[CVE-2026-64381][MODERATE 7.0] smb: client: Fix next buffer leak in receive_encrypted_standard() [ Upstream

[CVE-2026-64381][MODERATE 7.0] smb: client: Fix next buffer leak in receive_encrypted_standard() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64381
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: smb: client: Fix next buffer leak in receive_encrypted_standard() [ Upstream
Commit: 94e4f672db029414b9888b5137a7559f1febf2d8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94e4f672db029414b9888b5137a7559f1febf2d8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64381
Analysis date: Sat, 25 Jul 2026 07:11:23 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or compromised SMB server can trigger a kernel memory leak in the encrypted SMB client receive path by causing `receive_encrypted_standard()` to allocate `next_buffer` before the `MAX_COMPOUND` limit check fails, potentially leading to resource-exhaustion DoS after repeated responses.

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

The original Linux kernel CVE announcement for CVE-2026-64381 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64381

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.

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

CVE-2026-64381	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'A memory leak in the SMB client encrypted receive path can occur because next_buffer is allocated before the MAX_COMPOUND limit is checked in receive_encrypted_standard(). A malicious or compromised SMB server can return encrypted compounded responses that hit the compound PDU limit, causing the function to return before the allocated buffer is attached to server smallbuf or server bigbuf. For the CVSS the PR:N is used because the attacker does not need an account or local privileges on the victim system once the SMB client connection to the server exists. The base case is limited availability impact because each trigger leaks memory but does not immediately prove full system failure. The paranoid case treats sustained server controlled triggering as potential kernel memory exhaustion and denial of service. The issue is network reachable from the SMB server side but typically requires the victim client to connect to a malicious or compromised SMB share. No UAF, OOB write, arbitrary write, information disclosure, or privilege escalation primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	DISK SIMPLEFIX LEAK IMPROVEONLY SERVERTOCLIENT  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A malicious or compromised SMB server can influence the encrypted SMB response stream after the client connects.
* Availability impact realistic: +1. The bug leaks a kernel buffer on the error path and repeated triggering can consume memory.
* No memory corruption bonus. The patch shows a leak of `next_buffer`, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No C/I impact. There is no visible disclosure or integrity primitive.
* No privilege escalation signal. The primitive is resource exhaustion only.

Paranoid additional interpretation:

* Reliable kernel crash / strong DoS: +1. Sustained server-controlled triggering may lead to kernel memory exhaustion.
* Broad/common subsystem exposure: +1. SMB/CIFS client is a common enterprise filesystem client, although exploitation requires an active client connection to a malicious or compromised server.

Resource leak cap respected: this is not raised above 5 because the issue is a memory leak without memory corruption.

## 3. Reachability analysis

The attacker is realistically a malicious SMB server, a compromised SMB server, or an attacker able to tamper with the SMB session. The victim must have an SMB client connection that reaches the affected encrypted compounded response handling path. No local account on the victim is required by the attacker once the client connects to the server.

This is not a generic unsolicited Internet attack against an idle host. It is network-triggerable from the server side of an SMB client/server relationship. Namespaces and containers do not materially reduce the server-side trigger requirement, but mounted SMB shares inside containers or services may increase practical exposure if they connect to attacker-controlled infrastructure.

The path is not guaranteed default-active on every system because it requires SMB client use, encrypted responses, and crafted compound PDU behavior. However, SMB client usage is common enough that this should not be auto-closed as a purely theoretical issue.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than an Important-class memory corruption vulnerability. The patch supports a resource leak and possible memory-exhaustion DoS, but it does not support privilege escalation, information disclosure, arbitrary write, UAF, double-free, or object confusion.

The theoretical worst case is sustained kernel memory exhaustion from a malicious server. The realistic impact is denial of service against clients that connect to such a server. Because the trigger is network-mediated and attacker-controlled from the SMB server side, manual review is justified even though the primitive is only a leak.

## 5. One-sentence report phrase

A malicious or compromised SMB server can trigger a kernel memory leak in the encrypted SMB client receive path by causing `receive_encrypted_standard()` to allocate `next_buffer` before the `MAX_COMPOUND` limit check fails, potentially leading to resource-exhaustion DoS after repeated responses.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-mediated SMB client exposure plus sustained kernel memory leak makes this actionable for triage, but the patch does not show memory corruption or privilege escalation, so it should be reviewed as DoS/resource exhaustion rather than Important-class LPE.

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

Patch: smb: client: Fix next buffer leak in receive_encrypted_standard() [ Upstream
Commit: 94e4f672db029414b9888b5137a7559f1febf2d8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94e4f672db029414b9888b5137a7559f1febf2d8

Changed files:
  fs/cifs/smb2ops.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=94e4f672db029414b9888b5137a7559f1febf2d8

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

The original Linux kernel CVE announcement for CVE-2026-64381 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64381

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:H/PR:N/UI:R/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 3
  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: NO
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).