[CVE-2026-98901][MODERATE 7.0] ksmbd: fix lease break and ack state handling [ Upstream

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

CVE: CVE-2026-98901
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ksmbd: fix lease break and ack state handling [ Upstream
Commit: 17699905a381621c1cee5b6528feb05acedc86a4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=17699905a381621c1cee5b6528feb05acedc86a4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98901
Analysis date: Tue, 27 Oct 2026 02:14:44 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A remote authenticated SMB client can corrupt ksmbd lease-break state and undermine cache coherence during conflicting writes or truncation, potentially causing stale data or lost updates.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98901 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98901

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-2026-98901	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98901 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE_KPANIC, Aegis: MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N';*CWE-20;CWE-841;CWE-754;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'This is a protocol state and cache coherence flaw rather than a demonstrated memory corruption issue. ksmbd could skip valid leases containing WRITE_CACHING during writes or truncates, and failed or invalid lease break acknowledgments could be handled without preserving the pending break correctly. A remote SMB client able to obtain a lease and perform conflicting file operations may cause stale cache use, lost updates, or corruption of accessible shared files. For the CVSS the PR:L value is selected because reliable triggering normally requires an authenticated SMB session with permission to open or modify the affected file. AC:H reflects the required lease negotiation, break acknowledgment, and conflicting operation sequence. There is no supported UAF, OOB access, kernel arbitrary write, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 2 with primitive class NONE and manual review REQUIRED. Its independently reconciled impact was MODERATE. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	REMOTE DANGER DISK SKIP  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; stronger memory-corruption or cross-authority effects are unsupported.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Remote network reachability: an authenticated SMB client can submit lease acknowledgments and conflicting write/truncate operations.
* +1 Attacker-influenced active structured state: client-controlled `LeaseState` affected the server’s live lease/oplock transition and cache-coherence state.
* +1 Integrity impact: the supplied analysis supports stale caching, lost updates, or corruption of files writable through the share.
* +1 Practical chain: lease acquisition, invalid or failed break acknowledgment, and a conflicting file operation form a specific coherence-failure sequence.
* No timing penalty: the sequence is complex but client-driven; no narrow uncontrollable race is shown.
* No UAF, OOB write, arbitrary kernel write, disclosure, panic, protected-file overwrite, or privilege escalation is shown.
* No authorization-bypass points: the patch validates SMB lease-state transitions, not filesystem permission or capability decisions.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: there is no heap OOB write or shared-backing write primitive.

3. Reachability analysis

The supported actor is a remote authenticated SMB user with permission to obtain a lease and modify the affected shared file. Host administration and host UID 0 are not required. No namespace capability is relevant. Triggering requires negotiated lease state, a break acknowledgment, and conflicting I/O, but these are protocol operations controlled by the client rather than an unsupported external race.

4. Primitive and severity reading

The primitive is attacker-influenced corruption of active lease/cache-coherence state that is later consumed by ksmbd during writes or truncation. The fix accepts only valid acknowledgment states that are subsets of the server’s break target, applies that state directly, and preserves pending breaks after failed acknowledgments.

This is CLASS_B semantic/state corruption, not memory corruption. The shown effect is integrity loss within accessible shared files. Cross-permission victim-file corruption, kernel compromise, and LPE remain unproven. ImportantCandidate is YES under the active-state/trust-boundary rule because remote client-controlled state reaches later privileged server file processing with unresolved cross-client integrity consequences. Auto-downgrade is therefore unsafe.

5. One-sentence report phrase

A remote authenticated SMB client can corrupt ksmbd lease-break state and undermine cache coherence during conflicting writes or truncation, potentially causing stale data or lost updates.

6. Manual review advice

MANUAL CHECK REQUIRED

Review cross-client file-integrity consequences and whether malformed acknowledgments can affect data belonging to another share user.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ksmbd: fix lease break and ack state handling [ Upstream
Commit: 17699905a381621c1cee5b6528feb05acedc86a4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=17699905a381621c1cee5b6528feb05acedc86a4

Changed files:
  fs/smb/server/oplock.c
  fs/smb/server/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=17699905a381621c1cee5b6528feb05acedc86a4

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

The original Linux kernel CVE announcement for CVE-2026-98901 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98901

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:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N
  The Best / paranoid CVSS score: 5.3

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: 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).