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Response snapshot Oct 30, 2026, 19:11 UTC

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VulnerabilityCVE-2026-89638

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

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.

At a glance

Exploited
Not known
Severity
7.8 High (CVSS 3.1)
Chance of exploitation
0.2% in the next 30 days
Sources
1 source

What to do

Check the vendor advisory for affected versions and fixes.

Read NVD CVE-2026-89638 (opens a new tab)

What happened

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:

Continue reading

/* 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.

Scores and how they were measured

CVSS base severity

7.8

Version 3.1As of

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.2%

Scored Percentile 7.3%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    NVD CVE-2026-89638 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  2. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  3. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  4. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  5. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-89638, scored 2026-10-30. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 30). Linux kernel: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions (CVE-2026-89638). https://kevwire.com/record/CVE-2026-89638

MLA

"Linux kernel: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions (CVE-2026-89638)." KEVWire, 30 Oct. 2026, kevwire.com/record/CVE-2026-89638.

Chicago

KEVWire. "Linux kernel: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions (CVE-2026-89638)." Last modified October 30, 2026. https://kevwire.com/record/CVE-2026-89638.

Evidence summary (plain text)

Linux kernel: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions (Sep 11, 2026).
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.
CVSS 7.8 (version 3.1; evidence as of Oct 30, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-89638).
EPSS 0.2% for the following 30 days, scored Oct 30, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-89638&date=2026-10-30).
Sources:
NVD CVE-2026-89638: https://nvd.nist.gov/vuln/detail/CVE-2026-89638
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-30T13:06:45.042Z&lastModEndDate=2026-10-30T14:50:57.069Z&resultsPerPage=500&startIndex=500
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-30T12:22:16.957Z&lastModEndDate=2026-10-30T14:05:57.373Z&resultsPerPage=500&startIndex=500
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-30T11:45:59.009Z&lastModEndDate=2026-10-30T13:20:57.281Z&resultsPerPage=500&startIndex=500
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-89638&date=2026-10-30
Record ID: CVE-2026-89638.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 30, 2026, 14:52 UTC. Ingestion is not an independent editorial investigation.

NVD status: Received. Publication in the NVD is not evidence of exploitation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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