We propose a Hardware Trojan (HT) attack in wireless Integrated Circuits (ICs) that aims at leaking sensitive information within a legitimate transmission. The HT is hidden inside the transmitter modulating the sensitive information into the preamble of each transmitted frame which is used for the synchronization of the transmitter with the receiver. The data leakage does not affect synchronization and is imperceptible by the inconspicuous nominal receiver as it does not incur any performance penalty in the communication. A knowledgeable rogue receiver, however, can recover the data using signal processing that is too expensive and impractical to be used during run-time in nominal receivers. The HT mechanism is designed at circuit-level and is embedded entirely into the digital section of the RF transceiver having a tiny footprint. The proposed HT attack is demonstrated with measurements on a hardware platform. We demonstrate the stealthiness of the attack, i.e., its ability to evade defenses based on testing and run-time monitoring, and the robustness of the attack, i.e., the ability of the rogue receiver to recover the leaked information even under unfavorable channel conditions.