---
id: CVE-2026-31504
title: 'net: fix fanout UAF in packet_release() via NETDEV_UP race'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: fix fanout UAF in packet_release() via NETDEV_UP race

  `packet_release()` has a race window where `NETDEV_UP` can re-register a
  socket into a fanout group's `arr[]…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    ee642b1962caa9aa231c01abbd58bc453ae6b66e
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    42cfd7898eeed290c9fb73f732af1f7d6b0a703e
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    1b4c03f8892d955385c202009af7485364731bb9
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    654386baef228c2992dbf604c819e4c7c35fc71b
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    75fe6db23705a1d55160081f7b37db9665b1880b
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    d0c7cdc15fdf8c4f91aca1928e52295d175b6ec6
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    ceccbfc6de720ad633519a226715989cfb065af1
  - >-
    Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 <
    42156f93d123436f2a27c468f18c966b7e5db796
  - Linux 3.1
published: '2026-04-22'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T08:47:30.796Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-31504'
references:
  - url: 'https://git.kernel.org/stable/c/ee642b1962caa9aa231c01abbd58bc453ae6b66e'
  - url: 'https://git.kernel.org/stable/c/42cfd7898eeed290c9fb73f732af1f7d6b0a703e'
  - url: 'https://git.kernel.org/stable/c/1b4c03f8892d955385c202009af7485364731bb9'
  - url: 'https://git.kernel.org/stable/c/654386baef228c2992dbf604c819e4c7c35fc71b'
  - url: 'https://git.kernel.org/stable/c/75fe6db23705a1d55160081f7b37db9665b1880b'
  - url: 'https://git.kernel.org/stable/c/d0c7cdc15fdf8c4f91aca1928e52295d175b6ec6'
  - url: 'https://git.kernel.org/stable/c/ceccbfc6de720ad633519a226715989cfb065af1'
  - url: 'https://git.kernel.org/stable/c/42156f93d123436f2a27c468f18c966b7e5db796'
tags:
  - cve.org
epss: 0.00184
epssPercentile: 0.07035
ingestedAt: '2026-09-08T15:33:26.990Z'
---

## Overview

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

net: fix fanout UAF in packet_release() via NETDEV_UP race

`packet_release()` has a race window where `NETDEV_UP` can re-register a
socket into a fanout group's `arr[]` array. The re-registration is not
cleaned up by `fanout_release()`, leaving a dangling pointer in the fanout
array.
`packet_release()` does NOT zero `po->num` in its `bind_lock` section.
After releasing `bind_lock`, `po->num` is still non-zero and `po->ifindex`
still matches the bound device. A concurrent `packet_notifier(NETDEV_UP)`
that already found the socket in `sklist` can re-register the hook.
For fanout sockets, this re-registration calls `__fanout_link(sk, po)`
which adds the socket back into `f->arr[]` and increments `f->num_members`,
but does NOT increment `f->sk_ref`.

The fix sets `po->num` to zero in `packet_release` while `bind_lock` is
held to prevent NETDEV_UP from linking, preventing the race window.

This bug was found following an additional audit with Claude Code based
on CVE-2025-38617.

## Affected

- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < ee642b1962caa9aa231c01abbd58bc453ae6b66e`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < 42cfd7898eeed290c9fb73f732af1f7d6b0a703e`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < 1b4c03f8892d955385c202009af7485364731bb9`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < 654386baef228c2992dbf604c819e4c7c35fc71b`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < 75fe6db23705a1d55160081f7b37db9665b1880b`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < d0c7cdc15fdf8c4f91aca1928e52295d175b6ec6`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < ceccbfc6de720ad633519a226715989cfb065af1`
- `Linux >= ce06b03e60fc19c680d1bf873e779bf11c2fc518 < 42156f93d123436f2a27c468f18c966b7e5db796`
- `Linux 3.1`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.