  Covert channel networks are a well-known method for circumventing the
security measures organizations put in place to protect their networks from
adversarial attacks. This paper introduces a novel method based on bit-rate
modulation for implementing covert channels between devices connected over a
wide area network. This attack can be exploited to exfiltrate sensitive
information from a machine (i.e., covert sender) and stealthily transfer it to
a covert receiver while evading network security measures and detection
systems. We explain how to implement this threat, focusing specifically on
covert channel networks and their potential security risks to network
information transmission. The proposed method leverages bit-rate modulation,
where a high bit rate represents a '1' and a low bit rate represents a '0',
enabling covert communication. We analyze the key metrics associated with
covert channels, including robustness in the presence of legitimate traffic and
other interference, bit-rate capacity, and bit error rate. Experiments
demonstrate the good performance of this attack, which achieved 5 bps with
excellent robustness and a channel capacity of up to 0.9239 bps/Hz under
different noise sources. Therefore, we show that bit-rate modulation
effectively violates network security and compromises sensitive data.
