---
id: CVE-2026-97984
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

  This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to
  avoid a possible overflow of…
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

  This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to
  avoid a possible overflow of…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 427faee167bce4ccb0b22f6815a9ac509e31d4c3 <
    1d56a00ec91f2885c64ea5b8cea18cb746b91bbb
  - >-
    Linux >= 427faee167bce4ccb0b22f6815a9ac509e31d4c3 <
    ce66352d3eb21825410af8b91db7cc4bb8e22c98
  - >-
    Linux >= 427faee167bce4ccb0b22f6815a9ac509e31d4c3 <
    e01b7c176d9af2b7c22bb3fe12cf9a7904f99fdc
  - >-
    Linux >= 427faee167bce4ccb0b22f6815a9ac509e31d4c3 <
    0ae10b6be49b425827659b23bcce498f80eb7182
  - Linux 5.15
published: '2026-09-25'
updated: '2026-09-25'
sourceUpdated: '2026-09-25T11:17:26.443'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-97984'
references:
  - url: 'https://git.kernel.org/stable/c/0ae10b6be49b425827659b23bcce498f80eb7182'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/1d56a00ec91f2885c64ea5b8cea18cb746b91bbb'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/ce66352d3eb21825410af8b91db7cc4bb8e22c98'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/e01b7c176d9af2b7c22bb3fe12cf9a7904f99fdc'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
tags:
  - nvd
  - cve.org
ingestedAt: '2026-09-25T11:06:38.868Z'
---

## Overview

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

net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to
avoid a possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to
IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE, and a large packet is sent over
a netdev with an unusually large MTU.

Steps to reproduce (included in the new selftest):

1. Set device MTU bigger than IP6_MAX_MTU. cork->base.fragsize will be
   set to that MTU in ip6_setup_cork.
2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It
   lets maxnonfragsize be set to device MTU (cork->fragsize) in
   __ip6_append_data, rather than to IP6_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of
   IPv6 header). Device MTU allows it (it's only one byte bigger than
   IP6_MAX_MTU, and the device MTU is bigger than that).
4. The UDP length in the built packet is 65536, which overflows the
   16-bit length field and triggers the WARN in udp_set_len_short.

To avoid breaking sending UDP jumbograms over raw IPv6 sockets, limit
the change to UDP sockets only.

The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to
predate git history (verified reproduction on 2.6.21), was fixed later,
and then reappeared in commit 427faee167bc ("net: ipv6: introduce
ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The
overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in
commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE").

## Remediation

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