[CVE-2026-64009][IMPORTANT] xfrm: Check for underflow in xfrm_state_mtu [ Upstream

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

CVE: CVE-2026-64009
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: xfrm: Check for underflow in xfrm_state_mtu [ Upstream
Commit: 8014f70c4e6e5ab101ae3860a614e65e988372e3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8014f70c4e6e5ab101ae3860a614e65e988372e3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64009
Analysis date: Sun, 19 Jul 2026 11:56:09 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An integer underflow in xfrm_state_mtu can cause ESP output to pass a negative padding length as a huge size_t to memset, producing a kernel out-of-bounds zero write and a local crash or potential memory-corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-64009 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64009

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-2026-64009	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-191;*CWE-787;CWE-195;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'Manual review is recommended. This is not a resource leak or warning-only issue. The patch context describes a concrete OOB write primitive caused by unsigned underflow and signed-to-unsigned conversion in the ESP output path, so paranoid CIA:HHH is defensible while base CIA:LLH keeps the assessment grounded.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	WRITE KASAN OOB LINUS 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  The description explicitly states that a nobody user can configure the required ESP tunnel state and trigger the path. In typical kernels this depends on XFRM and network namespace capability behavior, but user namespaces can make this reachable to an unprivileged local user.

* Constrained kernel buffer overwrite: +1
  The primitive is a concrete kernel out-of-bounds write via memset at skb_tail_pointer. The written value is zero and the immediate target is not fully attacker-selected, so conservative scoring treats it as constrained rather than arbitrary.

* Weak LPE concern: +1
  The bug is a real OOB write in kernel networking memory. Practical privilege escalation is not demonstrated, but corruption beyond a simple NULL dereference is present.

* Reliable kernel crash / strong DoS: +1
  KASAN reports a huge write size caused by sign extension, and the expected result is a reliable kernel crash or memory corruption.

* Confidentiality impact plausible: +1
  Not directly demonstrated, but kernel heap corruption can plausibly affect adjacent object state. This is weaker than a proven information leak.

* Integrity impact plausible: +1
  The zero overwrite can corrupt kernel objects or metadata adjacent to the skb buffer.

* Common networking / packet-processing path: +1
  XFRM/IPsec ESP output is a kernel networking data path. It should not be treated as rare-only even though it requires specific runtime configuration.

Paranoid delta:

* Memory corruption, strong corruption primitive instead of constrained overwrite: +1 additional
  The patch and report describe a concrete OOB write of a massive size in kernel memory. For paranoid triage, this is strong enough to keep as an Important candidate.

## 3. Reachability analysis

The trigger is local, not directly remote. An attacker must arrange XFRM/IPsec ESP state with a large authentication overhead, small MTU, and large TFCPAD, then send traffic through the tunnel. The description says this is possible as a nobody user, likely relying on network namespace behavior where an unprivileged user can obtain CAP_NET_ADMIN inside a new namespace if user namespaces are enabled.

If unprivileged user namespaces or delegated network administration are disabled, practical reachability may become CAP_NET_ADMIN-gated and the conservative score should be reduced. If they are enabled, the bug is a local unprivileged kernel memory corruption issue.

The path is not public-Internet remote by packet traffic alone. The vulnerable state must first be configured locally.

Call-site confidence: high. The commit message gives the downstream use in esp_output and esp_output_fill_trailer, including the signed-to-size_t conversion and KASAN write size.

## 4. Severity interpretation

This is stronger than an ordinary Moderate issue because the patch fixes a concrete integer underflow leading to an OOB kernel write. It is not merely validation cleanup, warning suppression, or resource exhaustion.

Realistic impact is at least local DoS through kernel crash. Theoretical privilege escalation is plausible because the primitive is kernel memory corruption, but the payload is zero-filled and the target is not shown to be attacker-controlled. Therefore the conservative result is Actionable Moderate / Important boundary, while the paranoid result is a Strong Important candidate.

## 5. One-sentence report phrase

An integer underflow in xfrm_state_mtu can cause ESP output to pass a negative padding length as a huge size_t to memset, producing a kernel out-of-bounds zero write and a local crash or potential memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a concrete local kernel OOB write in the XFRM/ESP output path, with plausible privilege-escalation concern despite the currently demonstrated impact being crash-oriented.

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

Patch: xfrm: Check for underflow in xfrm_state_mtu [ Upstream
Commit: 8014f70c4e6e5ab101ae3860a614e65e988372e3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8014f70c4e6e5ab101ae3860a614e65e988372e3

Changed files:
  net/xfrm/xfrm_state.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=8014f70c4e6e5ab101ae3860a614e65e988372e3

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

The original Linux kernel CVE announcement for CVE-2026-64009 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64009

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: 7
  Paranoid ActionableScore: 8

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: NO
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).