---
id: CVE-2026-80840
title: 'ipv6: seg6: clear IPv4 control block on IPIP decapsulation'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ipv6: seg6: clear IPv4 control block on IPIP decapsulation

  End.DX4 and End.DT4 decapsulate an IPv4 packet through
  decap_and_validate() and send it directly to IPv4 rou…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    eb0f422487228e140f3d609b032ac61aedcab8fa
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    f52f1e75716d2ee49e013edf204ac92337c72fd8
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    0e3f01fe2e704e76af4385b8a1742641885a191c
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    3e4476e58343fb8f2fffced9e22d935376b17aaf
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    bf1c1151560d11036a144d917fa4c131831342d7
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    f4be3b391265e24c7720fc867c50062b436acf33
  - >-
    Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 <
    44930446dde45a7a90fe1446fa38eb0e2c561646
  - Linux 4.14
published: '2026-09-04'
updated: '2026-10-19'
sourceUpdated: '2026-10-19T08:44:33.475Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-80840'
references:
  - url: 'https://git.kernel.org/stable/c/10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667'
  - url: 'https://git.kernel.org/stable/c/eb0f422487228e140f3d609b032ac61aedcab8fa'
  - url: 'https://git.kernel.org/stable/c/9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c'
  - url: 'https://git.kernel.org/stable/c/f52f1e75716d2ee49e013edf204ac92337c72fd8'
  - url: 'https://git.kernel.org/stable/c/0e3f01fe2e704e76af4385b8a1742641885a191c'
  - url: 'https://git.kernel.org/stable/c/3e4476e58343fb8f2fffced9e22d935376b17aaf'
  - url: 'https://git.kernel.org/stable/c/bf1c1151560d11036a144d917fa4c131831342d7'
  - url: 'https://git.kernel.org/stable/c/f4be3b391265e24c7720fc867c50062b436acf33'
  - url: 'https://git.kernel.org/stable/c/44930446dde45a7a90fe1446fa38eb0e2c561646'
tags:
  - cve.org
epss: 0.00205
epssPercentile: 0.09771
ingestedAt: '2026-10-19T09:16:01.498Z'
---

## Overview

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

ipv6: seg6: clear IPv4 control block on IPIP decapsulation

End.DX4 and End.DT4 decapsulate an IPv4 packet through
decap_and_validate() and send it directly to IPv4 routing. The inner
packet therefore bypasses ip_rcv_core(), which normally clears IPCB
before IPv4 interprets skb->cb.

The skb instead retains IP6CB data from the outer packet. IP6CB and
IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps
IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and
ts.

The sender can make the stale optlen byte nonzero with a valid outer
extension-header chain. The reproducers put an eight-byte Destination
Options header immediately after the 40-byte IPv6 header and before the
Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled
Destination Options offset in both lastopt and nhoff, setting them to
40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees
optlen = 40 and rr = 40.

Both tcp_v4_save_options() and __ip_options_echo() skip option copying
when optlen is zero. Here optlen is 40, so the TCP SYN path allocates
room for 40 bytes of option data and calls __ip_options_echo(). The
stale rr value makes that function read inner packet byte 41 as the
Record Route option length. The reproducers set that sender-controlled
byte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte
option-data area.

Separate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5
kernel both produced:

  BUG: KASAN: slab-out-of-bounds in __ip_options_echo()
  Write of size 255

The relevant End.DX4 call path is:

  __ip_options_echo
  tcp_v4_route_req
  tcp_conn_request
  tcp_v4_conn_request
  tcp_rcv_state_process
  tcp_v4_do_rcv
  tcp_v4_rcv
  ip_protocol_deliver_rcu
  ip_local_deliver_finish
  ip_local_deliver
  input_action_end_dx4_finish
  input_action_end_dx4

The relevant End.DT4 call path is:

  __ip_options_echo
  tcp_v4_route_req
  tcp_conn_request
  tcp_v4_conn_request
  tcp_rcv_state_process
  tcp_v4_do_rcv
  tcp_v4_rcv
  ip_protocol_deliver_rcu
  ip_local_deliver_finish
  ip_local_deliver
  input_action_end_dt4

tcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so
it does not appear as a separate frame.

When decap_and_validate() handles IPPROTO_IPIP, save the ingress
interface from IP6CB, clear IPCB, and restore the saved value. Doing
this in the common decapsulation path covers End.DX4, End.DT4, and
End.DT46's IPv4 arm.

Use IP6CB(skb)->iif rather than skb->skb_iif. These actions run after
l3mdev processing, which can replace skb_iif with the L3 master;
IP6CB iif still records the receiving interface set at IPv6 ingress.

## Affected

- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < eb0f422487228e140f3d609b032ac61aedcab8fa`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < 9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < f52f1e75716d2ee49e013edf204ac92337c72fd8`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < 0e3f01fe2e704e76af4385b8a1742641885a191c`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < 3e4476e58343fb8f2fffced9e22d935376b17aaf`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < bf1c1151560d11036a144d917fa4c131831342d7`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < f4be3b391265e24c7720fc867c50062b436acf33`
- `Linux >= 891ef8dd2a8d14e4e73a81dcdb135b574c57f556 < 44930446dde45a7a90fe1446fa38eb0e2c561646`
- `Linux 4.14`

## Remediation

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