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Fencer Vulns
high 7.8

CVE-2026-89638 json

smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

published 2026-09-11 58 d ago modified 2026-10-30 9 d ago also EUVD-2026-76550

cvss
7.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 7%
public exploits
0
none known

Summary

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

smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

When a file has the setuid or setgid bit set and is written to, the VFS strips those bits and issues a setattr with ATTR_KILL_SUID/ATTR_KILL_SGID together with an ATTR_MODE carrying the already-cleared mode.

Both cifs_setattr_unix() and cifs_setattr_nounix() unconditionally dropped ATTR_MODE in that case:

/* skip mode change if it's just for clearing setuid/setgid */ if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) attrs->ia_valid &= ~ATTR_MODE;

This is fine for the default mount, where the mode is only emulated via the DOS read-only attribute and cannot represent the setuid/setgid bits anyway. However, with the "cifsacl" or "modefromsid" mount options the mode is stored on the server through an ACL (id_mode_to_cifs_acl()), with the SMB3.1.1 POSIX extensions the mode is sent to the server directly, and with the SMB1 Unix extensions (cifs_setattr_unix) the mode is sent via CIFSSMBUnixSetPathInfo(). In all those cases dropping ATTR_MODE means the cleared mode is never pushed to the server, so the setuid/setgid bit survives the write.

This is a security issue: on local filesystems the setuid bit is stripped when a file is written, but over these cifs.ko mounts the bit persists on the server, potentially allowing an unexpected privilege escalation on subsequent execution.

Fix this in two places:

1. cifs_setattr_nounix(): only take the "skip mode change" shortcut when the mode is emulated via the DOS read-only attribute (i.e. neither cifsacl/modefromsid nor the SMB3.1.1 POSIX extensions are in effect), so that the cleared mode is propagated to the server in the ACL / POSIX cases.

2. cifs_setattr_unix(): this function is only called when Unix extensions are in effect, so the mode is always stored on the server. Remove the shortcut entirely so that the cleared mode is always pushed.

Patch / fix

  • Linux Linux 5.10.273, 5.15.224, 6.1.191, 6.6.160, 6.12.113, 6.18.51, 7.2.4, 7.3
  • linux Debian 13 6.12.113-1
  • linux Debian 14 7.2.6-1
  • linux-6.12 Debian 12 6.12.113-1~deb12u1

debian advisories DEBIAN-CVE-2026-89638

  • linux Ubuntu 26.04 LTS 7.0.0-42.42
  • linux-aws Ubuntu 26.04 LTS 7.0.0-1016.17
  • linux-gcp Ubuntu 26.04 LTS 7.0.0-1016.18
  • linux-ibm Ubuntu 26.04 LTS 7.0.0-1017.17
  • linux-riscv Ubuntu 26.04 LTS 7.0.0-42.42.1
  • linux-oracle Ubuntu 26.04 LTS 7.0.0-1015.16
  • linux-hwe-7.0 Ubuntu 24.04 LTS 7.0.0-42.42~24.04.1
  • linux-oem-7.0 Ubuntu 26.04 LTS 7.0.0-1017.17

ubuntu advisories UBUNTU-CVE-2026-89638

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • kernel-64kb SLE Module for Basesystem 15 SP7 6.4.0-150700.53.84.1
  • kernel-syms SLE Module for Development Tools 15 SP7 6.4.0-150700.53.84.1
  • kernel-azure SLE Module for Public Cloud 15 SP7 6.4.0-150700.53.84.1
  • kernel-source SLE Module for Development Tools 15 SP7, SLE Module for Basesystem 15 SP7 6.4.0-150700.53.84.1
  • kernel-default SLE Workstation Extension 15 SP7, SLE Module for Legacy 15 SP7, SLE Module for Basesystem 15 SP7 +2 more 6.4.0-150700.53.84.1
  • kernel-zfcpdump SLE Module for Basesystem 15 SP7 6.4.0-150700.53.84.1
  • kernel-obs-build SLE Module for Development Tools 15 SP7 6.4.0-150700.53.84.1
  • kernel-default-base SLE Module for Basesystem 15 SP7 6.4.0-150700.53.84.1.150700.17.47.1

suse advisories SUSE-SU-2026:5121-1SUSE-SU-2026:4706-1SUSE-SU-2026:4668-1SUSE-SU-2026:4667-1SUSE-SU-2026:4666-1SUSE-SU-2026:4665-1

