[CVE-2026-64401][MODERATE 7.0] smb: client: resolve SWN tcon from live registrations [ Upstream

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

CVE: CVE-2026-64401
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: resolve SWN tcon from live registrations [ Upstream
Commit: 51d18db392e5386a7bb9e816d611f14e600cca3c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=51d18db392e5386a7bb9e816d611f14e600cca3c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64401
Analysis date: Sat, 25 Jul 2026 09:13:35 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An SMB/CIFS client SWN registration can retain a stale cached `tcon` pointer after the original tree connection is freed, allowing later witness notification handling to dereference freed kernel memory and potentially crash the system or, in the worst case, create a UAF-based privilege escalation risk.

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

The original Linux kernel CVE announcement for CVE-2026-64401 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64401

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: MODERATE 7.0
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-2026-64401	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A use-after-free can occur in the SMB/CIFS client witness notification path because an SWN registration may keep a cached tcon pointer after the original tcon has been unregistered and freed. A second mount of the same net and share can keep the registration alive while the cached tcon no longer represents a live object, so later notification handling may take locks or references through freed memory. For the CVSS the PR:N is used for the paranoid network server scenario, because an attacker may only need control of the SMB server or witness notification source while the client already has a CIFS mount. UI:R applies because the victim client must already be mounted or configured to communicate with that server. The issue is network-influenced but not a generic Internet packet trigger, and exploitation is more realistic inside environments where SMB shares and witness notifications are used. Impact is at least denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel use-after-free memory corruption. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64401 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	DISK UAF SERVERTOCLIENT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **6**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Network-triggerable or network-influenced path: +2**
  The vulnerable path is SMB/CIFS client SWN notification handling. A malicious or compromised SMB witness/server side can plausibly influence notification timing once the client is mounted or configured.

* **Memory corruption, weak/indirect corruption candidate: +1**
  The commit explicitly describes a stale cached `tcon` pointer and use-after-free, but does not show attacker-controlled reclaim, type confusion, callback control, or arbitrary write.

* **Real lifetime corruption: +1**
  The core issue is object lifetime misuse: `cifs_swn_reg` can outlive the first `tcon` while still caching a pointer to it.

* **Reliable kernel crash / strong DoS: +1**
  Taking `tc_lock` or incrementing `tc_count` through a freed `tcon` can plausibly crash the kernel.

* **Hard or special lifecycle condition: -1**
  Triggering requires a same-share second mount keeping the registration alive after the first `tcon` is freed, plus a later notification.

* **Configuration-specific SWN exposure: -1**
  SMB/CIFS is common, but SWN witness notification handling is not a generic default packet-processing path.

Paranoid additions:

* **Weak LPE concern: +1**
  Because this is a real kernel UAF involving a live object pointer used for locking and reference management, privilege escalation cannot be dismissed, even though no reclaim or overwrite primitive is demonstrated.

* **Confidentiality impact plausible: +1**
  Plausible only in the worst case due to UAF-based memory corruption, not because the patch shows a direct info leak.

* **Integrity impact plausible: +1**
  Plausible only in the worst case due to UAF-based memory corruption, not because the patch shows a direct write primitive.

## 3. Reachability analysis

The most realistic trigger requires an SMB/CIFS client using SWN witness notifications and a lifecycle where multiple same-share `tcon` objects share one registration. A local privileged actor may be needed to create or remove mounts in many deployments, but the dangerous notification path can be influenced by a malicious or compromised SMB witness/server once the victim client is already mounted or configured.

Namespaces and containers matter because CIFS mounting usually requires elevated privileges such as `CAP_SYS_ADMIN`, but reduced-capability service accounts or containerized mount helpers may make this more practically reachable than full host-root. This is not a generic Internet packet trigger. It is more realistic in enterprise SMB environments where clients mount attacker-controlled or compromised SMB shares.

Call-site confidence: **medium**. The commit message gives strong lifetime details, but the full diff and all callers are not included.

## 4. Severity interpretation

This should not be treated as an ordinary low Moderate issue. The conservative interpretation is a lifecycle-dependent UAF with likely DoS, giving a borderline manual-review score. The paranoid interpretation is Actionable Moderate because the stale `tcon` pointer is a real kernel object lifetime bug in a network-influenced client path.

Theoretical privilege escalation is plausible due to UAF, but realistic exploitation evidence is not demonstrated. There is no shown attacker-controlled reclaim, arbitrary write, callback hijack, or type confusion, so this should not be scored as a high-end Important issue solely from the patch.

## 5. One-sentence report phrase

An SMB/CIFS client SWN registration can retain a stale cached `tcon` pointer after the original tree connection is freed, allowing later witness notification handling to dereference freed kernel memory and potentially crash the system or, in the worst case, create a UAF-based privilege escalation risk.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the patch explicitly fixes a real use-after-free involving SMB/CIFS client lifetime management and a network-influenced notification path. Even though exploitation beyond DoS is not demonstrated, UAF in this context should not be auto-closed.

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

Patch: smb: client: resolve SWN tcon from live registrations [ Upstream
Commit: 51d18db392e5386a7bb9e816d611f14e600cca3c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=51d18db392e5386a7bb9e816d611f14e600cca3c

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=51d18db392e5386a7bb9e816d611f14e600cca3c

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

The original Linux kernel CVE announcement for CVE-2026-64401 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64401

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 4
  Paranoid ActionableScore: 6

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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).