[CVE-2026-97555][MODERATE 7.0] smb: client: fix heap overflow in DACL owner/group rewrite

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

CVE: CVE-2026-97555
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: fix heap overflow in DACL owner/group rewrite
Commit: fb736ab4fb8c4fdb8b2f9ccd3a62e03d11549ca1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb736ab4fb8c4fdb8b2f9ccd3a62e03d11549ca1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97555
Analysis date: Fri, 25 Sep 2026 15:15:52 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97555 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97555

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-2026-97555	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97555 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-122;CWE-131;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'The SMB client can underallocate a security descriptor when rewriting an existing DACL because the allocation can be based on the original on disk DACL size while replacement owner or group SIDs may be substantially longer. replace_sids_and_copy_aces can then write expanded ACE records beyond the allocated heap buffer, and KASAN confirms a slab out-of-bounds write in build_sec_desc. At minimum this can corrupt kernel heap memory and crash the system. Because the primitive is a heap out-of-bounds write whose total expansion can grow across multiple ACE records, privilege escalation or broader confidentiality and integrity impact cannot be excluded without manual analysis. For the CVSS the PR:L value is used for the paranoid score because a local user or constrained service with delegated rights to initiate the relevant ownership or ACL rewrite may not have full administrative control of the host. AC:H applies because reliable triggering depends on an affected CIFS mount, a suitable original DACL containing shorter SIDs, longer replacement SIDs from cifs.idmap, and execution of the vulnerable rewrite path. The issue is not directly triggerable by ordinary network traffic alone. A malicious SMB server may be able to influence the source DACL, but a local ownership or ACL operation is normally also required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-97555 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 5c3564852c58 YES  NO NO unknown 	MAYBE	WRITE KASAN OOB DISK SERVERTOCLIENT LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative manual-review safety floor: 4 -> 5 because a concrete kernel heap out-of-bounds write is established and exploitation leverage remains unresolved.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Strong corruption primitive. `replace_sids_and_copy_aces()` writes expanded ACE records beyond a heap allocation sized from the shorter original DACL. ACE count, original SID layout, and resulting expansion affect the overwrite.
* +1 Reliable kernel crash or strong DoS. KASAN directly reports a slab out-of-bounds write, making kernel failure realistic.
* +1 Important filesystem security path. The corruption occurs while constructing CIFS security descriptors during owner or group ACL rewriting.
* Total: 4.

Paranoid additional signal:

* +1 Weak LPE concern. Expansion can continue across multiple ACE records rather than being limited to a single invalid access, creating a defensible possibility of corrupting adjacent slab data. Target selection, useful adjacent-object placement, and exploit reliability are not established.
* Total: 5.

Important non-awarded signals:

* No UAF or repeated release/refcount operation exists.
* Controlled reclaim, arbitrary or directed write, callback control, and credential corruption are not demonstrated.
* Local unprivileged triggering is not established.
* Direct remote reachability is not established because a local ownership or ACL rewrite is also required.
* Confidentiality and integrity bonuses are not awarded merely because the heap overwrite could theoretically affect sensitive objects.
* No extra Availability point is awarded because it would duplicate the same crash consequence.
* No race penalty applies because this is a deterministic allocation-size mismatch, not a timing race.
* No privilege penalty applies because host-admin-equivalent privilege for the complete trigger is not positively established.

3. Reachability analysis

The vulnerable operation is reached through an owner, group, mode, or ACL rewrite on an affected CIFS mount. A suitable source DACL must contain shorter SIDs while the replacement SIDs returned through `cifs.idmap` are longer.

The supplied KASAN trace confirms execution by `chown`, but it does not resolve whether every usable trigger requires host root, `CAP_CHOWN`, file ownership, delegated filesystem-management rights, or another less-privileged operation that reaches the same rewrite path. Therefore `HostAdminRequired=UNKNOWN`.

An existing CIFS mount is a prerequisite and normally involves prior system configuration, but the evidence does not show that the triggering actor must also be the actor that created the mount. No user-namespace or namespace-scoped complete trigger chain is established.

