Researchers Expose GPON Optical Network Eavesdropping via Modified ONU Devices at DEF CON

At DEF CON 34, researchers presented an attack technique called GPWN that exploits fundamental characteristics of GPON (Gigabit Passive Optical Network) infrastructure to eavesdrop on neighboring subscribers' downstream traffic.

The passive nature of these networks means that all clients connected to the same optical splitter receive identical data; separation occurs only at the Optical Network Unit (ONU) through software filtering of GEM frames carrying 12-bit port identifiers. Using approximately one hundred dollars' worth of hardware, the researchers modified commodity Realtek RTL960x SFP modules to override these filters.

They sequentially disabled multiple protective mechanisms: GEM port identifier reassignment, VLAN tagging enforcement, internal switch forwarding rules, and various checksum validations. The result was the ability to read traffic destined for as many as 128 other subscribers on the same splitter—an apparent hardware limit of consumer-grade ONUs.

Although most traffic is encrypted, the researchers showed that DNS queries remain visible and can indicate when a residence is unoccupied. In certain deployments, cellular base-station traffic and unencrypted SIP voice calls were also observable. Importantly, only downstream traffic could be intercepted; upstream traffic remained protected.

The presentation further examined the possibility of compromising the provider-side Optical Line Terminal (OLT). Analysis of the VSOL G100S OLT revealed multiple vulnerabilities that could allow root-level command execution and subsequent compromise of customer routers.

Physical access to splitters located in public spaces was also noted as a realistic attack vector that would enable bidirectional traffic interception. Recommended mitigations include mandatory provider-side encryption and consistent use of VPNs by subscribers.

Additional items covered in the same report include the Head Mare group's compromise of TrueConf conferencing servers, the EvilFontTool font-based evasion technique, an OpenAI disclosure regarding an accidental AI-driven attack on Hugging Face, a new Spectre-v2 bypass affecting Intel and AMD processors, and the 18-year-old SCTPhantom (CVE-2026-64564) Linux kernel vulnerability.