[CVE-2026-64296][IMPORTANT] exfat: bound uniname advance in exfat_find_dir_entry()

[CVE-2026-64296][IMPORTANT] exfat: bound uniname advance in exfat_find_dir_entry()

[AL-KERNEL]

CVE: CVE-2026-64296
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: exfat: bound uniname advance in exfat_find_dir_entry()
Commit: 72a2589d82eb001c94b74bcfe6f9a599bd9bef60
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72a2589d82eb001c94b74bcfe6f9a599bd9bef60
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64296
Analysis date: Sat, 25 Jul 2026 11:48:24 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted exFAT directory can make `exfat_find_dir_entry()` advance its Unicode name pointer beyond the output buffer and perform an out-of-bounds read and fixed NUL write, causing at least local kernel crash risk and requiring manual review for possible memory corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-64296 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64296

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: IMPORTANT
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-2026-64296	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-125;*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 'An out-of-bounds read and write in exfat_find_dir_entry can occur when a crafted exFAT directory contains many TYPE_EXTEND filename fragments with early NUL characters. The pointer used for the output name advances by a fixed 15 character step per entry, while the length guard tracks only the shorter extracted fragment length, allowing the pointer to move beyond the p_uniname name buffer. For the CVSS the PR:L is used because a local user may be able to trigger parsing through an already mounted or automatically mounted exFAT filesystem, although systems that restrict mounting to administrators may have a higher practical privilege requirement. The issue is not network reachable and requires local access to a crafted filesystem image or removable media. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel memory corruption, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB DISK IMPROVEONLY LINUS KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1. A local user may trigger parsing by accessing an already mounted or automatically mounted crafted exFAT filesystem.
* Constrained kernel buffer overwrite: +1. The bug causes an out-of-bounds write, but the written value is a fixed NUL and the target is constrained by the filename buffer layout.
* Reliable kernel crash / strong DoS: +1. Out-of-bounds access in kernel filesystem parsing can realistically crash the kernel.
* Filesystem/storage metadata corruption reachable by local user: +1. The trigger is crafted exFAT directory metadata.
* Integrity impact plausible: +1. The out-of-bounds write can corrupt adjacent kernel memory, even if the payload is constrained.

Paranoid additions / interpretation:

* Memory corruption, strong corruption primitive: +2 instead of constrained +1. The patch explicitly fixes an out-of-bounds read and write past `p_uniname->name`.
* Weak LPE concern: +1. Privilege escalation is not demonstrated, but kernel OOB write from crafted filesystem metadata is enough to require manual review.

No remote/network score is applied. No Dirty-Pipe-like or page-cache ownership bypass signal is evident.

## 3. Reachability analysis

The bug is triggered through crafted exFAT directory entries with many short `TYPE_EXTEND` filename fragments containing early NUL characters. It is not network reachable. The realistic attacker is local and needs a crafted filesystem image, removable media, or access to a system where such media is automatically mounted or already mounted.

Mounting filesystems directly usually requires administrator privileges, but desktop automount, removable media workflows, or delegated environments can make parsing reachable from a low-privileged local user. Containers usually do not lower the requirement unless they can cause the host or a privileged helper to mount or scan the image.

Call-site confidence: high for the OOB read/write primitive because the changed code and bounds check are directly visible. Medium for practical reachability because it depends on mount and automount policy.

## 4. Severity interpretation

This is more than an ordinary Moderate because the patch fixes a concrete kernel out-of-bounds read and write, not just a validation warning or resource leak. Realistic impact is likely local DoS, because the overwrite is constrained and no arbitrary write, reclaim, callback control, or demonstrated LPE chain is shown. However, kernel filesystem metadata parsing bugs with OOB writes have enough historical risk to be treated as Actionable Moderate at minimum and a Strong Important candidate for manual triage.

## 5. One-sentence report phrase

A crafted exFAT directory can make `exfat_find_dir_entry()` advance its Unicode name pointer beyond the output buffer and perform an out-of-bounds read and fixed NUL write, causing at least local kernel crash risk and requiring manual review for possible memory corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a concrete kernel OOB read/write in filesystem parsing reachable through crafted metadata. The practical LPE path is not proven, but the memory corruption primitive is sufficient to avoid auto-closure.

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

Patch: exfat: bound uniname advance in exfat_find_dir_entry()
Commit: 72a2589d82eb001c94b74bcfe6f9a599bd9bef60
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72a2589d82eb001c94b74bcfe6f9a599bd9bef60

Commit description:

In exfat_find_dir_entry(), each TYPE_EXTEND (file name) entry advances the
output pointer by a fixed amount while the loop guard only tracks the
accumulated name length:

	if (++order == 2)
		uniname = p_uniname->name;
	else
		uniname += EXFAT_FILE_NAME_LEN;
	len = exfat_extract_uni_name(ep, entry_uniname);
	name_len += len;
	unichar = *(uniname+len);
	*(uniname+len) = 0x0;

uniname grows by EXFAT_FILE_NAME_LEN (15) per name entry, but name_len
grows only by the actual extracted length, which is shorter when a name
fragment contains an early NUL.  The only guard is
`name_len >= MAX_NAME_LENGTH`, so a crafted directory with many short
name fragments lets uniname run far past the
p_uniname->name[MAX_NAME_LENGTH + 3] buffer while name_len stays small,
causing an out-of-bounds read and write at *(uniname+len).

The sibling extractor exfat_get_uniname_from_ext_entry() already stops
on a short fragment (the lockstep `len != EXFAT_FILE_NAME_LEN` guard
added in commit d423345 ("exfat: check if filename entries exceeds
max filename length")); exfat_find_dir_entry() never got the
equivalent.  Track the per-entry write offset as a count and reject a
fragment once the offset, or the offset plus the extracted length, would
exceed MAX_NAME_LENGTH, before forming the output pointer.

Fixes: ca06197 ("exfat: add directory operations")
Cc: stable@vger.kernel.org
Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/exfat/dir.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=72a2589d82eb001c94b74bcfe6f9a599bd9bef60

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

The original Linux kernel CVE announcement for CVE-2026-64296 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64296

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).