# CVE-2026-85032: Zephyr MCTP over USB use-after-free in packet buffer handling

Source: https://junglewise.ai/threats/cve-2026-85032-the-mctp-over-usb-device-class-in-subsys-pmci-mctp-mctp-usb-c  
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-2026-85032 · Severity: medium · CVSS 4.8 · Published 2026-11-03

Zephyr's MCTP (Management Component Transport Protocol) over USB implementation has a race condition that allows an attacker with USB host access to trigger a use-after-free vulnerability. By repeatedly sending partial MCTP frames and forcing USB configuration changes or bus resets, an attacker can corrupt memory or cause a denial of service without needing credentials. This could lead to system instability or data corruption on devices using the MCTP subsystem.

Technologies: [Zephyr Project Zephyr OS](https://junglewise.ai/threats/technologies/zephyr-os). Vendors: [Zephyr Project](https://junglewise.ai/threats/vendors/zephyr-project).

## Quick answers

**Is CVE-2026-85032 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-85032 being exploited in the wild.

**How severe is CVE-2026-85032?** Medium severity, with a CVSS score of 4.8, published 3 November 2026.

**Which products and versions are affected?** Zephyr Project Zephyr OS versions with CONFIG_MCTP_USB enabled.

**Is there a fix for CVE-2026-85032?** Yes: fixed in Zephyr commit with k_work_cancel_sync() replacement (specific version not provided in advisory). Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-85032 attacked?** It is attacked over the network, in an operating system.

**Am I affected by CVE-2026-85032?** Check whether you run Zephyr Project Zephyr OS at an affected version. 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.

## How to fix

- Fixed in Zephyr commit with k_work_cancel_sync() replacement (specific version not provided in advisory)

## Technical details

The vulnerability exists in subsys/pmci/mctp/mctp_usb.c where RX handling is split across two threads: the system workqueue running mctp_usb_class_out_work() parses incoming data into a packet buffer (usb->rx_pkt), while a USB device stack thread can invoke mctp_usb_class_disable(). The disable handler used k_work_cancel() which does not wait for in-progress handlers, then immediately freed the packet buffer via mctp_pktbuf_free(). On SMP systems or with preemptible workqueues, this creates a window where the worker can write to freed memory; on cooperative-only systems, the worker may resume after the free and dereference a NULLed pointer. The fix replaces k_work_cancel() with k_work_cancel_sync() to ensure the worker completes before memory is freed.

## Affected products

- Zephyr Project Zephyr OS versions with CONFIG_MCTP_USB enabled

## Timeline

- 2026-11-03: disclosed

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