A team of researchers has shown that, with the right expertise and access, ordinary fiber optic internet cables can be turned into covert listening devices.

The Hong Kong Polytechnic University study, Hiding an Ear in Plain Sight, presented at this year’s NDSS, an academic conference focussed on cybersecurity, demonstrates that fiber-optic cables used to carry internet data can also pick up sound.

The technique relies on the fact that optical fibers are sensitive to tiny vibrations. When sound waves, such as human speech, travel through the air, they cause microscopic changes in the shape of the fiber.

These changes alter the light signals traveling inside the cable. By measuring those changes with an off-the-shelf technology called Distributed Acoustic Sensing (DAS), the researchers claimed that it is possible to reconstruct the original sound.

The paper set out to demonstrate this privacy problem within optical fibers “that can be exploited to eavesdrop on personal information.”

Secret sauce: the sensory receptor

To make an attack practical, the researchers designed a small device called a “sensory receptor” onto which the optical fiber is wound.

This involves wrapping the fiber around a small 65mm cylinder, which amplifies sound vibrations.

“This approach [sensory receptor deployment] achieves both a directional transformation and an accumulative effect: changes in the cylinder’s diameter (caused by sound waves) translate into stretching and contracting forces along the fiber’s length, while the coiling allows a longer fiber segment to be subjected to the strain."

Hiding an Ear in Plain Sight: NDSS paper submitted by Hong Kong Polytechnic University study

Researchers say this amplifies airborne sound, enabling the system to recover speech with surprising clarity.

In controlled and real-world office tests, the method retained more than 80% of the conversation content at short distances, with relatively low transcription errors.

With this setup, the researchers claim to recover a large proportion of spoken conversations at short distances – typically within a few meters.

The researchers suggest disguising the sensory receptor as an ordinary optical fiber box.

“This subtle camouflage allows the sensory receptor to blend in with other networking equipment for home/business, reducing the risk of raising suspicion,” they said.

Evading detection

Perhaps the most concerning aspect of this attack for defenders is its ability to evade detection. Unlike traditional bugs or microphones, their system does not require power at the listening location and does not emit radio signals.

This makes the system invisible to standard bug-detection tools and resistant to common countermeasures, such as ultrasonic jammers.

However, there are limitations to this type of covert eavesdropping. The researchers note that the fiber must be relatively close to the sound source for the speech to be clear, and performance decreases with distance and background noise.

The researchers assume that carrying out such covert operations also requires physical access to fiber infrastructure and specialized equipment, which can be expensive and difficult to obtain.

Because of these constraints, they suggest the method is most likely to be used in targeted situations – an inside bad actor, “such as a technician or subcontractor,” or “by attackers impersonating these roles, or through compromised third-party service providers.”

Such use cases would lend themselves well to corporate espionage, surveillance in sensitive environments such as government or diplomatic offices, or inside threats involving telecom workers with access to network infrastructure.

However, one point the researchers make in the paper is worth bearing in mind: in many office and private residential buildings, multiple, often legacy optical fibers are installed, belonging to different internet service providers, which could provide snoopers with a channel to spy.

“Usually, only one fiber is in active use, while the others remain unused (which are also known as dark fibers), running along walls, ceilings, and other interior structures. These fibers could potentially serve as unintended channels for eavesdropping,” the report states.

It’s not the only trusted infrastructure with spying potential to hit the headlines this year. Last month, Cybernews reported on a GitHub project that claims to use WiFi to see through walls.

However, it was not as thorough as this Hong Kong University paper, though, with commentators dismissing WiFi-DensePose as “AI Slop,” adding that the project lacked the signal-processing pipeline required to extract CSI from WiFi hardware.

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