---
id: CVE-2026-63984
title: 'ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()

  ipv6_rpl_srh_decompress() computes:

      outhdr->hdrlen = (((n + 1) * sizeof(struct in6_addr)) >> 3);

  hdrl…
severity: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    75b3680047bf09af8e7e471a7a6ddf2ce5847f56
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    fd238c51b0fa5390cceca9f1ac5a9ffda8063eed
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    3618b34942b76471d044369bfd30d58c39068bf1
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    97e06791368c01f0ad2a4b3269c2abe19485ca32
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    de02fc049352af5a9595f015511222d0a85c326b
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    6fe1cb312038516cb4d9fa089d700af7059f1a64
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    c0487a9c1e116cf349e2d1f302d9019670460858
  - >-
    Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 <
    9d5e7a46a9f6d8f503b41bfefef70659845f1679
  - Linux 5.7
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: total
  timestamp: '2026-10-16T15:34:39.184194Z'
published: '2026-07-19'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T15:42:16.382Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-63984'
references:
  - url: 'https://git.kernel.org/stable/c/75b3680047bf09af8e7e471a7a6ddf2ce5847f56'
  - url: 'https://git.kernel.org/stable/c/fd238c51b0fa5390cceca9f1ac5a9ffda8063eed'
  - url: 'https://git.kernel.org/stable/c/3618b34942b76471d044369bfd30d58c39068bf1'
  - url: 'https://git.kernel.org/stable/c/97e06791368c01f0ad2a4b3269c2abe19485ca32'
  - url: 'https://git.kernel.org/stable/c/de02fc049352af5a9595f015511222d0a85c326b'
  - url: 'https://git.kernel.org/stable/c/6fe1cb312038516cb4d9fa089d700af7059f1a64'
  - url: 'https://git.kernel.org/stable/c/c0487a9c1e116cf349e2d1f302d9019670460858'
  - url: 'https://git.kernel.org/stable/c/9d5e7a46a9f6d8f503b41bfefef70659845f1679'
tags:
  - cve.org
epss: 0.00514
epssPercentile: 0.42494
ingestedAt: '2026-10-16T17:24:46.787Z'
---

## Overview

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

ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()

ipv6_rpl_srh_decompress() computes:

    outhdr->hdrlen = (((n + 1) * sizeof(struct in6_addr)) >> 3);

hdrlen is __u8. For n >= 127 the result exceeds 255 and silently
truncates. With n=127 (cmpri=15, cmpre=15, pad=0, hdrlen=16):

    (128 * 16) >> 3 = 256, truncated to 0 as __u8

The caller in ipv6_rpl_srh_rcv() then places the compressed header
at buf + ((ohdr->hdrlen + 1) << 3). With hdrlen=0 this is buf + 8,
but the decompressed region occupies buf[0..2055] (8-byte header
plus 128 full addresses). The compressed header overlaps the
decompressed data, and ipv6_rpl_srh_compress() writes into this
overlap, corrupting the routing header of the forwarded packet.

The existing guard at exthdrs.c:546 checks (n + 1) > 255, which
prevents n+1 from overflowing unsigned char (the segments_left
field), but does not prevent the computed hdrlen from overflowing
__u8. n=127 passes because 128 <= 255, yet hdrlen=256 does not
fit.

Tighten the bound to (n + 1) > 127. This caps n at 126, giving
hdrlen = (127 * 16) >> 3 = 254, which fits in __u8. The compressed
header then lands at buf + ((254 + 1) << 3) = buf + 2040, exactly
past the decompressed region (buf[0..2039]). No overlap. 127
segments is well beyond any realistic RPL deployment.

## Affected

- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < 75b3680047bf09af8e7e471a7a6ddf2ce5847f56`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < fd238c51b0fa5390cceca9f1ac5a9ffda8063eed`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < 3618b34942b76471d044369bfd30d58c39068bf1`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < 97e06791368c01f0ad2a4b3269c2abe19485ca32`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < de02fc049352af5a9595f015511222d0a85c326b`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < 6fe1cb312038516cb4d9fa089d700af7059f1a64`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < c0487a9c1e116cf349e2d1f302d9019670460858`
- `Linux >= 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3 < 9d5e7a46a9f6d8f503b41bfefef70659845f1679`
- `Linux 5.7`

## Remediation

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