---
id: CVE-2026-31712
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ksmbd: require minimum ACE size in smb_check_perm_dacl()

  Both ACE-walk loops in smb_check_perm_dacl() only guard against an
  under-sized remaining buffer, not against a…
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ksmbd: require minimum ACE size in smb_check_perm_dacl()

  Both ACE-walk loops in smb_check_perm_dacl() only guard against an
  under-sized remaining buffer, not against a…
severity: high
cvss: 8.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H'
cwe:
  - CWE-787
vendor: linux
product: linux_kernel
affected:
  - 'linux_kernel >= 5.15, < 6.12.84'
  - 'linux_kernel >= 6.13, < 6.18.25'
  - 'linux_kernel >= 6.19, < 7.0.2'
patched:
  - linux_kernel 7.0.2
published: '2026-05-01'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T02:19:19.810'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-31712'
references:
  - url: 'https://git.kernel.org/stable/c/151b1799861fde38087c08f613abc2843ef597b0'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/282cbbb476b9f35793452bc461934af4c7eca169'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/325d4ac11f526cb8964cff14548ccf02d8c756d8'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/90089584b2e25c4510b7b987387b4405f0673ece'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/95e5aa3c3261da8c95b27d7aecf8ee39b9f86a4c'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/d07b26f39246a82399661936dd0c853983cfade7'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/f20adc4ef7428bc485ee83fd1a592252fb87718b'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-784472.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
tags:
  - nvd
  - cve.org
epss: 0.00315
epssPercentile: 0.2278
ingestedAt: '2026-10-15T03:30:12.868Z'
---

## Overview

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

ksmbd: require minimum ACE size in smb_check_perm_dacl()

Both ACE-walk loops in smb_check_perm_dacl() only guard against an
under-sized remaining buffer, not against an ACE whose declared
`ace->size` is smaller than the struct it claims to describe:

  if (offsetof(struct smb_ace, access_req) > aces_size)
      break;
  ace_size = le16_to_cpu(ace->size);
  if (ace_size > aces_size)
      break;

The first check only requires the 4-byte ACE header to be in bounds;
it does not require access_req (4 bytes at offset 4) to be readable.
An attacker who has set a crafted DACL on a file they own can declare
ace->size == 4 with aces_size == 4, pass both checks, and then

  granted |= le32_to_cpu(ace->access_req);               /* upper loop */
  compare_sids(&sid, &ace->sid);                         /* lower loop */

reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at
offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES
* 4 bytes).

Tighten both loops to require

  ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE

which is the smallest valid on-wire ACE layout (4-byte header +
4-byte access_req + 8-byte sid base with zero sub-auths).  Also
reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES
before letting compare_sids() dereference sub_auth[] entries.

parse_sec_desc() already enforces an equivalent check (lines 441-448);
smb_check_perm_dacl() simply grew weaker validation over time.

Reachability: authenticated SMB client with permission to set an ACL
on a file.  On a subsequent CREATE against that file, the kernel
walks the stored DACL via smb_check_perm_dacl() and triggers the
OOB read.  Not pre-auth, and the OOB read is not reflected to the
attacker, but KASAN reports and kernel state corruption are
possible.

## Affected

- `linux_kernel >= 5.15, < 6.12.84`
- `linux_kernel >= 6.13, < 6.18.25`
- `linux_kernel >= 6.19, < 7.0.2`

## Remediation

Upgrade past the affected range:

- `linux_kernel 7.0.2`