# CVE-2026-120831: libslirp queue pointer corruption in if_output()

Source: https://junglewise.ai/threats/cve-2026-120831-a-flaw-was-found-in-libslirp-in-if-output-when-moving-a-session  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-03. Updated: 2026-11-03  
License: CC BY 4.0 with a link back

CVE-2026-120831 · Severity: medium · CVSS 6.5 · Published 2026-11-03

libslirp is a user-mode networking library used in virtual machine hosts to handle guest network traffic. A queue pointer corruption bug in libslirp can be triggered by a guest generating burst traffic, causing the host's networking process to crash and denying network service to all virtual machines on that host.

Vendors: [Freedesktop.Org](https://junglewise.ai/threats/vendors/freedesktop-org).

## Quick answers

**Is CVE-2026-120831 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-120831 being exploited in the wild.

**How severe is CVE-2026-120831?** Medium severity, with a CVSS score of 6.5, published 3 November 2026.

**Which products and versions are affected?** freedesktop.org libslirp.

**Is there a fix for CVE-2026-120831?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-120831 attacked?** It is attacked from the adjacent network, in a software library.

**Am I affected by CVE-2026-120831?** Check whether you run Freedesktop.Org Libslirp. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An out-of-bounds write vulnerability exists in the if_output() function of libslirp when moving a session from the fast queue to the batch queue. The code incorrectly uses ifm->m_nextpkt instead of the session queue head when unlinking and reinserting packets, corrupting internal queue pointers. A guest virtual machine generating intentional burst traffic can trigger this condition to crash the host networking process without requiring authentication or user interaction.

## Affected products

- freedesktop.org libslirp

## Timeline

- 2026-11-03: disclosed

## References

- [https://access.redhat.com/downloads/content/package-browser/](https://access.redhat.com/downloads/content/package-browser/)
- [https://access.redhat.com/security/cve/CVE-2026-120831](https://access.redhat.com/security/cve/CVE-2026-120831)

## Related threats

- [freedesktop.org libslirp heap OOB read in TCP urgent data handling](https://junglewise.ai/threats/cve-2026-9539-freedesktop-org-libslirp-heap-oob-read-in-tcp-urgent-data-handling)