[CVE-2026-64448][MODERATE 7.0] smb: client: restrict implied bcc[0] exemption to responses without data area [ Upstream

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

CVE: CVE-2026-64448
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: restrict implied bcc[0] exemption to responses without data area [ Upstream
Commit: 8d0bbc78046d264bbf6a574ea6f9072258a43e35
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8d0bbc78046d264bbf6a574ea6f9072258a43e35
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64448
Analysis date: Sat, 25 Jul 2026 10:03:21 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious SMB server can exploit an unconditional implied bcc[0] length exemption in smb2_check_message() to make the SMB client read past the receive buffer during NEGOTIATE or SESSION_SETUP, causing a kernel crash and warranting manual review due to the concrete OOB read primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64448 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64448

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-2026-64448	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-130;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'Manual check is recommended because the bug is network influenced and has a concrete OOB read primitive in kernel parsing code. Base score treats the realistic effect as DoS only. Paranoid score keeps the same bug class and raises confidentiality to low only, not HHH, because the patch evidence shows reads past the receive buffer but no write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	READ KASAN OOB DISK HARDWARE SERVERTOCLIENT KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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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 non-conforming SMB server can trigger the issue through crafted NEGOTIATE or SESSION_SETUP responses before session establishment.

* Reliable kernel crash / strong DoS: +1
  KASAN shows slab-out-of-bounds reads in kernel SMB authentication parsing paths. Realistic impact is at least client kernel crash or Oops during mount.

* Broad/common subsystem exposure: +1
  SMB/CIFS client is a commonly deployed network filesystem client, although exploitation requires the client to connect to the malicious server.

* Confidentiality impact plausible, paranoid only: +1
  The bug is a concrete OOB read primitive in kernel receive-buffer parsing. However, no direct attacker-visible leak path is demonstrated, so this is C:L only in the paranoid score.

No strong memory corruption, OOB write, UAF, type confusion, arbitrary write, or LPE primitive is shown by the patch.

## 3. Reachability analysis

The attacker can be a malicious SMB server or an attacker able to redirect the victim to such a server. The vulnerable path is reachable during SMB2 NEGOTIATE and SESSION_SETUP, before authentication completes. The attacker does not need privileges on the client system, but the victim normally must initiate a mount or connection, so practical triggering often requires user/admin action or an existing automount workflow. Namespaces do not materially change the remote-server attack model, although local mount permission policy affects who can cause the client to connect.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is network-influenced kernel protocol parsing with a demonstrated OOB read. It does not look like an Important-class LPE because the primitive is read-only and no write, reclaim, callback, refcount, or type-confusion path is shown. Conservative handling is DoS-focused. Paranoid handling keeps C:L due to the OOB read but does not raise integrity or privilege-escalation impact.

## 5. One-sentence report phrase

A malicious SMB server can exploit an unconditional implied bcc[0] length exemption in smb2_check_message() to make the SMB client read past the receive buffer during NEGOTIATE or SESSION_SETUP, causing a kernel crash and warranting manual review due to the concrete OOB read primitive.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the bug is remotely influenced, occurs in kernel network filesystem parsing before session establishment, and has a concrete KASAN-confirmed out-of-bounds read, even though current evidence supports DoS and limited confidentiality concern rather than LPE.

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

Patch: smb: client: restrict implied bcc[0] exemption to responses without data area [ Upstream
Commit: 8d0bbc78046d264bbf6a574ea6f9072258a43e35
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8d0bbc78046d264bbf6a574ea6f9072258a43e35

Changed files:
  fs/cifs/smb2misc.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=8d0bbc78046d264bbf6a574ea6f9072258a43e35

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

The original Linux kernel CVE announcement for CVE-2026-64448 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64448

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).