Security Researchers Turn Fibre Optic Broadband Lines into Spying Microphones

A team of security researchers from Hong Kong have demonstrated a covert way of turning standard fibre optic broadband cables, such as those used to connect homes (FTTP), into passive and undetectable microphones, which can be used to eavesdrop on conversations people may be having inside a property (house, office etc.).

Firstly, it’s important to remember that the idea of using optical fibre cables, which transmit data using pulses of laser light, to detect and monitor nearby events is nothing new. Scientists have already found ways of turning such cables into Earthquake and Tsunami monitors (here and here), which work by detecting changes in the polarization of light as it moves through the cable.

More recently we’ve also seen these fibre sensing technologies being improved and adapted to detect other things, such as cases of sabotage against vital subsea cables (here), while Openreach are separately helping to trial an approach for helping to locate water and gas leaks (here). So far, so positive.

The catch is that this same sort of technology could also potentially be used for more worrying purposes, which has just been demonstrated at the Network and Distributed System Security (NDSS) Symposium 2026 in San Diego, California – under the title: “Hiding an Ear in Plain Sight: On the Practicality and Implications of Acoustic Eavesdropping with Telecom Fiber Optic Cables” (here).

Summary of the Research Paper

Optical fibers are widely regarded as reliable communication channels due to their resistance to external interference and low signal loss.

This paper demonstrates a critical side channel within telecommunication optical fiber that allows for acoustic eavesdropping. By exploiting the sensitivity of optical fibers to acoustic vibrations, attackers can remotely monitor sound-induced deformations in the fiber structure and further recover information from the original sound waves.

This issue becomes particularly concerning with the proliferation of Fiber-to-the-Home (FTTH) installations in modern buildings. Attackers with access to one end of an optical fiber can use commercially available Distributed Acoustic Sensing (DAS) systems to tap into the private environment surrounding the other end. However, because the optical fiber alone is not sensitive enough to airborne sound, we introduce a “Sensory Receptor” that improves acoustic capture.

Our results demonstrate the ability to recover critical information, such as human activities, indoor localization, and conversation contents, raising important privacy concerns for fiber-optic communication networks.

Now before everybody jumps into panic mode, it’s worth noting that the Internet Protocol (IP) based data connectivity that normally runs over such cables is already something that can be much more problematic than this for your average home broadband connection (e.g. hackers taking control of your router/network/computer or Amazon’s Alexa listening in on your conversations).

In fact, it would probably be a lot easier for hackers to snoop on you digitally via the internet than to try and directly convert the physical optical fibre cable that connects your home into an acoustic vibration sensing Microphone. In addition, the method demonstrated can currently only recover 80% of speech at a source-to-receptor distance of 2 metres, which is rather limiting but may be improved.

The team were also able to use this method to identify a person’s location, with a strong degree of accuracy, within a building. On top of that they could also identify other activities, such as typing, coughing or alarms etc.

Depending on the setup, the attacker themselves could be as much as 50 metres away, while requiring no line of sight to the victim’s environment, and the method is also resistant to commercial ultrasonic jammers. But the commercial kit required to make all this work is highly specialised and very expensive, often costing many thousands of pounds. Deploying this method also risks breaking the cable.

In short, the team have demonstrated that it’s possible, which is a first step and today’s limitations may well be overcome in the future. But for now, an enterprising hacker can probably find many other ways to snoop on your activity by breaching your home network or device via the internet, and they could be half a world away while doing all that instead of just 50m.