[CVE-2026-99110][MODERATE 7.0] smb: client: fix heap overflow in cifs_do_set_acl()

[CVE-2026-99110][MODERATE 7.0] smb: client: fix heap overflow in cifs_do_set_acl()

[AL-KERNEL]

CVE: CVE-2026-99110
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: fix heap overflow in cifs_do_set_acl()
Commit: d96f3365ce61c5f39602955af43c8174548dee98
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d96f3365ce61c5f39602955af43c8174548dee98
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99110
Analysis date: Wed, 28 Oct 2026 23:55:35 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A permitted CIFS ACL setter can trigger an attacker-shaped heap overflow because CIFS wire ACL entries are larger than the xattr-sized buffer validation permits.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99110 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99110

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-2026-99110	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99110 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-122;*CWE-787;CWE-131;CWE-197;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A local user who can set a POSIX ACL on a file in an affected CIFS mount can provide an ACE count that passes the smaller xattr format size check while posix_acl_to_cifs writes the larger CIFS wire representation into the request buffer. This provides a concrete kernel heap out of bounds write primitive and can cause a host wide kernel crash. Carefully shaped corruption may affect adjacent heap objects and could plausibly lead to kernel privilege escalation, although reliable exploitation depends on heap layout and use of the SMB Unix POSIX ACL path. Sizes above USHRT_MAX can also truncate the 16 bit data_count and cause the server to receive a partial ACL, creating an additional authorization integrity risk. For the CVSS the PR:L value is selected because reliable triggering requires a local user context with permission to modify ACLs on a file in the affected CIFS mount. The base score uses AC:H because converting the overflow into controlled kernel compromise requires heap shaping, while the paranoid score uses AC:L as the highest still defensible interpretation for an attacker controlled heap write. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.8 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 3.3 and ActionableScore 2 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. 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	DISK INIT SIMPLEFIX LEAK IMPROVEONLY SERVERTOCLIENT LINUS PACKET  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_A
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; no additional unsupported exploit effects awarded.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Confirmed attacker-influenced heap OOB write: validation permits roughly `4 + count*8`, while `posix_acl_to_cifs()` writes `6 + count*10` into the same request buffer. ACE count and serialized ACL data are materially user-influenced.
* +1 — Local below-host-admin trigger: supplied evidence grounds triggering by a user permitted to set a POSIX ACL on a file in an affected CIFS mount.
* +1 — Plausible host-wide kernel crash from overwriting adjacent heap storage.
* +1 — Weak exploitation leverage: variable-length, attacker-shaped CIFS ACE serialization extends into adjacent heap memory. The adjacent object and target are not selected, so strong/arbitrary-write leverage is not awarded.
* No extra memory-layout point: the size mismatch is the same fact underlying the OOB-write score.
* No C/I or LPE points: controlled reclaim, a named adjacent victim, directed write, disclosure, credentials corruption, or a completed escalation chain is not shown.
* The 16-bit `data_count` truncation can send a partial ACL, but unauthorized modification of a protected victim is not established.
* NEW52 B5-R does not apply: no retry-accumulated index or missing retry reset is shown.
* B4 authorization-state preservation does not apply: this is not a demonstrated DENY-to-ALLOW guard bypass.

3. Reachability analysis

A local user with write/ACL authority over a file on a mounted CIFS filesystem can supply the ACL count and entries. That is below host-administrator privilege and does not require host UID 0. Mount creation may require prior administration, but the subsequent vulnerable ACL operation is grounded as user-accessible. Namespace capability scope is not established. No race, unusual hardware, or retry sequence is required; the CIFS Unix POSIX ACL feature and a suitable mount are prerequisites.

4. Primitive and severity reading

The primitive is a real variable-length heap OOB write into the SMB request allocation, followed by normal request construction/transmission. It is CLASS_A because attacker-influenced serialized ACE bytes overwrite beyond the allocated request area. A crash is plausible; adjacent-object corruption and LPE remain credible but unproven. The separate partial-ACL server effect is shown at the protocol level, but a cross-user authorization impact is unresolved and is not scored as an authorization bypass. ImportantCandidate is YES and automatic downgrade is disallowed because the concrete heap-write primitive requires exploitability review.

5. One-sentence report phrase

A permitted CIFS ACL setter can trigger an attacker-shaped heap overflow because CIFS wire ACL entries are larger than the xattr-sized buffer validation permits.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm reachable ACL count limits, request allocation class, overwrite extent, and adjacent-object opportunities before assigning final Important severity.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb: client: fix heap overflow in cifs_do_set_acl()
Commit: d96f3365ce61c5f39602955af43c8174548dee98
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d96f3365ce61c5f39602955af43c8174548dee98

Commit description:

cifs_set_acl() validates ACL size using posix_acl_xattr_size():

        4 + (count * 8)  // 4-byte header + 8 bytes per ACE

cifs_do_set_acl() then calls posix_acl_to_cifs() to write the CIFS
wire format into the same buffer:

        6 + (count * 10)  // 6-byte header + 10 bytes per ACE

An ACL that passes the xattr-based check in cifs_set_acl() can
overflow the heap when posix_acl_to_cifs() writes the larger CIFS
format.

Validate the CIFS format size against the remaining buffer space and
USHRT_MAX before converting--data_count is __u16, so sizes above
USHRT_MAX truncate the on-wire packet length, causing the server to
apply a partial ACL.  Replace MaxDataCount = 1000 with
min(CIFSMaxBufSize, USHRT_MAX).

Fixes: dc1af4c ("cifs: implement set acl method")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/cifssmb.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=d96f3365ce61c5f39602955af43c8174548dee98

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

The original Linux kernel CVE announcement for CVE-2026-99110 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99110

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:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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).