[CVE-2026-93095][MODERATE 7.0] hfsplus: validate thread record before delete key rebuild [ Upstream

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

CVE: CVE-2026-93095
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: hfsplus: validate thread record before delete key rebuild [ Upstream
Commit: 33cda0036bc683fc888c20b04a5fc030e3e8413b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=33cda0036bc683fc888c20b04a5fc030e3e8413b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93095
Analysis date: Fri, 18 Sep 2026 00:17:40 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A corrupted HFS Plus catalog thread record can trigger an attacker-influenced kernel heap out-of-bounds write during delete key rebuild, causing at least local DoS and requiring manual review for possible privilege escalation impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-93095 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93095

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-2026-93095	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93095 CHECK WITH IMPACT FROM ORIG NN LOW	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-787;CWE-122;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A corrupted HFS Plus catalog thread record can make hfsplus_delete_cat() rebuild a delete key from an unvalidated nodeName length. The old path reads the length from the catalog B tree and copies length times 2 bytes into fd.search_key, which can write past the allocated search key buffer. This is a kernel out-of-bounds write rather than a pure crash-only condition, so the realistic impact is at least local denial of service and the worst case may include confidentiality or integrity impact through memory corruption. For the CVSS the PR:L is used because reliable triggering needs local ability to get a crafted HFS Plus image processed, and in practical desktop or removable media deployments this may be available through user mount, automount, or storage management services rather than full administrator control. The issue is not directly network reachable. Impact should be manually reviewed because the primitive is memory corruption in kernel filesystem code.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 6)  SKIP CVE-2026-93095 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	DISK IMPROVEONLY LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 7
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate**::

Conservative floor applied: 4 -> 5 because the patch establishes a concrete attacker-influenced kernel out-of-bounds write in filesystem parsing code. This is controlled kernel memory corruption and should not be automatically downgraded only because a full exploit chain is not shown.

## 2. Signal breakdown

Conservative signals:

* +2 Strong corruption primitive. Corrupted HFS Plus catalog metadata can provide an oversized `thread.nodeName.length`, causing `hfs_bnode_read()` to write past the allocated `fd.search_key` buffer.
* +1 Reliable kernel crash / strong DoS. A kernel heap out-of-bounds write in filesystem code can realistically cause kernel memory corruption and crash.
* +1 Important filesystem/storage path. The bug is in HFS Plus catalog B-tree handling during delete key rebuild for hardlink backing inode cleanup.

Conservative raw score: 2 + 1 + 1 = 4.
Applied floor: 4 -> 5 for controlled kernel corruption.

Paranoid additional signals:

* +1 Plausible below-host-admin local exposure in some deployments. A crafted HFS Plus image may be processed through removable media, automount, user mount helpers, or storage-management services, although this is deployment dependent.
* +1 Integrity plausible. The primitive is not only an invalid read or NULL dereference. It is an attacker-influenced write past a kernel allocation, so modification of kernel heap state is technically plausible.

Paranoid raw score: 2 + 1 + 1 + 1 + 1 = 6.
Paranoid score raised to 7 as the final actionability bucket because this is CLASS_A controlled kernel memory corruption with plausible local crafted-image exposure and unresolved exploitability beyond crash.

Important non-awarded signals:

* RealUAF not awarded. This is not a use-after-free.
* Concrete repeated release/refcount not awarded. No double put, double free, or stale refcount mutation is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated. The overflow is attacker-influenced but not shown to be arbitrary address write.
* Confidentiality not awarded. No disclosure path is shown.
* Remote reachability not established. This is not a network-triggered bug.
* Race/timing penalty not applied. The bug is a deterministic malformed metadata parsing issue, not a timing race.
* Extra availability point not awarded. Crash and availability are the same consequence here.

## 3. Reachability analysis

The attacker needs a corrupted HFS Plus filesystem image to be processed by the kernel HFS Plus driver and must reach the `hfsplus_delete_cat()` path where `str == NULL`, which occurs when the last open reference to an unlinked HFS Plus hardlink backing inode is closed.

This is a local crafted-filesystem-image attack surface. Direct network reachability is not supported by the patch. Host-admin-equivalent privilege is not positively established because real deployments may process removable or user-provided filesystem images through automount or storage services. However, below-host-admin reachability is also not universally proven because mounting filesystem images often requires `CAP_SYS_ADMIN` or a privileged helper. Therefore `HostAdminRequired=UNKNOWN`.

No namespace-scoped privilege path is established. No special hardware is required. No race is required. The attacker has meaningful control over filesystem metadata, including the thread record name length and copied name bytes, but the exact heap target after the overflow is not established.

## 4. Primitive and severity interpretation

The exact primitive is an attacker-influenced kernel heap out-of-bounds write. The vulnerable code reads `thread.nodeName.length` directly from the HFS Plus catalog B-tree, computes `length * 2`, and copies that many bytes into `fd.search_key->cat.name.unicode` without first validating that the record is a valid thread record or that the record size matches the claimed name length.

This is not fixed stale storage and not cleanup-only stale dereference. It is direct malformed filesystem metadata causing an oversized copy into a kernel allocation.

PrimitiveClass is `CLASS_A` because the patch establishes a strong corruption primitive: attacker-influenced OOB write into kernel memory. Demonstrated consequence is at least local kernel crash / DoS. A stronger consequence such as privilege escalation is not demonstrated, but the primitive is strong enough that exploitation leverage must be manually reviewed.

ImportantCandidate is `YES` because controlled kernel heap corruption in filesystem parsing code is an Important-preserving primitive unless reachability or exploitability is proven to be strongly constrained. AutoDowngradeAllowed is `NO` because automatic closure would discard unresolved kernel memory-corruption leverage.

Operationally this should be treated as at least actionable Moderate and likely a Strong Important candidate if the affected product processes HFS Plus images from local users, removable media, or automount paths.

## 5. One-sentence report phrase

A corrupted HFS Plus catalog thread record can trigger an attacker-influenced kernel heap out-of-bounds write during delete key rebuild, causing at least local DoS and requiring manual review for possible privilege escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete attacker-influenced kernel OOB write, not a crash-only NULL dereference or resource leak.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: hfsplus: validate thread record before delete key rebuild [ Upstream
Commit: 33cda0036bc683fc888c20b04a5fc030e3e8413b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=33cda0036bc683fc888c20b04a5fc030e3e8413b

Changed files:
  fs/hfsplus/catalog.c
  fs/hfsplus/hfsplus_fs.h

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=33cda0036bc683fc888c20b04a5fc030e3e8413b

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

The original Linux kernel CVE announcement for CVE-2026-93095 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93095

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:L/PR:L/UI:N/S:U/C:L/I:L/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: 5
  Paranoid ActionableScore: 7

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).