---
id: CVE-2026-89631
title: 'smb: client: reject a tree connect response whose byte count is too small'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  smb: client: reject a tree connect response whose byte count is too small

  CIFSTCon() bounds its strnlen() over the byte area with the server's
  ByteCount minus two, whi…
severity: critical
cvss: 9.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-125
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 <
    d837645a719151ed392d4d9cab4571c2753dab12
  - >-
    Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 <
    2ae219762ebac929bf6ab3473681f10ed566af08
  - >-
    Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 <
    3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d
  - >-
    Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 <
    411e484fe71a7f1028de617447edad9cb6d9d68a
  - >-
    Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 <
    65deb18359341141d37dc86fc7853511be3c87a7
  - Linux 2.6.30
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-16T19:24:39.378892Z'
published: '2026-09-11'
updated: '2026-10-21'
sourceUpdated: '2026-10-21T12:48:03.355Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-89631'
references:
  - url: 'https://git.kernel.org/stable/c/d837645a719151ed392d4d9cab4571c2753dab12'
  - url: 'https://git.kernel.org/stable/c/2ae219762ebac929bf6ab3473681f10ed566af08'
  - url: 'https://git.kernel.org/stable/c/3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d'
  - url: 'https://git.kernel.org/stable/c/411e484fe71a7f1028de617447edad9cb6d9d68a'
  - url: 'https://git.kernel.org/stable/c/65deb18359341141d37dc86fc7853511be3c87a7'
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89631.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-89631'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2532089'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-89631'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-89631'
  - url: >-
      https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89631.mbox
  - url: 'https://access.redhat.com/errata/RHSA-2026:85321'
  - url: 'https://access.redhat.com/errata/RHSA-2026:85773'
  - url: 'https://access.redhat.com/errata/RHSA-2026:85774'
tags:
  - cve.org
  - csaf
  - vex
  - red-hat
epss: 0.00457
epssPercentile: 0.3948
patched:
  - 10base_smtv
  - enterprise_linux_appstream_v_10
  - enterprise_linux_baseos_v_10
  - enterprise_linux_baseos_v_8
  - enterprise_linux_codeready_linux_builder_v_10
  - enterprise_linux_crb_v_8
  - enterprise_linux_hidden_v_10
  - enterprise_linux_real_time_for_nfv_v_10
  - enterprise_linux_nfv_v_8
  - enterprise_linux_real_time_v_10
  - enterprise_linux_rt_v_8
scores:
  cna: 9.1
  vendor: 8.1
ingestedAt: '2026-09-14T15:23:07.474Z'
---

## Overview

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

smb: client: reject a tree connect response whose byte count is too small

CIFSTCon() bounds its strnlen() over the byte area with the server's
ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int
and converts to a huge size_t.  The later subtraction wraps the __u16
bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of
up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of the
slab object, and the bytes reach userspace through tcon->nativeFileSystem
in /proc/fs/cifs/DebugData.

Reject a byte area too small for what the parser consumes.  Two bytes is
the least it can consume, and no conformant response carries fewer.  The
new trace point is the 129th smb_eio_trace entry, which __mode(byte)
cannot represent, so the attribute goes with it.

## Affected

- `Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 < d837645a719151ed392d4d9cab4571c2753dab12`
- `Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 < 2ae219762ebac929bf6ab3473681f10ed566af08`
- `Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 < 3be89e8039a85fa3b6cd5f9dcc4c1459eb356c2d`
- `Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 < 411e484fe71a7f1028de617447edad9cb6d9d68a`
- `Linux >= cc20c031bb067eb3280a1c4b5c42295093e24863 < 65deb18359341141d37dc86fc7853511be3c87a7`
- `Linux 2.6.30`

## Remediation

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

## Vendor advisories

- **RHSA-2026:85321** · Red Hat · fixed in: 10Base-SMTV, Red Hat Enterprise Linux AppStream (v. 10), Red Hat Enterprise Linux BaseOS (v. 10), Red Hat Enterprise Linux CodeReady Linux Builder (v. 10), Red Hat Enterprise Linux Hidden (v. 10), Red Hat Enterprise Linux Real Time for NFV (v. 10), … · released 2026-10-19 · [advisory](https://access.redhat.com/errata/RHSA-2026:85321)
- **RHSA-2026:85773** · Red Hat · fixed in: Red Hat Enterprise Linux BaseOS (v. 8), Red Hat Enterprise Linux CRB (v. 8) · released 2026-10-20 · [advisory](https://access.redhat.com/errata/RHSA-2026:85773)
- **RHSA-2026:85774** · Red Hat · fixed in: Red Hat Enterprise Linux NFV (v. 8), Red Hat Enterprise Linux RT (v. 8) · released 2026-10-20 · [advisory](https://access.redhat.com/errata/RHSA-2026:85774)
- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux 9 · no fix planned: Red Hat Enterprise Linux 9 · updated 2026-10-20 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89631.json)