[CVE-2026-89638][IMPORTANT] smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89638
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions [ Upstream
Commit: b10015807e4c628095d1d1d1c9307efc8cdd9e1b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b10015807e4c628095d1d1d1c9307efc8cdd9e1b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89638
Analysis date: Sat, 12 Sep 2026 05:27:43 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89638 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89638

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89638	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89638 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-281;CWE-269;CWE-266;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A permission preservation flaw in the CIFS/SMB client can leave setuid/setgid bits set on the server after a file is written. The problem affects mounts where mode is stored on the server, including cifsacl, modefromsid, SMB3.1.1 POSIX extensions, or SMB1 Unix extensions. A local user who can write to an affected setuid or setgid file on such a mount may later execute it with unexpectedly preserved privileges, causing privilege escalation on the client system. For the CVSS the PR:L is selected because exploitation requires a local user or process on the client that can access the mounted share and perform file operations, rather than unauthenticated network traffic. The SMB server is network based, but the vulnerable condition is reached through local filesystem operations on a mounted share and is not scored as a direct network attack. Impact can include high confidentiality, integrity, and availability compromise if the preserved setuid/setgid file executes with elevated privileges.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	DISK SERVERTOCLIENT LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6. No floor or cap applied.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap applied.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Conservative signals:

* +2: Concrete security-boundary / permission semantics bypass. VFS intends to clear setuid/setgid on write, but CIFS/SMB client dropped ATTR_MODE and failed to propagate the cleared mode to the server for ACL/POSIX/Unix-extension mode storage.
* +1: Local below-host-admin trigger. A local user or process with write access to an affected mounted share can trigger the write path without being host administrator.
* +1: Practical exploit chain strongly implied. Write to affected setuid/setgid file -> bits remain on server -> later execution may run with unexpected elevated privileges.
* +1: Integrity impact is plausible. Preserved setuid/setgid mode changes execution privilege semantics contrary to local filesystem security expectations.
* +1: Important filesystem/security metadata path. The bug affects setuid/setgid mode-bit handling, which is security-relevant filesystem metadata.

Paranoid difference:

* +2 instead of conservative +1 practical-chain signal: Strong LPE plausibility. The patch explicitly states this may allow unexpected privilege escalation on subsequent execution, and setuid/setgid preservation is a direct privilege mechanism.
* No additional stacking for the same LPE consequence beyond this replacement.

Important non-awarded signals:

* RealUAF not awarded. No UAF, stale pointer, double free, refcount misuse, or memory corruption primitive is established.
* Remote reachability not awarded. The SMB server is network based, but the vulnerable condition is reached through local filesystem operations on an already mounted share.
* Arbitrary write not awarded. The primitive is permission preservation, not attacker-directed kernel memory corruption.
* Confidentiality and availability were not separately stacked in conservative scoring. They may follow from successful privilege escalation, but counting them separately would risk double-counting the same setuid/setgid consequence.
* Race/timing penalty not applied. This is not a race.
* Host-admin penalty not applied. Host-admin may be required to create the mount, but the exploit trigger after setup is local file access, and the supplied evidence supports PR:L-style reachability.

3. Reachability analysis

The evaluated attacker is a local user or local process on the CIFS/SMB client system. The attacker needs access to an affected CIFS/SMB mount and write permission to a file that has setuid or setgid semantics preserved on the server. The vulnerable mount configuration includes cifsacl, modefromsid, SMB3.1.1 POSIX extensions, or SMB1 Unix extensions, because in these cases the mode is stored on the server and the cleared mode must be propagated.

This is not scored as a direct network attack. Network transport is involved because the filesystem is SMB-backed, but attacker-controlled packets alone are not the described trigger. The security-relevant action is a local write followed by possible local execution from the mounted filesystem.

Host administrator rights are not required for the file write/execute trigger once the affected mount exists and permissions allow access. Prior administrative setup of the mount may exist in real deployments, and nosuid mount policy can mitigate the issue, but those deployment constraints do not erase the established below-host-admin trigger path.

Namespace-scoped privilege is not relevant from the supplied evidence. No special hardware, race condition, sysctl, or boot-time feature is required beyond the affected CIFS/SMB client mode-handling configuration.

4. Primitive and severity interpretation

The exact primitive is a filesystem permission-preservation flaw. The kernel client fails to send the VFS-cleared mode to the SMB server when setuid/setgid bits should be stripped after write. This leaves security-relevant mode bits intact on the server.

This is not active-object UAF, fixed stale storage, or cleanup/destructor memory corruption. There is no concrete repeated release/refcount operation. The primitive is a CLASS_A security-boundary issue because a mandatory privilege-stripping rule can be bypassed in affected configurations.

The demonstrated consequence is persistence of setuid/setgid bits after write. The security consequence is potential privilege escalation on subsequent execution. The main uncertainty is deployment-specific: mount options, nosuid policy, server mode support, and whether an attacker can write to a suitable setuid/setgid file. Those constraints affect exploitability but do not reduce the bug to ordinary Moderate, because the patch explicitly fixes a privilege-preserving behavior.

ImportantCandidate=YES because the bug directly affects setuid/setgid privilege semantics and has a plausible LPE consequence. AutoDowngradeAllowed=NO because automatically closing this as ordinary low/moderate would discard a concrete security-boundary primitive with explicit privilege-escalation relevance.

Likely operational severity: Important in affected deployments where CIFS/SMB mounts use server-stored mode semantics and allow setuid/setgid execution without nosuid.

5. One-sentence report phrase

A CIFS/SMB client permission handling flaw can preserve setuid or setgid bits after writes on server-backed mode mounts, allowing a local user with write access to an affected file to execute it later with unexpected privileges.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly describes a security issue with potential privilege escalation, and real impact depends on mount options, nosuid policy, server mode semantics, and writable setuid/setgid file exposure.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions [ Upstream
Commit: b10015807e4c628095d1d1d1c9307efc8cdd9e1b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b10015807e4c628095d1d1d1c9307efc8cdd9e1b

Changed files:
  fs/smb/client/inode.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b10015807e4c628095d1d1d1c9307efc8cdd9e1b

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89638 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89638

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 6
  Paranoid ActionableScore: 7

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).