Most wireless routers are equipped with a series of LEDs to indicate things like network connectivity and activity, though if a router has been compromised with malware, those blinking lights could reveal more than the owner bargained for. Using specially crafted malware, an attacker could instruct those LEDs to transmit data in a binary format.

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Source: Hot Hardware – Malware Turns Routers Into Network Security Hack Beacons With xLED Data Exfiltration