[CVE-2026-68457][IMPORTANT] ksmbd: use opener credentials for FSCTL mutations [ Upstream

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

CVE: CVE-2026-68457
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ksmbd: use opener credentials for FSCTL mutations [ Upstream
Commit: a8c18434e1f0d9f7989170bdb0490c0160baf065
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a8c18434e1f0d9f7989170bdb0490c0160baf065
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68457
Analysis date: Sat, 15 Aug 2026 12:36:41 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
ksmbd FSCTL mutation operations used worker credentials instead of opener file credentials for VFS revalidation, allowing a remote SMB user with a valid handle to potentially bypass file permission or LSM checks and modify file data or attributes.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68457 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68457

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
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CVE-2026-68457	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H';CWE-863;CWE-269;CWE-284;CWE-250;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H';BEST CVSS score: '8.3';DESCR 'ksmbd FSCTL mutation paths used current worker credentials when calling VFS helpers for xattr, fallocate, and file attributes instead of the credentials captured when the SMB file handle was opened. A remote SMB client with access to the share could trigger SET_SPARSE, SET_ZERO_DATA, or SET_COMPRESSION on an open handle and cause permission revalidation to run under the wrong credentials. For the CVSS the PR:L value is used because reliable triggering normally requires an authenticated or otherwise authorized SMB session with access to an open file handle, while guest shares may be a configuration dependent exception. The issue is network reachable through the SMB server control path rather than packet data processing alone. Impact is mainly integrity and availability because the operations can change file attributes or zero and punch file data, causing policy bypass or data loss. Confidentiality is rated low only in the paranoid score because credential confusion and LSM bypass may expose limited metadata or policy side effects, but there is no direct read primitive or kernel memory disclosure shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-68457 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE DISK SKIP  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: **6**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* **Remote reachable / network-triggerable: +2**
  The affected path is reachable through ksmbd over SMB, via FSCTL requests on an open SMB handle.

* **Security-boundary bypass with practical sensitive-object access: +2**
  The bug causes VFS permission, ownership, and LSM revalidation to run under ksmbd worker credentials instead of the opener credentials. This is a practical credential and permission-model bypass, even without memory corruption.

* **Integrity impact plausible: +1**
  SET_SPARSE, SET_ZERO_DATA, and SET_COMPRESSION can mutate file state, xattrs, file attributes, or file contents. SET_ZERO_DATA and punch-hole behavior can cause unauthorized data modification or data loss.

* **Availability impact realistic: +1**
  Unauthorized zeroing or hole punching can destroy or corrupt file data from the application perspective. This is persistent data availability impact, not merely a transient service crash.

* **Practical exploit chain demonstrated or strongly implied: +1 paranoid only**
  The commit directly explains the chain: SMB handle operation -> VFS helper -> revalidation under wrong credentials -> mutation allowed under unintended credentials. This supports the paranoid score.

Not triggered:

* **Generic memory corruption: +0**
  No UAF, OOB write, double-free, type confusion, or refcount primitive is shown.

* **Privilege escalation plausible: +0**
  No kernel code execution or local root escalation primitive is indicated.

* **Reliable kernel crash / strong DoS: +0**
  The impact is data mutation or policy bypass, not a kernel panic path.

## 3. Reachability analysis

A remote SMB client can trigger the affected operations if it can authenticate or otherwise access the ksmbd share and obtain an open file handle. Anonymous or guest shares could reduce the practical privilege requirement, but the conservative assumption is an authenticated SMB user, equivalent to PR:L. The bug is network reachable when ksmbd is enabled and exposed, but ksmbd is not universally enabled by default on all Linux systems. Containers and namespaces do not materially reduce the attack requirement unless they provide access to the SMB service or delegated file access through ksmbd.

Call-site confidence: **high**. The patch shows the affected FSCTL mutation paths and the exact credential fix using `override_creds(fp->filp->f_cred)` around VFS xattr, fallocate, and fileattr helpers.

## 4. Severity interpretation

This behaves more like an **Important candidate** than an ordinary Moderate because it is a network-reachable server-side authorization bug that can apply file mutations under the wrong credentials. The realistic impact is not kernel memory corruption or direct privilege escalation, but it can bypass intended VFS or LSM checks and cause unauthorized file attribute changes or data loss. The conservative score treats it as actionable Moderate. The paranoid score treats the wrong-credential VFS mutation as a strong enough security-boundary bypass to require Important-level manual review.

## 5. One-sentence report phrase

ksmbd FSCTL mutation operations used worker credentials instead of opener file credentials for VFS revalidation, allowing a remote SMB user with a valid handle to potentially bypass file permission or LSM checks and modify file data or attributes.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is network reachable and affects authorization boundaries in a file server. Even without memory corruption, the wrong-credential execution model can cause practical integrity and availability impact, so it should not be auto-closed.

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

Patch: ksmbd: use opener credentials for FSCTL mutations [ Upstream
Commit: a8c18434e1f0d9f7989170bdb0490c0160baf065
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a8c18434e1f0d9f7989170bdb0490c0160baf065

Changed files:
  fs/smb/server/smb2pdu.c
  fs/smb/server/vfs.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=a8c18434e1f0d9f7989170bdb0490c0160baf065

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

The original Linux kernel CVE announcement for CVE-2026-68457 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68457

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:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
  The Best / paranoid CVSS score: 8.3

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).