---
id: "CVE-2026-98773"
kind: "vulnerability"
aliases: ["CVE-2026-98773"]
title: "Linux kernel USB cdc-acm: use-after-free when a modem is unplugged"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["cdc-acm", "infrastructure", "linux-kernel", "usb-serial"]
not_affected_if: ["no-cdc-acm-devices"]
published_at: "2026-10-18T16:17:31+00:00"
first_seen_at: "2026-10-23T21:57:16+00:00"
updated_at: "2026-10-25T05:15:29+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98773", "first_seen_at": "2026-10-23T21:57:16+00:00"}]
---

# Linux kernel USB cdc-acm: use-after-free when a modem is unplugged

A race in the Linux kernel's USB CDC ACM serial driver lets the port shutdown path resubmit URBs while a disconnect is freeing them. The sources describe a use-after-free in the control and read callbacks that touch freed DMA memory.

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

## How to check

- unknown

## Fix

- unknown

## Timeline

- 2026-10-23T21:57:16+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98773

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

USB: cdc-acm: skip URB restart in port_shutdown if disconnected

acm_port_shutdown() is called from the tty-port machinery whenever the
last user closes the port.  acm_disconnect() sets acm->disconnected and
calls tty_port_tty_vhangup(), which eventually triggers port_shutdown().
At that point acm_disconnect() has already called acm_poison_urbs() and
is about to free the control and read URBs.

However, acm_port_shutdown() unconditionally calls acm_unpoison_urbs()
and, when the ALWAYS_POLL_CTRL quirk is set, immediately re-submits the
ctrlurb and all read URBs.  If the concurrent acm_disconnect() then
frees those URBs and their DMA buffers while they are in-flight, the
result is a use-after-free in acm_ctrl_irq() or acm_read_bulk_callback()
accessing the freed DMA memory.

Fix by checking acm->disconnected after draining the delayed anchor and
returning early: if we are in a disconnect path the URBs must remain
poisoned and must not be resubmitted.

The early return is placed after the delayed-anchor drain loop because
that loop only cancels write-side accounting (wb->use = false and
autopm counter) and does not touch the hardware path.

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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_