[CVE-2026-72137][IMPORTANT] xfrm: nat_keepalive: avoid double free on send error [ Upstream

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

CVE: CVE-2026-72137
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: xfrm: nat_keepalive: avoid double free on send error [ Upstream
Commit: d0a4dc7efa825bce60a8da8f7d43c864a159abde
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0a4dc7efa825bce60a8da8f7d43c864a159abde
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72137
Analysis date: Sat, 15 Aug 2026 17:20:37 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An skb ownership bug in XFRM NAT keepalive handling can double free a keepalive packet after IPv4 or IPv6 output path handoff, causing at least a local kernel crash and warranting manual review for possible heap corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72137 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72137

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-2026-72137	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-416;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'xfrm NAT keepalive handling can double free an skb when an IPv4 or IPv6 send helper returns an error after the skb has already been handed to the output path. The first free can occur inside the networking stack after ownership transfer, while nat_keepalive_send previously freed the same skb again on any helper error. For the CVSS the PR:L is used for the paranoid score because a reduced capability root, container root, or service with CAP_NET_ADMIN in a relevant network namespace may be able to configure XFRM NAT keepalive state and trigger send error paths. The issue is not directly Internet reachable through packet traffic, but it is in the network stack and may be reachable from local or delegated network administration contexts. Impact is at least denial of service via kernel crash. Because the primitive is a double free of an skb, worst case memory corruption may allow confidentiality and integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	DANGER SIMPLEFIX SKB LINUS IPV6 KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The patch fixes an skb double free after ownership may have been transferred to `ip_build_and_send_pkt()` or `ip6_xmit()`.
* Real lifetime corruption: +1. The bug is specifically an ownership and lifetime mismatch for an skb.
* Reliable kernel crash / strong DoS: +1. Double freeing an skb can realistically crash the kernel.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Creating and managing the relevant XFRM/IPsec NAT keepalive state is normally privileged.
* Weak LPE concern: +1. Double free is a real corruption primitive, but the patch does not show controlled reclaim, controlled overwrite, callback control, or a demonstrated LPE path.
* Important network subsystem context: +1. XFRM/IPsec is not ordinary application code and skb lifetime bugs in this area deserve manual review, although this is not directly packet-triggered from the Internet.

Paranoid additional interpretation:

* Confidentiality impact plausible: +1. In worst case, skb double free could become broader heap corruption, though not demonstrated.
* Integrity impact plausible: +1. In worst case, allocator or object lifetime corruption may affect kernel integrity.
* Privilege penalty reduced from -2 to -1. Delegated CAP_NET_ADMIN, container root, or service accounts managing network namespaces may make the practical attacker less than full host root.

## 3. Reachability analysis

The realistic trigger is local or delegated network-administration control over XFRM/IPsec NAT keepalive state plus conditions that make the IPv4 or IPv6 send helper return an error after skb ownership handoff. This is not directly remote reachable through ordinary inbound packet traffic. In typical host deployments the path requires CAP_NET_ADMIN, but containers, network namespaces, VPN/IPsec managers, or reduced-capability service accounts may lower the practical privilege boundary. The path is configuration-dependent, but XFRM/IPsec is a standard kernel networking subsystem and should not be treated as a rare hardware-only issue.

Call-site confidence: high. The provided diff includes both the caller `nat_keepalive_send()` and the IPv4/IPv6 helper behavior that determines skb ownership.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is not just a warning, leak, or validation-only fix. The concrete primitive is a double free of an skb after possible networking-stack ownership transfer. Realistic impact is likely local privileged or delegated-privilege DoS, but the memory lifetime corruption class makes LPE theoretically plausible enough for manual review. The conservative score is borderline due to CAP_NET_ADMIN requirements, while the paranoid score reaches Strong Important candidate because reduced-privilege network administration contexts and skb allocator behavior may increase practical risk.

## 5. One-sentence report phrase

An skb ownership bug in XFRM NAT keepalive handling can double free a keepalive packet after IPv4 or IPv6 output path handoff, causing at least a local kernel crash and warranting manual review for possible heap corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is a concrete skb double free in kernel networking lifetime handling. Even though typical triggering requires CAP_NET_ADMIN and is not directly remote, double free bugs can be underestimated during automated triage.

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

Patch: xfrm: nat_keepalive: avoid double free on send error [ Upstream
Commit: d0a4dc7efa825bce60a8da8f7d43c864a159abde
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0a4dc7efa825bce60a8da8f7d43c864a159abde

Changed files:
  net/xfrm/xfrm_nat_keepalive.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=d0a4dc7efa825bce60a8da8f7d43c864a159abde

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

The original Linux kernel CVE announcement for CVE-2026-72137 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72137

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:H/PR:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 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).