---
id: "CVE-2026-120596"
kind: "vulnerability"
aliases: ["CVE-2026-120596"]
title: "Nut/OS TCP stack: one crafted packet freezes all TCP services"
severity: "high"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: [{"name": "Nut/OS", "fixed": [], "affected": "<= 5.1", "ecosystem": "vendor"}]
ecosystems: []
tags: ["embedded", "ethernut", "infrastructure", "nut-os", "tcp-stack"]
not_affected_if: ["not-on-network", "nut-os-newer-than-5.1"]
published_at: "2026-11-02T17:17:40+00:00"
first_seen_at: "2026-11-02T17:24:12+00:00"
updated_at: "2026-11-02T17:27:23+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-120596", "first_seen_at": "2026-11-02T17:24:12+00:00"}, {"source": "analyst", "url": null, "first_seen_at": "2026-11-02T17:27:23+00:00"}]
---

# Nut/OS TCP stack: one crafted packet freezes all TCP services

A malformed TCP option with a zero length byte sends the Nut/Net TCP parser into an infinite loop in Nut/OS up to version 5.1. A single unauthenticated packet to any open TCP port stops all TCP services until the device is rebooted.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Nut/OS affected <= 5.1 fixed ?

## How to check

- Find the Nut/OS version your firmware was built with (the Nut/OS source tree or Configurator build); 5.1 and earlier are affected per NVD.
- Check whether net/tcpsm.c NutTcpInputOptions rejects a TCP option whose length byte is 0; if it does not, the build is exposed.
- Check whether the device is reachable on any open TCP port from untrusted networks. Also check any other product built on the Nut/Net stack.

## Timeline

- 2026-11-02T17:24:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-120596
- 2026-11-02T17:27:23+00:00 analyst — NVD page returned no text; description taken from the signal. Ethernut download page lists stable 5.1 and beta 5.2.4; no vendor advisory or fixed release found, so the fix is unknown (settling it needs a diff of NutTcpInputOptions in 5.2.x or the SVN repository). Nothing verified. No PoC or exploitation found; no social search done (obscure product).

## Documents

- [describes] NVD CVE-2026-120596 https://nvd.nist.gov/vuln/detail/CVE-2026-120596
- [mentions] Ethernut Download (stable 5.1, beta 5.2.4) http://www.ethernut.de/en/download/

## From the sources

A remotely exploitable denial-of-service vulnerability exists in the TCP options parser of the Nut/Net TCP/IP stack included in Nut/OS, developed by Ethernut/Egnite. Nut/OS releases up to version 5.1 are affected. An unauthenticated attacker can send a single TCP segment containing an unknown TCP option with a length byte of zero to any open TCP port on a vulnerable device. When processed, this malformed option triggers an infinite loop in the function NutTcpInputOptions, permanently blocking the TCP processing thread and rendering all TCP-based services unresponsive until the device is rebooted.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_