[CVE-2026-98126][LOW] smb/client: validate new EOF for zero range [ Upstream

[CVE-2026-98126][LOW] smb/client: validate new EOF for zero range [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98126
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: smb/client: validate new EOF for zero range [ Upstream
Commit: 3673f057b64abfa957e8ae84448db69369a5091a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3673f057b64abfa957e8ae84448db69369a5091a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98126
Analysis date: Sat, 26 Sep 2026 16:36:07 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98126 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98126

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-98126	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98126 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-770;*CWE-20;CWE-693;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An SMB client file size limit bypass occurs because smb3_zero_range can extend EOF through FALLOC_FL_ZERO_RANGE without checking RLIMIT_FSIZE when FALLOC_FL_KEEP_SIZE is not used. A local process with write access to a file on a CIFS mount can create or extend a file beyond its configured file size limit. For the CVSS the PR:L because a local user process with access to the mounted share and permission to modify the target file is required. The issue is not directly network reachable even though the affected filesystem uses SMB transport. Impact is primarily a local resource policy bypass and may contribute to storage exhaustion if repeated operations can consume shared filesystem capacity. Server side quotas, access controls, and available space limits can still restrict the practical impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	DISK SERVERTOCLIENT LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger. A local process with write permission for a file on an already mounted CIFS share can invoke fallocate with FALLOC_FL_ZERO_RANGE and extend EOF.
* +1 Resource-policy bypass. The operation bypasses RLIMIT_FSIZE enforcement for this specific SMB client file-extension path.

Important non-awarded signals:

* No UAF, stale reference, refcount misuse, OOB access, or other memory-corruption primitive is established.
* No controlled reclaim, arbitrary write, information disclosure, or privilege-escalation primitive is established.
* No direct network trigger is established. SMB is used as transport by the mounted filesystem, but the vulnerable operation is a local fallocate request.
* No additional resource-exhaustion point is awarded. The patch demonstrates bypass of a per-process file-size limit, but does not establish bypass of server quotas, filesystem free-space limits, ACLs, or host-wide resource accounting.
* No confidentiality or integrity point is awarded. The process must already have permission to modify the target file, and the patch does not show modification of another principals data.
* No host-admin penalty is applied. The attacker does not need host-admin privilege to perform the vulnerable operation when a writable CIFS mount is already available.
* No rare-subsystem penalty is applied. CIFS mounts require prior configuration, but a user-accessible mounted share is a normal deployment scenario and is not itself proof that the triggering user requires host administration.

3. Reachability analysis

The evaluated attacker is a local user or process with write access to a file on an existing CIFS mount. The attacker invokes the normal fallocate interface with FALLOC_FL_ZERO_RANGE without FALLOC_FL_KEEP_SIZE and selects a range extending beyond the current EOF.

HostAdminRequired is NO because the triggering operation itself can be performed by a non-administrative user who has ordinary write access to the mounted share. Mounting or configuring CIFS normally requires prior administrative setup, but that setup is not part of the attacker trigger once the writable mount is exposed.

NamespaceScopedPrivilege is UNKNOWN. The supplied patch does not establish whether the relevant CIFS mount and all required filesystem operations can be created and used entirely in an attacker-controlled namespace. Namespace support is not needed for the demonstrated local trigger.

No special hardware, race condition, boot option, or root-controlled sysctl is required by the supplied evidence. Server-side quotas, permissions, capacity limits, and SMB server policy can limit practical storage impact.

4. Primitive and severity interpretation

The exact defect is a missing RLIMIT_FSIZE check before smb3_zero_range sends an SMB request that increases a file EOF. This is a logic and resource-policy enforcement bug, not a memory-lifetime or memory-safety vulnerability.

The operation may allow a process to exceed its configured file-size limit for a writable file on a CIFS mount. It does not establish a cross-user write, ACL bypass, server quota bypass, filesystem ownership violation, COW violation, or access to protected data. The process is already authorized to modify the target file.

PrimitiveClass is NONE because no meaningful corruption, lifetime, authorization decision bypass, or ownership primitive is established. The demonstrated consequence is a local file-size-limit bypass and potentially increased use of shared storage. A host-wide storage exhaustion outcome is possible only under additional deployment-specific conditions and is not demonstrated by the patch.

ImportantCandidate is NO because the issue lacks a concrete kernel-corruption primitive, security-boundary bypass, or cross-principal impact. AutoDowngradeAllowed is YES because the available evidence supports ordinary low-impact resource-policy handling rather than unresolved Important-class exploitation leverage.

5. One-sentence report phrase

A local user with write access to a CIFS mounted file can bypass RLIMIT_FSIZE through FALLOC_FL_ZERO_RANGE and extend EOF beyond the configured per-process file size limit.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a bounded local RLIMIT_FSIZE enforcement omission with no demonstrated memory corruption, privilege escalation, authorization bypass against another principal, or server-side quota bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb/client: validate new EOF for zero range [ Upstream
Commit: 3673f057b64abfa957e8ae84448db69369a5091a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3673f057b64abfa957e8ae84448db69369a5091a

Changed files:
  fs/smb/client/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=3673f057b64abfa957e8ae84448db69369a5091a

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

The original Linux kernel CVE announcement for CVE-2026-98126 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98126

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).