ACK frame to pretend to be its intended receiver.