---
id: CVE-2026-98393
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

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

  net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

  This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a
  possible overflow of UDP length that …
severity: none
published: '2026-10-14'
updated: '2026-10-14'
sourceUpdated: '2026-10-14T14:17:50.193'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-98393'
references:
  - url: 'https://git.kernel.org/stable/c/81f540827d243b8dad76c1f44e70ea31719f393c'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/83fedc4741210f75be8ca5b487985a242bf6f18d'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/b83641e0ab8b20eefcc4cdc5a059f897375291a2'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
tags:
  - nvd
ingestedAt: '2026-10-14T15:36:13.977Z'
---

## Overview

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

net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a
possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to
IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an
unusually large MTU.

Steps to reproduce:

1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will
   be set to that MTU in ip_setup_cork.
2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be
   set to device MTU (cork->fragsize) in __ip_append_data, rather than
   to IP_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of
   IPv4 header). Device MTU allows it (it's only one byte bigger than
   IP_MAX_MTU + IPv4 header, 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.

Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward
always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.

The Fixes tag points at the first commit where I could reproduce the
overflow with IPv4 and IP_PMTUDISC_PROBE.

## Remediation

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