[CVE-2026-93828][LOW] exfat: fix handling of damaged volume in exfat_create_upcase_table() [ Upstream

[CVE-2026-93828][LOW] exfat: fix handling of damaged volume in exfat_create_upcase_table() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-93828
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: exfat: fix handling of damaged volume in exfat_create_upcase_table() [ Upstream
Commit: 60ace93811609e4dd883e9de5fb4462ed834160a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=60ace93811609e4dd883e9de5fb4462ed834160a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93828
Analysis date: Thu, 24 Sep 2026 14:37:19 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93828 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93828

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: LOW
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-2026-93828	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93828 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:N/I:N/A:H';CWE-190;*CWE-835;**CWE-400;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A malformed exFAT volume with an upcase table size of zero can cause an integer underflow when the kernel calculates the number of table sectors. The resulting very large sector count can make exfat_load_upcase_table loop indefinitely and cause a denial of service during volume mount or automount. For the CVSS the PR:L paranoid score assumes a local user can provide a crafted filesystem image to an available mount or automount path. The base case is physical media based and requires user interaction to mount the affected volume. No confidentiality or integrity impact is supported because the patch indicates an unbounded loop and does not demonstrate a memory corruption primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	DISK HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel DoS. A zero upcase-table size underflows in `tbl_size - 1`, produces an extremely large `num_sectors` value, and can make `exfat_load_upcase_table()` loop indefinitely.
* +1 Broadly deployed filesystem exposure. exFAT support is commonly available for removable-media and filesystem-image mounting, and the malformed metadata is consumed during mount initialization.
* No local-unprivileged-trigger bonus. A local user may be able to provide a crafted image to an existing automount workflow, but the supplied context does not establish that the complete mount or automount path is available below host-admin privilege on the evaluated system.
* No memory-corruption signal. The patch establishes integer underflow and unbounded iteration, not UAF, OOB access, invalid free, refcount corruption, or an attacker-controlled write.
* No confidentiality or integrity signal. No disclosure path, modification primitive, ownership violation, or authorization bypass is shown.
* No extra Availability point. The indefinitely running mount path is the same DoS consequence already counted above.
* No remote-reachability signal. The malformed exFAT metadata must be presented through a locally accessible block device, filesystem image, or an already configured automount path.

3. Reachability analysis

The direct trigger is a crafted or corrupted exFAT volume whose upcase-table directory entry has a zero size. The most straightforward scenario is a removable device or locally supplied filesystem image being mounted by a user, desktop automounter, or storage-management service.

The patch does not establish that an ordinary unprivileged user can independently perform the full mount operation. Mounting may require host-level `CAP_SYS_ADMIN`, while desktop automount environments can expose a lower-privilege path through a separate trusted service. Therefore, `HostAdminRequired` is `UNKNOWN`, not `YES` and not `NO`.

No network namespace, user namespace, delegated capability, special device capability, or complete namespace-scoped trigger sequence is established. No special hardware is required beyond normal block-device or image-mounting capability. This is not a timing race.

4. Primitive and severity interpretation

The exact defect is an integer underflow in the sector-count calculation:

`num_sectors = ((tbl_size - 1) >> blksize_bits) + 1`

When `tbl_size` is zero, unsigned subtraction yields a very large value. The subsequent upcase-table loading path can iterate indefinitely. This is an invalid-state and DoS condition, classified as `CLASS_C`.

There is no active structured stale object, no UAF, no repeated release or refcount mutation, no controlled reclaim, and no attacker-controlled memory-corruption primitive. The demonstrated effect is CPU consumption or a stuck mount-related kernel path. The fix rejects zero-size upcase tables, records a filesystem error, marks the volume read-only, and falls back to the default upcase table.

`ImportantCandidate=NO` because no unresolved corruption, ownership, authorization, or security-boundary primitive exists. `AutoDowngradeAllowed=YES` because this is a bounded DoS-only issue with no meaningful exploitation leverage beyond availability impact.

5. One-sentence report phrase

A malformed exFAT volume with a zero sized upcase table can trigger an integer underflow during mount and cause the kernel to loop indefinitely while loading the table, resulting in a local denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch demonstrates a malformed-filesystem DoS through an integer-underflow-induced loop, with no evidence of memory corruption, information disclosure, integrity impact, privilege escalation, or security-boundary bypass.

7. Call-site confidence

Call-site confidence: medium

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

Patch: exfat: fix handling of damaged volume in exfat_create_upcase_table() [ Upstream
Commit: 60ace93811609e4dd883e9de5fb4462ed834160a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=60ace93811609e4dd883e9de5fb4462ed834160a

Changed files:
  fs/exfat/nls.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=60ace93811609e4dd883e9de5fb4462ed834160a

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

The original Linux kernel CVE announcement for CVE-2026-93828 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93828

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:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).