A malicious SMB server may influence the source DACL, ACE count, and short SID representation. It cannot be treated as independently remote-triggerable from the supplied evidence because the vulnerable rewrite also requires a local operation and long replacement SIDs from local id mapping. This may therefore be a compound server-plus-local-operation scenario.

No special hardware, boot option, or race timing is required. CIFS ACL handling and appropriate mount options must be active.

4. Primitive and severity interpretation

The exact primitive is a heap out-of-bounds write caused by allocating from the original DACL size while emitting larger replacement ACE records. This is active heap-buffer corruption, not stale storage, a UAF, an invalid read, or passive metadata inconsistency.

The observed write is four bytes immediately beyond an 884-byte allocation, but the root cause is not inherently restricted to four bytes. Each rewritten ACE may expand toward `sizeof(struct smb_ace)`, and expansion can accumulate across multiple ACEs. This supports CLASS_A because a concrete heap overwrite with input-dependent length and structured contents is established.

The demonstrated consequence is kernel heap corruption and likely crash or DoS. Privilege escalation remains an unresolved possibility rather than an established result. The evidence does not establish target selection, controlled heap placement, arbitrary data, controlled reclaim, or a practical LPE chain.

`ImportantCandidate=YES` because automatically treating a variable-length kernel heap overwrite as an ordinary Moderate issue could discard meaningful exploitation risk. `AutoDowngradeAllowed=NO` and manual review is required to determine attacker control over DACL contents, replacement SID bytes, overflow length, slab placement, and reachable adjacent object types.

5. One-sentence report phrase

The CIFS client can underallocate a rewritten security descriptor and copy expanded SID based ACE records beyond the heap buffer, causing kernel memory corruption and a crash while leaving privilege escalation potential unresolved.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a variable-length kernel heap overwrite, but the supplied context does not resolve trigger privilege, overwrite controllability, heap placement, or whether adjacent active objects can be usefully corrupted.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: fix heap overflow in DACL owner/group rewrite
Commit: fb736ab4fb8c4fdb8b2f9ccd3a62e03d11549ca1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb736ab4fb8c4fdb8b2f9ccd3a62e03d11549ca1

Commit description:

When id_mode_to_cifs_acl rewrites an existing DACL, it allocates a
buffer sized according to the on-disk DACL length reported by
dacl_ptr->size. However, replace_sids_and_copy_aces may rewrite each
ACE with a new owner/group SID obtained from the cifs.idmap upcall.
Those SIDs can have up to SID_MAX_SUB_AUTHORITIES (15) sub-authorities,
making each ACE up to 76 bytes (sizeof(struct smb_ace)).

If the original DACL contains short SIDs (e.g., 1 sub-authority) while
the replacement SIDs are long, the rewritten ACEs overflow the
allocation.

Fix this by always budgeting for worst-case SID expansion: allocate
sizeof(struct smb_acl) plus num_aces * sizeof(struct smb_ace), which
covers the smb_acl header and room for every ACE at maximum SID size.
This replaces the previous split logic that used dacl_ptr->size for
cifsacl mounts but num_aces * sizeof(struct smb_ace) for mode_from_sid
mounts: both paths can trigger the same rewrite and need the same
headroom.

KASAN reports this as:
  BUG: KASAN: slab-out-of-bounds in build_sec_desc+0x1e8a/0x2680 [cifs]
  Write of size 4 at addr ffff8881a5e25374 by task chown/5298
  ...
  The buggy address is located 0 bytes to the right of
   allocated 884-byte region [ffff8881a5e25000, ffff8881a5e25374)

Cc: stable@vger.kernel.org
Fixes: bc3e9dd ("cifs: Change SIDs in ACEs while transferring file ownership.")
Assisted-by: Kiro:claude-opus-4.6
Signed-off-by: Bjoern Doebel <doebel@amazon.de>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Fixes: 5c3564852c58 ("cifs: Minimize the number of cifs_acl memory allocations")
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/client/cifsacl.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=fb736ab4fb8c4fdb8b2f9ccd3a62e03d11549ca1

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

The original Linux kernel CVE announcement for CVE-2026-97555 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97555

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).