  • kernel-source openSUSE Tumbleweed 7.2.7-1.1

opensuse advisories openSUSE-SU-2026:11880-1

  • kernel AlmaLinux 10 6.12.0-211.69.1.el10_2
  • rv AlmaLinux 10 6.12.0-211.69.1.el10_2
  • perf AlmaLinux 10 6.12.0-211.69.1.el10_2
  • rtla AlmaLinux 10 6.12.0-211.69.1.el10_2
  • libperf AlmaLinux 10 6.12.0-211.69.1.el10_2
  • kernel-rt AlmaLinux 10 6.12.0-211.69.1.el10_2
  • kernel-64k AlmaLinux 10 6.12.0-211.69.1.el10_2
  • kernel-core AlmaLinux 10 6.12.0-211.69.1.el10_2

almalinux advisories ALSA-2026:91565ALSA-2026:90365ALSA-2026:89082ALSA-2026:89083

  • kernel Rocky Linux 9 5.14.0-687.61.1.el9_8.cloud.1.0
  • kernel Rocky Linux 10 6.12.0-211.69.1.el10_2
  • kernel Rocky Linux 9 5.14.0-687.61.1.el9_8
  • kernel-rt Rocky Linux 8 4.18.0-553.178.1.rt7.519.el8_10
  • kernel Rocky Linux 8 4.18.0-553.178.1.el8_10

rocky linux advisories RXSA-2026:90365RLSA-2026:91565RLSA-2026:90365RLSA-2026:89082RLSA-2026:89083

