[CVE-2026-43038][IMPORTANT] ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach() [ Upstream

[CVE-2026-43038][IMPORTANT] ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-43038
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach() [ Upstream
Commit: c438ba010171b70bad22fc18b1d5bdc3627476e8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c438ba010171b70bad22fc18b1d5bdc3627476e8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43038
Analysis date: Wed, 20 May 2026 22:42:50 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted ICMPv4 error can leave IPv4 control-buffer metadata interpreted as IPv6 metadata in `ip6_err_gen_icmpv6_unreach`, potentially driving bogus HAO parsing and a network-triggered OOB write or kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-43038 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43038

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-2026-43038	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-843;CWE-665;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A stale IPv4 skb control buffer can be interpreted as an IPv6 control buffer in ip6_err_gen_icmpv6_unreach. A crafted ICMPv4 error with a CIPSO option may set a bogus IPv6 dsthao offset and make icmp6_send search attacker controlled inner IPv6 data for a fake HAO option. If the HAO length checks are insufficient, mip6_addr_swap may perform a 16-byte swap past packet data into skb metadata, creating a plausible network triggered out-of-bounds write. For the CVSS the PR:N is used because the attacker only needs packet reachability to the affected tunnel error path and no local account on the target. AC:H is used because practical triggering depends on a configured IPv6 over IPv4 error path and carefully crafted ICMPv4 and inner IPv6 contents. Impact is at least remote denial of service and in the paranoid interpretation may include confidentiality and integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE NETWORK SKB IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The described trigger is a forged ICMPv4 error packet reaching the IPv6-over-IPv4 ICMP error generation path.
* Memory corruption, weak/indirect corruption candidate: +1. The bug is stale skb control-buffer interpretation, where IPv4 `inet_skb_parm` is reused as IPv6 `inet6_skb_parm`.
* Reliable kernel crash / strong DoS: +1. A bogus `dsthao` offset may drive invalid parsing and potentially unsafe address swapping.
* Common networking packet-processing path: +1. The affected code is in IPv6 ICMP error handling, though the specific trigger depends on tunnel/error-path conditions.
* Availability impact realistic: +1. Invalid packet metadata interpretation can plausibly crash or destabilize the kernel.
* Rare/configuration-dependent trigger: -1. Practical exploitation appears to require a configured SIT or IPv6-over-IPv4 error path, crafted ICMPv4, CIPSO option overlap, and suitable inner IPv6 contents.

Paranoid additional signals:

* Memory corruption, strong corruption primitive: +2 instead of +1. The commit explicitly discusses a possible 16-byte swap past packet data into `skb_shared_info`, which is an OOB write candidate.
* Weak privilege/escalation concern: +1. No reliable exploit primitive is demonstrated, but skb metadata corruption from a network packet is serious enough to review as more than DoS.
* Confidentiality impact plausible: +1. If skb metadata corruption is reachable, secondary disclosure paths cannot be excluded.
* Integrity impact plausible: +1. A network-triggered OOB write into skb-related memory can plausibly corrupt kernel state.
* Broad/default networking exposure: +1. IPv4/IPv6 ICMP handling is broadly deployed, even if this exact SIT-related path is conditional.

## 3. Reachability analysis

The likely attacker is remote or adjacent-network, able to send crafted ICMPv4 error packets toward a host using the affected IPv6-over-IPv4 error handling path. No local account is required, so PR:N is reasonable for CVSS-style interpretation.

Namespaces do not materially reduce the network risk if the vulnerable host network stack receives the crafted packets. Containers may matter only if the vulnerable tunnel configuration is inside a namespace and reachable by an attacker.

The path is not a generic IPv6 receive fast path for all systems. Realistic triggering depends on SIT or similar IPv6-over-IPv4 error handling, CIPSO option placement, and carefully shaped encapsulated IPv6 content. This supports high attack complexity but does not eliminate the remote attack surface.

## 4. Severity interpretation

This should not be treated as ordinary Moderate. The conservative view is Actionable Moderate because it is remotely reachable and may cause a kernel crash through networking metadata confusion.

The paranoid view is a Strong Important candidate because the patch context explicitly raises a possible OOB write into `skb_shared_info`. The exploitability of that primitive is not proven, and the trigger is configuration-dependent, but network-triggered skb metadata corruption is a high-risk kernel class that should not be auto-closed.

## 5. One-sentence report phrase

A crafted ICMPv4 error can leave IPv4 control-buffer metadata interpreted as IPv6 metadata in `ip6_err_gen_icmpv6_unreach`, potentially driving bogus HAO parsing and a network-triggered OOB write or kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network-triggerable under plausible tunnel conditions and the patch description identifies a possible OOB write into skb metadata, which is sufficient for Important-candidate manual review even though reliable exploitation is not demonstrated.

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

Patch: ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach() [ Upstream
Commit: c438ba010171b70bad22fc18b1d5bdc3627476e8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c438ba010171b70bad22fc18b1d5bdc3627476e8

Changed files:
  net/ipv6/icmp.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=c438ba010171b70bad22fc18b1d5bdc3627476e8

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

The original Linux kernel CVE announcement for CVE-2026-43038 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43038

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 5
  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: 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).