- Some researchers from Hong Kong universities demonstrate that fiber-optic internet cables can capture nearby sounds, with a small amplifier device hidden inside standard junction boxes recovering 80 percent of conversation content from over 50 meters away.
- The eavesdropping method requires no power or radio signals, this makes it invisible to traditional bug detection tools and completely resistant to ultrasonic jammers used in sensitive meeting rooms.
- Organizations can defend against the attack by installing polished fiber connectors and optical isolators, they should ensure installers do not leave excess fiber coiled inside rooms.
A group of researchers in Hong Kong has developed an innovative technique that allows hackers to listen in on conversations using traditional fiber optic internet cables. The researchers from the Hong Kong Polytechnic and the Chinese University of Hong Kong tested their method in real-world conditions.
They presented their study paper, titled ‘Hiding an Ear in Plain Sight,’ the NDSS Symposium 2026, one of the biggest cybersecurity conferences in the world. The results of this research showed how the glass fiber that transmits internet information to businesses and residential properties can collect sound, even private conversations.
How Sound Waves Travel Through Glass Fibers
Sound waves always create microscopic vibrations through any object, including the physical medium of a thin piece of glass fiber which connects an internet provider into a building. Each micro vibration slightly changes the laser light as it passes through the glass fiber.
By connecting a special type of monitoring device called a Distributed Acoustic Sensing (DAS) to the opposite end of the cable, an attacker will be able to read the vibrations in this case and reconstruct them into corresponding audible sounds or voice-like speech.
However, a standard fiber lying along a baseboard lacks enough sensitivity on its own. Sound fades too quickly in the air, and the fiber is too thin to pick it up without help. The researchers solved this problem by building a small device they call a “Sensory Receptor.”
This 65mm plastic cylinder holds approximately 15 meters of fiber wound tightly around it. The cylinder traps sound waves and amplifies them in huge amounts such that the fiber can register them.
The receptor is quite small to hide within the same plastic junction box that internet installers leave on walls to manage extra cable. Attackers could plant the device during routine installation or maintenance without raising suspicion.
The Attack Captures Conversations with High Accuracy
The research team tested their method in both laboratory and real office environments. Their results showed the system could recognize daily activities, including typing, walking, snoring, and washing dishes with 83 percent accuracy. Also, the attack could locate where a sound originated in a room, with accuracy within approximately one meter.
Most concerning, the researchers successfully recovered spoken words from several meters away. In a real office test, they hid the receptor inside a fiber box and positioned the attacker over 50 meters away in another room. The system recovered roughly 80 percent of the conversation content.
This attack differs fundamentally from traditional hidden microphones – the fiber-optic system requires no electricity, no batteries, and emits no radio signals. Professional bug sweeps that look for hidden mics or cameras cannot detect it. Ultrasonic jammers, which some boardrooms adopt against the use of phone microphones, have zero effect on this technique.
How Organizations Can Defend Against the Threat
The researchers recommend several countermeasures for sensitive environments. Organizations should install polished fiber connectors and optical isolators on transmission channels. Both measures make this attack much harder to execute.
Building managers should not let internet installers leave excess fiber coiled inside rooms. Instead, they should coil the cable inside walls or in sealed boxes outside the room. Keeping fiber runs away from desks and resonant walls also reduces risk. In high-security spaces, adding soundproofing to walls and ceilings where fiber runs provides additional protection.
The attack requires physical access to both ends of the target fiber. This means a rogue insider, a subcontractor, or someone impersonating a technician must plant the receptor. While the technique poses a real threat to government facilities, corporate boardrooms, and diplomatic offices, average home users face lower risk from this sophisticated eavesdropping method.
The threat to government and defence networks is not theoretical. In a separate but related development, Pakistan-linked hackers target Indian government and defence networks, demonstrating that state-sponsored actors are actively seeking ways to infiltrate sensitive systems, whether through fiber-optic eavesdropping or digital intrusion. Organizations securing their facilities must consider both physical and cyber threats.
