---
id: "CVE-2026-120831"
kind: "vulnerability"
aliases: ["CVE-2026-120831"]
title: "libslirp: queue corruption in if_output() lets a guest crash host networking"
severity: "medium"
cvss: 6.5
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["infrastructure", "libslirp", "qemu", "self-hosted", "virtualization"]
not_affected_if: ["no-user-mode-networking", "trusted-guests-only"]
published_at: "2026-11-03T02:16:53+00:00"
first_seen_at: "2026-11-03T02:47:45+00:00"
updated_at: "2026-11-03T02:48:57+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-120831", "first_seen_at": "2026-11-03T02:47:45+00:00"}]
---

# libslirp: queue corruption in if_output() lets a guest crash host networking

libslirp, the user-mode networking library used by QEMU and similar tools, corrupts its packet queue pointers when moving a session between queues. A guest that generates burst traffic can crash the host networking process, causing denial of service.

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

## How to check

- Check whether the host runs QEMU or another tool built on libslirp, for example with `qemu-system-x86_64 --version` and `ldd $(which qemu-system-x86_64) | grep slirp`
- Note whether guests use user-mode networking (`-netdev user`) rather than a bridge or tap device

## Timeline

- 2026-11-03T02:47:45+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-120831

## From the sources

A flaw was found in libslirp in if_output(). When moving a session from the fast queue to the batch queue, the code unlinked and reinserted an arbitrary packet via ifm->m_nextpkt instead of the session queue head, corrupting queue pointers. A guest generating burst traffic could crash the host networking process, causing a denial of service.

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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_