[CVE-2026-43037][IMPORTANT] ip6_tunnel: clear skb2->cb[] in ip4ip6_err() [ Upstream

[CVE-2026-43037][IMPORTANT] ip6_tunnel: clear skb2->cb[] in ip4ip6_err() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-43037
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6_tunnel: clear skb2->cb[] in ip4ip6_err() [ Upstream
Commit: ea9f65b27c8404e164848ebff1443310fd187629
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea9f65b27c8404e164848ebff1443310fd187629
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43037
Analysis date: Thu, 21 May 2026 00:28:54 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale `skb->cb[]` type confusion in the IPv4-over-IPv6 tunnel error path can let remote packet data drive an oversized copy into a fixed 40-byte kernel stack buffer, causing DoS and possible kernel memory corruption on reachable tunnel endpoints.

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

The original Linux kernel CVE announcement for CVE-2026-43037 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43037

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-43037	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-843;CWE-120;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '9.8';DESCR 'A stale skb control buffer in ip4ip6_err can make IPv6 receive metadata be interpreted as IPv4 IPCB state. This can set a bogus IPv4 record route option offset and make __ip_options_echo copy an attacker controlled option length into a fixed 40 byte stack buffer. For the CVSS the PR:N is used because an attacker does not need local credentials if the IPv4 over IPv6 tunnel error path is reachable over the network. The base score uses AC:H because exploitation depends on an ip6 tunnel configuration and the ICMP error path. The paranoid score uses AC:L for exposed tunnel endpoints because the bug provides attacker influenced stack buffer overflow in a network path. Impact may include kernel crash and potentially confidentiality or integrity compromise due to memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE SIMPLEFIX NETWORK SKB IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative score signals:

* Remote reachable / network-triggerable: +2
  The affected path is an IPv4-over-IPv6 tunnel error path. If the tunnel endpoint is reachable, packets can influence the vulnerable path remotely.

* Generic memory corruption, strong corruption primitive: +2
  The commit describes an attacker-influenced copy length being used to copy into `dopt->__data`, a fixed 40-byte stack buffer.

* Reliable kernel crash / strong DoS: +1
  A stack buffer overflow in the kernel networking path is a credible crash path.

* Confidentiality impact plausible: +1
  Stack corruption can plausibly expose or influence sensitive kernel state, although no specific leak primitive is shown.

* Integrity impact plausible: +1
  The overflow is attacker-influenced and occurs in kernel stack memory, so integrity impact is plausible.

* Availability impact realistic: +1
  Kernel crash is a realistic outcome.

* Common networking / packet-processing path: +1
  This is in IP tunnel and ICMP error processing, not a local-only ioctl or administrative path.

Subtractions:

* Configuration-dependent tunnel exposure: -1
  Triggering requires an IPv4-over-IPv6 tunnel/error-path configuration. This reduces broad exposure but does not make the primitive low risk.

Paranoid score difference:

* No rare/configuration penalty: +1 compared with conservative
  For exposed tunnel endpoints, the bug is remotely reachable and the primitive is strong enough that configuration dependence should not downgrade it below high-end Important handling.

## 3. Reachability analysis

An unauthenticated remote attacker may trigger the bug if they can send traffic that reaches an affected IPv4-over-IPv6 tunnel endpoint and causes the `ip4ip6_err()` error path. No local account or kernel privilege is required in that deployment model.

The path is not universally enabled by default because it depends on `ip6_tunnel` or IPv4-over-IPv6 tunnel use. Containers and namespaces are less central here than network exposure, although tunnel configuration may exist in network namespaces or infrastructure hosts. Realistic exploitation depends on reaching the tunnel endpoint and shaping packet data so that the stale IPv6 `skb->cb[]` contents are interpreted as IPv4 IPCB option state.

## 4. Severity interpretation

This behaves like an Important to possible high-end Important issue, not an ordinary Moderate. The patch and commit message describe a concrete memory corruption primitive: stale control-buffer type confusion leading to attacker-controlled length copy into a fixed-size kernel stack buffer.

The realistic demonstrated impact is kernel crash or denial of service. The theoretical but technically credible higher impact is kernel control-flow or data corruption because the overflow is on the kernel stack and is influenced by remote packet data. The main limiting factor is deployment exposure through configured tunnel endpoints.

## 5. One-sentence report phrase

A stale `skb->cb[]` type confusion in the IPv4-over-IPv6 tunnel error path can let remote packet data drive an oversized copy into a fixed 40-byte kernel stack buffer, causing DoS and possible kernel memory corruption on reachable tunnel endpoints.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a remotely reachable packet-path stack buffer overflow when the affected tunnel configuration is exposed, so it should not be auto-closed even if tunnel deployment is configuration-dependent.

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

Patch: ip6_tunnel: clear skb2->cb[] in ip4ip6_err() [ Upstream
Commit: ea9f65b27c8404e164848ebff1443310fd187629
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea9f65b27c8404e164848ebff1443310fd187629

Changed files:
  net/ipv6/ip6_tunnel.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=ea9f65b27c8404e164848ebff1443310fd187629

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

The original Linux kernel CVE announcement for CVE-2026-43037 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43037

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:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 8
  Paranoid ActionableScore: 9

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).