[CVE-2026-64323][MODERATE 7.0] udf: validate VAT header length against the VAT inode size

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

CVE: CVE-2026-64323
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: udf: validate VAT header length against the VAT inode size
Commit: 0ad2d09a8d66fa8dc6f9b70d660b5fb4478ea934
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ad2d09a8d66fa8dc6f9b70d660b5fb4478ea934
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64323
Analysis date: Sat, 25 Jul 2026 14:54:56 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A crafted UDF VAT image can set an attacker-controlled header length larger than the VAT inode size, causing integer underflow in the VAT entry count and an out-of-bounds kernel read during mount, with realistic local DoS impact and possible limited confidentiality concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64323 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64323

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: NO

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-2026-64323	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-191;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.1';DESCR 'A crafted UDF image with a virtual VAT partition can set an attacker controlled VAT 2.0 lengthHeader that is larger than the VAT inode size. This makes the VAT entry count underflow to a very large value and can bypass the normal block count validation in udf_get_pblock_virt15. On the ICB inline path the kernel can then read past the inode inline data using iinfo i_data plus the attacker controlled start offset, resulting in a kernel out of bounds read. For the CVSS the PR:L is used because practical triggering requires a local user or local service path able to cause the crafted UDF image to be mounted, rather than a remote packet only trigger. The issue is not network reachable by itself. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, limited confidentiality impact is possible because the primitive is an out of bounds kernel read, but there is no strong evidence of arbitrary write or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	OOB DISK LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user may be able to supply a crafted UDF image through removable media, loop image handled by a privileged mount helper, desktop automount, or delegated mount path.
* Reliable kernel crash / strong DoS: +1. The patch and commit describe an attacker-controlled VAT header causing an out-of-bounds read during mount.
* Important filesystem/storage path: +1. The bug is in UDF filesystem metadata parsing during mount.
* Filesystem/storage metadata reachable by local user: +1. The trigger is attacker-controlled on-disk filesystem metadata.

Paranoid additional signal:

* Confidentiality impact plausible: +1. The primitive is a kernel out-of-bounds read from inode inline data. No clear user-visible disclosure path is shown, but limited confidentiality impact is technically plausible enough for manual triage.

Signals not applied:

* No remote reachable signal. The bug is not network-triggerable by itself.
* No generic strong memory corruption signal. The described primitive is read-only OOB, not OOB write, UAF, double-free, or arbitrary write.
* No privilege escalation signal. There is no demonstrated write primitive, reclaim primitive, object replacement, or type confusion.

## 3. Reachability analysis

Triggering requires mounting or otherwise causing the kernel to parse a crafted UDF image with a virtual VAT partition. In typical server deployments, mounting filesystems requires CAP_SYS_ADMIN, but practical local exposure may exist through removable media automount, desktop mount services, container or namespace delegation, or privileged services that mount user-provided images. The path is not remotely reachable as packet input. UDF support is a common filesystem feature, but actual exposure depends on whether untrusted users can provide mountable media or images.

Call-site confidence: high. The commit explains the relevant downstream use in udf_get_pblock_virt15 and the exact OOB read expression.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate. The realistic impact is local DoS through kernel crash during crafted filesystem mount. The theoretical higher impact is limited kernel memory disclosure due to an OOB read, but the patch does not show a direct disclosure channel or privilege escalation primitive. It should not be treated as Important without additional evidence, but it should not be auto-closed because it is an explicit attacker-controlled filesystem metadata OOB read.

## 5. One-sentence report phrase

A crafted UDF VAT image can set an attacker-controlled header length larger than the VAT inode size, causing integer underflow in the VAT entry count and an out-of-bounds kernel read during mount, with realistic local DoS impact and possible limited confidentiality concern.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.

Reason: this is an attacker-controlled filesystem metadata OOB read in kernel code. It is not a proven LPE, but the OOB read primitive and mount-service exposure possibilities justify manual triage rather than automatic closure.

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

Patch: udf: validate VAT header length against the VAT inode size
Commit: 0ad2d09a8d66fa8dc6f9b70d660b5fb4478ea934
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0ad2d09a8d66fa8dc6f9b70d660b5fb4478ea934

Commit description:

udf_load_vat() takes the virtual partition's start offset straight from
the on-disk VAT 2.0 header without checking it against the VAT inode
size:

	map->s_type_specific.s_virtual.s_start_offset =
		le16_to_cpu(vat20->lengthHeader);
	map->s_type_specific.s_virtual.s_num_entries =
		(sbi->s_vat_inode->i_size -
			map->s_type_specific.s_virtual.s_start_offset) >> 2;

lengthHeader is a fully attacker-controlled 16-bit value.  If it exceeds
the VAT inode size, the s_num_entries subtraction underflows to a huge
count, which defeats the "block > s_num_entries" bound in
udf_get_pblock_virt15(); and on the ICB-inline path that function reads

	((__le32 *)(iinfo->i_data + s_start_offset))[block]

so a large s_start_offset indexes past the inode's in-ICB data.  Mounting
a crafted UDF image with a virtual (VAT) partition then triggers an
out-of-bounds read.

Reject a VAT whose header length does not leave room for at least one
entry within the VAT inode.

Fixes: fa5e081 ("udf: Handle VAT packed inside inode properly")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Link: https://patch.msgid.link/20260612-b4-disp-9a2317ee-v1-1-fefef5736154@proton.me
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/udf/super.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=0ad2d09a8d66fa8dc6f9b70d660b5fb4478ea934

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

The original Linux kernel CVE announcement for CVE-2026-64323 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64323

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 6.1

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: 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).