[CVE-2026-64380][MODERATE 7.0] smb: client: harden POSIX SID length parsing [ Upstream

[CVE-2026-64380][MODERATE 7.0] smb: client: harden POSIX SID length parsing [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64380
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: harden POSIX SID length parsing [ Upstream
Commit: 171605aed68380c2fa75dff9b3a1ed427c50065b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=171605aed68380c2fa75dff9b3a1ed427c50065b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64380
Analysis date: Sat, 25 Jul 2026 10:59:52 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious SMB server can return a truncated POSIX SID that causes the Linux SMB client to perform a one byte out of bounds read in posix_info_sid_size(), potentially crashing the client kernel and warranting manual review due to network supplied parser input.

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

The original Linux kernel CVE announcement for CVE-2026-64380 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64380

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
======================================================================

CVE-2026-64380	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H';CWE-125;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'A one byte out of bounds read in the SMB client POSIX SID parser can occur because posix_info_sid_size reads sid(1) even when only one byte remains in the received buffer. A malicious or compromised SMB server can trigger this by returning a truncated POSIX SID in an SMB2 POSIX information response, causing the client kernel to read past the validated buffer boundary and potentially crash. For the CVSS the PR:N is used because the malformed data can be supplied by a remote SMB server or network attacker without a local account on the client. User interaction is set to UI:R because the victim client normally has to mount or access an SMB share that reaches this parsing path. Impact is primarily denial of service via kernel crash. For the paranoid score, limited confidentiality impact is kept because the bug class is an out of bounds read, but integrity impact is not raised because the patch does not show a write primitive or UAF.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK SIMPLEFIX IMPROVEONLY SERVERTOCLIENT KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  A malicious or compromised SMB server can supply a truncated POSIX SID in an SMB2 POSIX information response to the client.

* Reliable kernel crash / strong DoS: +1
  The helper reads one byte past the validated buffer boundary. This can plausibly lead to KASAN/Oops or kernel crash depending on buffer placement.

* Broad/default/common subsystem or broadly deployed exposure: +1
  SMB/CIFS client code is commonly deployed, although the POSIX info path depends on SMB POSIX extensions and client access to a server.

* Confidentiality impact plausible: +1 paranoid only
  The bug is a one byte OOB read from kernel memory. There is no demonstrated disclosure channel, so this is kept only in the paranoid score as C:L style concern.

Not counted:

* No strong memory corruption primitive. This is read-only OOB, not OOB write, UAF, double free, or type confusion.
* No integrity impact. The patch does not show attacker-controlled write or state corruption.
* No LPE signal. There is no reclaim, callback control, arbitrary write, or privilege boundary bypass.

## 3. Reachability analysis

The likely attacker is a malicious SMB server, compromised SMB server, or network attacker capable of influencing SMB responses. The victim client normally needs to mount or access an SMB share that reaches the SMB2 POSIX info parsing path, so this is not unsolicited packet-to-kernel exposure. No local privileges on the victim are required by the remote peer, but user interaction or an existing client workflow is typically needed. Containers and namespaces do not materially reduce the remote server side privilege requirement, but the vulnerable code executes in the host kernel SMB client if the host performs the mount or access.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than a clear Important vulnerability. Theoretical impact includes limited kernel memory disclosure concern because the primitive is an OOB read, but realistic evidence supports DoS much more strongly than confidentiality or privilege escalation. The issue deserves manual review because it is network-supplied parser input in a kernel client path, but it should not be upgraded to Important without evidence of a stable leak, write primitive, UAF, or LPE chain.

## 5. One-sentence report phrase

A malicious SMB server can return a truncated POSIX SID that causes the Linux SMB client to perform a one byte out of bounds read in posix_info_sid_size(), potentially crashing the client kernel and warranting manual review due to network supplied parser input.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable malicious-server input reaches kernel parsing code and triggers an OOB read. The visible primitive is limited, but this is not suitable for automatic closure because SMB client parser bugs can be deployment-sensitive and the paranoid score reaches actionable Moderate.

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

Patch: smb: client: harden POSIX SID length parsing [ Upstream
Commit: 171605aed68380c2fa75dff9b3a1ed427c50065b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=171605aed68380c2fa75dff9b3a1ed427c50065b

Changed files:
  fs/cifs/smb2pdu.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=171605aed68380c2fa75dff9b3a1ed427c50065b

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

The original Linux kernel CVE announcement for CVE-2026-64380 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64380

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:N/UI:R/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: 5

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).