---
id: CVE-2026-63920
title: 'ipv6: validate extension header length before copying to cmsg'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ipv6: validate extension header length before copying to cmsg

  ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}
  cmsgs (and their IPV6_2292* legacy c…
severity: high
cvss: 7.1
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    81394827dfb72772c50d0ae3bdfa094428a5d76d
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    931b4a1f13408c2507719890f78f7227c34a0282
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    08464413e628803bd10cb1df68d0138665f2f885
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    a29768d56eb3798c052ad3281b05596e695a17af
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    0d330eff318c0f44d4fb0ad2c2aef38f87f24c90
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    a35daeabb433686234b010ebf7b53778dbd6c9b8
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    eb18a1b1644e4cad978df2131e2bb9a2e6886992
  - >-
    Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 <
    dd433671fef381fdaf7b530c631e6b782d66e224
  - Linux 2.6.12
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-16T15:33:23.448048Z'
published: '2026-07-19'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T15:42:11.708Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-63920'
references:
  - url: 'https://git.kernel.org/stable/c/81394827dfb72772c50d0ae3bdfa094428a5d76d'
  - url: 'https://git.kernel.org/stable/c/931b4a1f13408c2507719890f78f7227c34a0282'
  - url: 'https://git.kernel.org/stable/c/08464413e628803bd10cb1df68d0138665f2f885'
  - url: 'https://git.kernel.org/stable/c/a29768d56eb3798c052ad3281b05596e695a17af'
  - url: 'https://git.kernel.org/stable/c/0d330eff318c0f44d4fb0ad2c2aef38f87f24c90'
  - url: 'https://git.kernel.org/stable/c/a35daeabb433686234b010ebf7b53778dbd6c9b8'
  - url: 'https://git.kernel.org/stable/c/eb18a1b1644e4cad978df2131e2bb9a2e6886992'
  - url: 'https://git.kernel.org/stable/c/dd433671fef381fdaf7b530c631e6b782d66e224'
tags:
  - cve.org
epss: 0.00171
epssPercentile: 0.06058
ingestedAt: '2026-10-16T17:24:46.775Z'
---

## Overview

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

ipv6: validate extension header length before copying to cmsg

ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}
cmsgs (and their IPV6_2292* legacy counterparts) by trusting the
on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.
The length was validated only at parse time (ipv6_parse_hopopts(),
etc.).  An nftables payload-write expression can rewrite hdrlen after
parsing and before the skb reaches recvmsg; the write itself is
in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040
bytes from an 8-byte header.  nftables is reachable from an
unprivileged user namespace, so this is an unprivileged
slab-out-of-bounds read:

  BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540
   put_cmsg+0x3ac/0x540
   udpv6_recvmsg+0xca0/0x1250
   sock_recvmsg+0xdf/0x190
   ____sys_recvmsg+0x1b1/0x620

Add ipv6_get_exthdr_len() which validates that at least two bytes
are accessible before reading the hdrlen field, then checks the
computed length against skb_tail_pointer(skb), returning 0 on
failure.  Extension headers are kept in the linear skb area by
pskb_may_pull() during input, so skb_tail_pointer() is the correct
bound.

Use ipv6_get_exthdr_len() at all non-AH call sites: the five
standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)
and the three standard cases in the extension-header walk loop
(DSTOPTS, ROUTING, default).  AH retains an inline bounds check
because its length formula differs ((ptr[1]+2)<<2).

The walk loop also gets a pre-read bounds check at the top to
validate ptr before any case accesses ptr[0] or ptr[1].

When the walk loop detects a corrupted header, return from the
function instead of continuing to process later socket options.

## Affected

- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 81394827dfb72772c50d0ae3bdfa094428a5d76d`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 931b4a1f13408c2507719890f78f7227c34a0282`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 08464413e628803bd10cb1df68d0138665f2f885`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < a29768d56eb3798c052ad3281b05596e695a17af`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 0d330eff318c0f44d4fb0ad2c2aef38f87f24c90`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < a35daeabb433686234b010ebf7b53778dbd6c9b8`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < eb18a1b1644e4cad978df2131e2bb9a2e6886992`
- `Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < dd433671fef381fdaf7b530c631e6b782d66e224`
- `Linux 2.6.12`

## Remediation

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