[CVE-2026-64379][MODERATE 7.0] smb: client: mask server-provided mode to 07777 in modefromsid [ Upstream

[CVE-2026-64379][MODERATE 7.0] smb: client: mask server-provided mode to 07777 in modefromsid [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64379
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: mask server-provided mode to 07777 in modefromsid [ Upstream
Commit: 5f6f2241034f189c69d4d0b5f8fe24a0c25b0c14
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f6f2241034f189c69d4d0b5f8fe24a0c25b0c14
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64379
Analysis date: Sat, 25 Jul 2026 06:30:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An SMB client using modefromsid may accept unmasked server-provided mode bits from an NFS mode SID, allowing a malicious SMB server to influence client-side file mode interpretation and potentially cause a limited metadata policy bypass.

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

The original Linux kernel CVE announcement for CVE-2026-64379 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64379

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: NO

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-2026-64379	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:L/I:L/A:L';*CWE-20;CWE-284;CWE-269;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:N';BEST CVSS score: '5';DESCR 'SMB client modefromsid handling accepted a server-provided mode value from sid sub_auth(2) without masking it to 07777 before merging it into cf_mode. A malicious or compromised SMB server could therefore inject non permission mode bits into the client side file attributes, potentially causing weaker than expected local mode interpretation or file type confusion. For the CVSS the PR:N is used because the attacker does not need a local account on the victim if they control or can tamper with the SMB server response. UI:R is used because the client normally has to mount or access the SMB share for the crafted metadata to be processed. The issue is network reachable through SMB protocol responses, but practical exploitation usually requires a malicious server, a compromised server, or a trusted network position and the modefromsid option. Impact is mainly integrity policy bypass, with only limited confidentiality or availability impact in the paranoid interpretation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	READ DISK SIMPLEFIX 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. The attacker-controlled value is supplied by an SMB server response and parsed by the SMB client.
* Integrity impact plausible: +1. The server can inject non-permission mode bits into client-side `cf_mode`, potentially weakening expected mode interpretation.
* Cross-boundary policy bypass: +1. The bug can affect the local client security interpretation of remote filesystem metadata.
* Important filesystem/storage path: +1. This is filesystem metadata handling in the SMB client.
* Legacy / non-default execution mode: -1. The issue requires `modefromsid` to be active, so it is not a default SMB client path.
* Paranoid confidentiality impact plausible: +1. If injected mode bits cause file-type or access-mode confusion in consumers of the metadata, limited confidentiality impact is defensible, but not proven.

No memory corruption, UAF, OOB write, refcount issue, or kernel crash primitive is shown by the patch.

## 3. Reachability analysis

A malicious or compromised SMB server, or an attacker able to tamper with SMB responses, can provide a crafted NFS mode SID value. The victim client must mount or access the SMB share with `modefromsid` active. No local account on the victim is required for the attacker in the server-controlled scenario, but user or administrator interaction is normally needed to establish or use the mount.

Namespaces and containers do not materially lower the trigger requirement unless a containerized workload is allowed to mount or consume attacker-controlled SMB shares. The path is network-influenced but configuration-dependent.

Call-site confidence: medium. The patch shows the affected parser and assignment to `cf_mode`, but not all downstream inode consumers.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an Important-class vulnerability. It is not memory corruption and does not show a direct LPE primitive. The realistic impact is client-side metadata or permission interpretation confusion caused by a remote SMB server. The paranoid score is higher because filesystem mode metadata can influence security decisions, but the patch does not justify treating it as arbitrary write, kernel memory corruption, or direct privilege escalation.

## 5. One-sentence report phrase

An SMB client using modefromsid may accept unmasked server-provided mode bits from an NFS mode SID, allowing a malicious SMB server to influence client-side file mode interpretation and potentially cause a limited metadata policy bypass.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue is network-influenced and affects security-relevant filesystem metadata, but it should not be escalated to Important without evidence of sensitive-object access, privilege escalation, or memory corruption.

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

Patch: smb: client: mask server-provided mode to 07777 in modefromsid [ Upstream
Commit: 5f6f2241034f189c69d4d0b5f8fe24a0c25b0c14
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f6f2241034f189c69d4d0b5f8fe24a0c25b0c14

Changed files:
  fs/cifs/cifsacl.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=5f6f2241034f189c69d4d0b5f8fe24a0c25b0c14

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

The original Linux kernel CVE announcement for CVE-2026-64379 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64379

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