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

At DEF CON 34, two researchers demonstrated how inexpensive hardware modifications allow interception of downstream traffic in GPON fiber networks shared among multiple subscribers. The passive optical design means all clients connected to the same splitter receive identical data streams, with separation handled only at the ONU level through 12-bit GEM port identifiers. By acquiring Realtek RTL960x-based SFP modules and systematically bypassing built-in restrictions on GEM port reassignment, VLAN tagging, switch forwarding, and checksum validation, the team achieved access to traffic intended for up to 128 neighboring users. Exposed data includes DNS queries that can reveal occupancy patterns, unencrypted SIP voice traffic, and in some cases traffic from nearby cellular base stations. The researchers also identified exploitable vulnerabilities in the VSOL G100S OLT device that could grant root-level control and compromise provider routers. Additional coverage in the same Kaspersky report examined Head Mare's compromise of TrueConf servers, EvilFontTool font-based evasion, an OpenAI presentation on an accidental AI attack against Hugging Face, and the SCTPhantom Linux kernel flaw present for 18 years.

HabrAugust 10, 2026Translated from Russian

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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.

Coldcard TrueConf Realtek RTL960x VSOL G100S SCTPhantom CVE-2026-64564

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