[CVE-2026-68100][MODERATE 7.0] ksmbd: validate num_subauth when copying ACE in set_ntacl_dacl

[CVE-2026-68100][MODERATE 7.0] ksmbd: validate num_subauth when copying ACE in set_ntacl_dacl

[AL-KERNEL]

CVE: CVE-2026-68100
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ksmbd: validate num_subauth when copying ACE in set_ntacl_dacl
Commit: e31fada5143784bc05c7ae44c79eed9b7a2e147e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e31fada5143784bc05c7ae44c79eed9b7a2e147e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68100
Analysis date: Mon, 10 Aug 2026 12:45:09 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An authenticated remote SMB client can store a malformed DACL and later trigger ksmbd to perform an attacker-indexed out-of-bounds heap read during DACL query handling, creating possible kernel memory disclosure and denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-68100 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68100

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.

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

CVE-2026-68100	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H';CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '8.1';DESCR 'A kernel heap out-of-bounds read in ksmbd can occur because set_ntacl_dacl copies ACE data from a stored security descriptor without validating sid.num_subauth. An authenticated SMB client can first store a malformed DACL with SMB2_SET_INFO and later trigger SMB2_QUERY_INFO for DACL on an inode with a POSIX access ACL, causing set_posix_acl_entries_dacl to read ntace sid sub_auth at an attacker controlled index. For the CVSS the PR:L is used because the attacker needs authenticated SMB access to the ksmbd share, but no local shell or administrator privileges on the victim are required. The issue is network reachable over SMB when ksmbd is exposed to the authenticated client. Impact is at least possible kernel memory disclosure through an out-of-bounds read and possible denial of service if the invalid read hits an unsafe memory boundary or hardened debug checks.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-68100 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE WRITE OOB DISK SKIP  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Trigger is over SMB against ksmbd via authenticated SMB2_SET_INFO followed by SMB2_QUERY_INFO.

* Confidentiality impact plausible: +1
  The bug is an attacker-controlled out-of-bounds heap read from `sid.sub_auth[]`.

* Availability impact realistic: +1
  The invalid heap read may crash the kernel depending on heap layout and hardening.

Paranoid additional signals:

* Memory corruption, weak/indirect corruption candidate: +1
  This is not an OOB write or UAF, but it is an unchecked attacker-controlled index into kernel heap data.

* Broadly deployed network service exposure: +1
  SMB file sharing is a common network-exposed service class, even though ksmbd itself is configuration-dependent.

Not awarded:

* No strong LPE plausibility: no write primitive, reclaim primitive, callback control, type confusion, or refcount takeover is shown.
* No integrity impact: the patch supports OOB read, not unauthorized write or ACL modification.
* No generic strong memory corruption: this is read-only OOB, not OOB write, UAF, double free, or arbitrary write.

Call-site confidence: high. The commit message gives the relevant chain: `SMB2_SET_INFO` stores malformed DACL, then `SMB2_QUERY_INFO` reaches `build_sec_desc()` -> `set_ntacl_dacl()` -> `set_posix_acl_entries_dacl()`.

## 3. Reachability analysis

An authenticated SMB client with access to a ksmbd share can trigger the issue remotely. No local shell or administrator privileges on the victim are required, so practical PR is low authentication-level access to SMB rather than host-level privilege.

Namespaces are not the main factor here. Exposure depends on whether ksmbd is enabled and reachable. Many general Linux systems do not expose ksmbd by default, but NAS, appliance, embedded, and file-server deployments may expose SMB services to internal networks.

Realistic exploitation conditions are credible: the attacker controls `sid.num_subauth` through a stored security descriptor and can later trigger the query path. The demonstrated primitive is an OOB heap read, not a proven privilege-escalation primitive.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is network-reachable after authentication and affects kernel heap memory in a server protocol path. It should be treated as Actionable Moderate at minimum, with Strong Important candidate handling for products where ksmbd is exposed to semi-trusted or untrusted SMB users.

Theoretical risk includes broader kernel memory disclosure or crash. Realistic evidence supports authenticated remote OOB read and possible DoS, but not direct privilege escalation or arbitrary code execution.

## 5. One-sentence report phrase

An authenticated remote SMB client can store a malformed DACL and later trigger ksmbd to perform an attacker-indexed out-of-bounds heap read during DACL query handling, creating possible kernel memory disclosure and denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable authenticated kernel heap OOB read in ksmbd should not be auto-closed, especially for products exposing ksmbd to internal or semi-trusted SMB clients.

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

Patch: ksmbd: validate num_subauth when copying ACE in set_ntacl_dacl
Commit: e31fada5143784bc05c7ae44c79eed9b7a2e147e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e31fada5143784bc05c7ae44c79eed9b7a2e147e

Commit description:

set_ntacl_dacl() copies each ACE from the attacker-controlled stored
security descriptor verbatim into the response DACL without checking
sid.num_subauth. The ACE bytes (including an unchecked num_subauth)
originate from an authenticated SMB2_SET_INFO(SecInfo=DACL) that is
stored raw via ksmbd_vfs_set_sd_xattr(); parse_dacl() rejects a bad ACE
with `break` rather than an error, so parse_sec_desc() still returns
success and the malformed SD reaches the xattr intact.

On a subsequent SMB2_QUERY_INFO(SecInfo=DACL) for an inode carrying a
POSIX access ACL, build_sec_desc() -> set_ntacl_dacl() ->
set_posix_acl_entries_dacl() walks the copied ACEs and reads

    ntace->sid.sub_auth[ntace->sid.num_subauth - 1]

with num_subauth taken straight from the stored SD. Since sub_auth[]
is fixed at SID_MAX_SUB_AUTHORITIES (15), a crafted num_subauth (e.g.
255) drives an out-of-bounds heap read of ~1 KB with an offset fully
controlled by an authenticated client.

The sibling functions already gate this field:
  parse_dacl()    -- num_subauth == 0 || > SID_MAX_SUB_AUTHORITIES
  parse_sid()     -- num_subauth > SID_MAX_SUB_AUTHORITIES
  smb_copy_sid()  -- min_t(u8, num_subauth, SID_MAX_SUB_AUTHORITIES)
set_ntacl_dacl() is the lone inconsistent path that omits the check.

Add the same num_subauth validation in set_ntacl_dacl() before copying
the ACE, matching the gate already enforced by parse_dacl().

Signed-off-by: Haofeng Li <lihaofeng@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  fs/smb/server/smbacl.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=e31fada5143784bc05c7ae44c79eed9b7a2e147e

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

The original Linux kernel CVE announcement for CVE-2026-68100 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68100

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:L/PR:L/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 8.1

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: 6

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).