AudioGap: An AirGapped Covert Channel Exploiting the Frequency Diversity of Audio IC Electromagnetic Leakage
Mohamed Alla Eddine Bahi, Maria Méndez Real, Erwan Nogues, Maxime Pelcat · 2025
This paper presents AudioGap, a novel Electromagnetic (EM) covert channel that exfiltrates data from airgapped systems by exploiting System Signals Auto Modulation (SSAM), a passive modulation phenomenon inherent in digital circuitry. Unlike conventional EM attacks that require fine control over hardware, AudioGap passively leverages SSAM interactions between a Local Oscillator (Lo) and nearby digital circuit to exfiltrate data.We demonstrate a covert channel based on Comb Frequency Division Multiplexing (CFDM) that achieves 2-bit-per-symbol-period transmission, effectively doubling the throughput of traditional On-Off Keying (OOK) and Binary Amplitude Shift Keying (B-ASK). Furthermore, we develop a harmonic-based frequency detection technique that reduces the search space by a factor of 30 compared to brute-force methods, significantly improving receiver frequency search.Experimental validation using a Realtek ALC3220 audio chip and Software Defined Radio (SDR) demonstrates data transmission up to 28 cm, with up to 25.8% lower Bit Error Rate (BER) compared to state-of-the-art modulation.