Specially-designed malware installed on a router or switch can take control over the device s LEDs and use them to transmit data in a binary format to a nearby attacker, who can capture it using simple video recording equipment. The attack scenario is the creation of a talented team of researchers from the Cyber Security Research Center at the Ben-Gurion University of the Negev in Israel, who previously researched other types of data exfiltration scenarios relying on hard drive LEDs, headphones, and others.
Source: https://www.bleepingcomputer.com/news/security/malware-uses-router-leds-to-steal-data-from-secure-networks/