  • kernel Azure Linux 3.0 6.6.161.1-1

azure linux advisories AZL-100353

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-89638 updated cvss 7.8

kernel: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

  • Red Hat Enterprise Linux 9 kernel-rt, kernel-rt-kvm, kernel-rt-core, kernel-rt-modules, kernel-rt-debug-kvm, kernel-rt-debug-core, +12 more
    affected: workaround, no fix available
  • Red Hat Enterprise Linux kernel-modules-internal 6.12.0-211.69.1.el10_2
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux AppStream (v. 9) rv 5.14.0-687.61.1.el9_8, perf 5.14.0-687.61.1.el9_8, rtla 5.14.0-687.61.1.el9_8, python3-perf 5.14.0-687.61.1.el9_8, kernel-devel-matched 5.14.0-687.61.1.el9_8, kernel-64k-devel-matched 5.14.0-687.61.1.el9_8, +7 more
    fixed: vendor fix RHSA-2026:90365
  • Red Hat Enterprise Linux AppStream (v. 10) rv 6.12.0-211.69.1.el10_2, perf 6.12.0-211.69.1.el10_2, rtla 6.12.0-211.69.1.el10_2, python3-perf 6.12.0-211.69.1.el10_2, kernel-devel-matched 6.12.0-211.69.1.el10_2, kernel-64k-devel-matched 6.12.0-211.69.1.el10_2, +7 more
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux BaseOS (v. 8) perf 4.18.0-553.178.1.el8_10, kernel 4.18.0-553.178.1.el8_10, bpftool 4.18.0-553.178.1.el8_10, kernel-core 4.18.0-553.178.1.el8_10, kernel-tools 4.18.0-553.178.1.el8_10, python3-perf 4.18.0-553.178.1.el8_10, +15 more
    fixed: vendor fix RHSA-2026:89083
  • Red Hat Enterprise Linux BaseOS (v. 9) kernel 5.14.0-687.61.1.el9_8, kernel-64k 5.14.0-687.61.1.el9_8, kernel-core 5.14.0-687.61.1.el9_8, kernel-tools 5.14.0-687.61.1.el9_8, kernel-modules 5.14.0-687.61.1.el9_8, kernel-64k-core 5.14.0-687.61.1.el9_8, +27 more
    fixed: vendor fix RHSA-2026:90365
  • Red Hat Enterprise Linux BaseOS (v. 10) kernel 6.12.0-211.69.1.el10_2, kernel-64k 6.12.0-211.69.1.el10_2, kernel-core 6.12.0-211.69.1.el10_2, kernel-tools 6.12.0-211.69.1.el10_2, kernel-modules 6.12.0-211.69.1.el10_2, kernel-64k-core 6.12.0-211.69.1.el10_2, +28 more
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) libperf 5.14.0-687.61.1.el9_8, kernel-debuginfo-common-s390x 5.14.0-687.61.1.el9_8, kernel-debuginfo-common-x86_64 5.14.0-687.61.1.el9_8, kernel-debuginfo-common-aarch64 5.14.0-687.61.1.el9_8, kernel-debuginfo-common-ppc64le 5.14.0-687.61.1.el9_8
    fixed: vendor fix RHSA-2026:90365
show 10 more
  • Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) libperf 6.12.0-211.69.1.el10_2, kernel-debuginfo-common-s390x 6.12.0-211.69.1.el10_2, kernel-debuginfo-common-x86_64 6.12.0-211.69.1.el10_2, kernel-debuginfo-common-aarch64 6.12.0-211.69.1.el10_2, kernel-debuginfo-common-ppc64le 6.12.0-211.69.1.el10_2
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux CRB (v. 8) kernel-debuginfo-common-x86_64 4.18.0-553.178.1.el8_10, kernel-debuginfo-common-aarch64 4.18.0-553.178.1.el8_10, kernel-debuginfo-common-ppc64le 4.18.0-553.178.1.el8_10
    fixed: vendor fix RHSA-2026:89083
  • Red Hat Enterprise Linux Hidden (v. 10) kernel-modules-internal 6.12.0-211.69.1.el10_2, kernel-debuginfo-common-x86_64 6.12.0-211.69.1.el10_2
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux NFV (v. 8) kernel-rt 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-kvm 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-core 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-modules 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-kvm 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-core 4.18.0-553.178.1.rt7.519.el8_10, +4 more
    fixed: vendor fix RHSA-2026:89082
  • Red Hat Enterprise Linux Real Time (v. 9) kernel-rt 5.14.0-687.61.1.el9_8, kernel-rt-64k 5.14.0-687.61.1.el9_8, kernel-rt-core 5.14.0-687.61.1.el9_8, kernel-rt-modules 5.14.0-687.61.1.el9_8, kernel-rt-64k-core 5.14.0-687.61.1.el9_8, kernel-rt-debug-core 5.14.0-687.61.1.el9_8, +14 more
    fixed: vendor fix RHSA-2026:90365
  • Red Hat Enterprise Linux Real Time (v. 10) kernel-rt 6.12.0-211.69.1.el10_2, kernel-rt-64k 6.12.0-211.69.1.el10_2, kernel-rt-core 6.12.0-211.69.1.el10_2, kernel-rt-modules 6.12.0-211.69.1.el10_2, kernel-rt-64k-core 6.12.0-211.69.1.el10_2, kernel-rt-debug-core 6.12.0-211.69.1.el10_2, +14 more
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux Real Time for NFV (v. 9) kernel-rt 5.14.0-687.61.1.el9_8, kernel-rt-core 5.14.0-687.61.1.el9_8, kernel-rt-modules 5.14.0-687.61.1.el9_8, kernel-rt-debug-core 5.14.0-687.61.1.el9_8, kernel-rt-modules-core 5.14.0-687.61.1.el9_8, kernel-rt-debug-modules 5.14.0-687.61.1.el9_8, +4 more
    fixed: vendor fix RHSA-2026:90365
  • Red Hat Enterprise Linux Real Time for NFV (v. 10) kernel-rt 6.12.0-211.69.1.el10_2, kernel-rt-core 6.12.0-211.69.1.el10_2, kernel-rt-modules 6.12.0-211.69.1.el10_2, kernel-rt-debug-core 6.12.0-211.69.1.el10_2, kernel-rt-modules-core 6.12.0-211.69.1.el10_2, kernel-rt-debug-modules 6.12.0-211.69.1.el10_2, +4 more
    fixed: vendor fix RHSA-2026:91565
  • Red Hat Enterprise Linux RT (v. 8) kernel-rt 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-core 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-modules 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-core 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-debug-modules 4.18.0-553.178.1.rt7.519.el8_10, kernel-rt-modules-extra 4.18.0-553.178.1.rt7.519.el8_10, +2 more
    fixed: vendor fix RHSA-2026:89082
  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
total
Assessed
23 d ago

Exploit prediction

0.18% chance of exploitation in the next 30 days, higher than 7% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Affected products

  • Linux Linux

Classification

Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.