Every half year or so security researchers from Israel's Ben-Gurion University come up with an innovative new way to steal data from air-gapped networks. Often these techniques are far-fetched and not very practical but they're very interesting nonetheless.
This time the Ben-Gurion University researchers came up with a way to turn a switch or a router into an espionage device. This is achieved by planting a piece of "xLED" malware that can control the device's LEDs to transmit data in binary format.
This malware will intercept specific data passing through the router, break it down into its binary format, and use a router LED to signal the data to a nearby attacker, with the LED turned on standing for a binary one and the LED turned off representing a binary zero.
An attacker with a clear line of sight to the equipment can record the blinking operation. This “attacker” can be a security camera, a company insider, recording equipment mounted on a drone, and various other setups where a video recording device has a clear sight of the router or switch’s blinking LEDs.
More details at Bleeping Computer. As noted above, it's an interesting technique but it doesn't make a whole lot of sense as it requires access to the router or switch.