[CVE-2026-93811][MODERATE REGULAR] ksmbd: Fix acl.sd_buf memory leak and invalid sd_size error handling [ Upstream

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

CVE: CVE-2026-93811
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ksmbd: Fix acl.sd_buf memory leak and invalid sd_size error handling [ Upstream
Commit: c3836f55e0dab96314774215e12d87f63fa919f2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c3836f55e0dab96314774215e12d87f63fa919f2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93811
Analysis date: Thu, 24 Sep 2026 14:32:46 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93811 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93811

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 REGULAR
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-2026-93811	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93811 CHECK WITH IMPACT FROM ORIG NN NONE	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:N/A:H';*CWE-401;*CWE-772;*CWE-20;CWE-252;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'ksmbd can leak acl.sd_buf when NTACL security descriptor decoding fails because the error path releases n.data but does not release the partially allocated descriptor buffer. An undersized decoded descriptor can also be returned as a successful result with a stale output value because rc remains zero after the invalid sd_size check. Repeated requests for an object with malformed NTACL metadata may accumulate kernel memory consumption and eventually cause denial of service. For the CVSS the PR:L value is used because a low privileged authenticated SMB client may be able to request security descriptor processing, although creation of the malformed xattr can require additional access or a preexisting corrupted object. The issue is potentially network reachable through ksmbd, but AC:H reflects the requirement for specially malformed security descriptor metadata and a reachable affected operation. The demonstrated primitive is a memory leak rather than UAF or an out of bounds write, so the supported impact is resource exhaustion without proven confidentiality or integrity compromise.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-93811 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE DISK INIT LEAK ERRORPATH UAF SKIP  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +1 Concrete persistent kernel resource leak. When `ndr_decode_v4_ntacl()` fails after allocating `acl.sd_buf`, the old error path frees `n.data` but permanently loses the descriptor buffer.
* No additional availability point. The patch establishes a leak, but immediate host-wide exhaustion or a kernel crash from a small number of requests is not demonstrated.

Paranoid signals:

* +1 Concrete persistent kernel resource leak.
* +1 Strong DoS through accumulation. If the same malformed NTACL metadata can be processed repeatedly, each request may lose another allocation and sustained requests can plausibly exhaust kernel memory.

Important non-awarded signals:

* Remote reachability +2 was not awarded because the complete network trigger is unresolved. A remote client may request security-descriptor processing, but the evidence does not establish that such a client can create or select the required malformed NTACL xattr.
* No UAF is established. The phrase `stale *pntsd` refers to an invalid output value, not demonstrated post-free use.
* No OOB read or write is established. Returning an undersized descriptor could be dangerous to a caller, but the supplied call-site context does not show an actual invalid access.
* No controlled reclaim, arbitrary write, disclosure, authorization bypass, or privilege escalation is demonstrated.
* No confidentiality or integrity points were awarded.
* The leak and its possible exhaustion consequence were not counted as multiple availability effects beyond the single paranoid DoS point.

3. Reachability analysis

The vulnerable function is in the ksmbd server path and may be reached while processing SMB security-descriptor operations. A remote authenticated SMB user may be able to request processing of an affected object, but the complete attacker-controlled chain is not established because malformed NTACL metadata must already exist or be creatable through another operation.

`HostAdminRequired=UNKNOWN` because the supplied evidence does not establish whether an ordinary SMB account can create the malformed xattr, whether share-level write or security-descriptor privileges suffice, or whether local administrative modification is required.

No evidence establishes that the complete operation is available through attacker-created user or network namespaces, so `NamespaceScopedPrivilege=UNKNOWN`. The system must have ksmbd configured with an accessible share, but no special hardware, boot mode, or race timing is involved. Once suitable malformed metadata exists, repeated triggering appears sequential and does not require winning a race.

4. Primitive and severity interpretation

The exact established primitive is a persistent kernel heap allocation leak on an error path. A second logic error can return success for an undersized security descriptor while leaving `*pntsd` set, but the supplied context does not show a subsequent OOB access or another memory-corruption sink.

This is neither fixed stale storage nor an active structured-object UAF. There is no release followed by use of a freed object and no repeated release or refcount mutation. The case is therefore `CLASS_C`, representing resource-exhaustion and invalid-state behavior without an independently useful corruption primitive.

The demonstrated consequence is incremental memory loss. Sustained kernel-memory exhaustion is a defensible paranoid consequence if a client can repeatedly reach the path. UAF, kernel read or write primitives, disclosure, and privilege escalation remain speculative and are not scored.

`ImportantCandidate=NO` because the patch establishes only a leak and invalid error handling. `AutoDowngradeAllowed=YES` because no unresolved corruption, ownership, authorization, or security-boundary primitive is present. Operational severity is Low to ordinary Moderate, subject to confirming remote repeatability.

5. One-sentence report phrase

A malformed ksmbd NTACL security descriptor can leak kernel memory or be returned with an invalid size, potentially allowing sustained authenticated requests to cause resource-exhaustion denial of service if remote control of the required xattr is confirmed.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether a low-privileged SMB client can create or select malformed NTACL xattrs and repeatedly invoke `ksmbd_vfs_get_sd_xattr()`, and inspect callers for use of an undersized successful `*pntsd`.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ksmbd: Fix acl.sd_buf memory leak and invalid sd_size error handling [ Upstream
Commit: c3836f55e0dab96314774215e12d87f63fa919f2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c3836f55e0dab96314774215e12d87f63fa919f2

Changed files:
  fs/smb/server/vfs.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=c3836f55e0dab96314774215e12d87f63fa919f2

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

The original Linux kernel CVE announcement for CVE-2026-93811 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93811

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:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  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: 1
  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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).