[CVE-2026-43186][IMPORTANT] ipv6: ioam: fix heap buffer overflow in __ioam6_fill_trace_data() [ Upstream

[CVE-2026-43186][IMPORTANT] ipv6: ioam: fix heap buffer overflow in __ioam6_fill_trace_data() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-43186
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: ioam: fix heap buffer overflow in __ioam6_fill_trace_data() [ Upstream
Commit: f4d9d4b8fd839719d564651671e24c62c545c23b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4d9d4b8fd839719d564651671e24c62c545c23b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43186
Analysis date: Tue, 09 Jun 2026 18:23:14 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted IPv6 IOAM packet can provide an inconsistent trace `nodelen` and `type`, causing `__ioam6_fill_trace_data()` to write beyond the allocated trace area into adjacent skb memory, leading to remote DoS and plausible kernel heap corruption impact in IOAM-enabled deployments.

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

The original Linux kernel CVE announcement for CVE-2026-43186 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43186

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
======================================================================

CVE-2026-43186	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-122;*CWE-20;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '9.8';DESCR 'A heap buffer overflow in the IPv6/IOAM receive path can occur because __ioam6_fill_trace_data() trusts trace nodelen from the incoming packet without validating it against the IOAM trace type field. A crafted packet can set nodelen to 0 while enabling multiple trace type bits, causing writes past the allocated trace area and into adjacent skb_shared_info memory. For the CVSS the PR:N means no local privileges are required on the victim when an attacker can send crafted IPv6/IOAM traffic to a configured target. The issue is network reachable in environments where IPv6 Hop-by-Hop IOAM processing is enabled and reachable, typically inside controlled telemetry or data center networks rather than the public Internet. Impact is at least remote denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel heap memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE OOB DANGER LINUS IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: **6**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* **Remote reachable / network-triggerable: +2**
  The bug is in the IPv6 IOAM receive path and can be triggered by a crafted IPv6 packet when IOAM Hop-by-Hop processing is enabled and reachable.

* **Memory corruption, strong corruption primitive: +2**
  The commit explicitly describes a heap buffer overflow where `__ioam6_fill_trace_data()` can write around 100 bytes past the allocated IOAM trace area into adjacent `skb_shared_info`.

* **Reliable kernel crash / strong DoS: +1**
  The described overflow corrupts adjacent heap memory and leads to a kernel panic.

* **Confidentiality impact plausible: +1**
  Conservative evidence mainly supports DoS, but kernel heap corruption in packet receive context can plausibly affect adjacent metadata or later object handling.

* **Integrity impact plausible: +1**
  The write crosses an allocation boundary into kernel heap metadata or adjacent skb state, so integrity impact is plausible even if no full exploit primitive is demonstrated.

* **Common packet-processing path scrutiny: +1, paranoid only**
  This is packet receive processing in the IPv6 stack. It is not a default Internet-facing path in most deployments, but network packet parsing plus heap overwrite warrants elevated triage.

Subtracted signals:

* **Rare / configuration-dependent exposure: -1**
  Practical triggering requires IPv6 IOAM processing and a relevant IOAM namespace or deployment where such Hop-by-Hop options are accepted. This is typically limited to controlled telemetry or data-center networks, not the public Internet.

## 3. Reachability analysis

A remote sender can trigger the vulnerable path by sending a crafted IPv6 packet with an inconsistent IOAM Pre-allocated Trace header. No local user account or victim-side privileges are required if the attacker can deliver IPv6 IOAM traffic to the configured target.

The main limiting factor is deployment exposure. IPv6 IOAM is not commonly enabled for arbitrary Internet traffic and is usually deployed inside controlled telemetry, service-provider, lab, or data-center environments. Network filtering of Hop-by-Hop options may also reduce reachability.

Namespaces may matter because IOAM namespaces and network namespace configuration determine whether the packet proceeds far enough to call the trace fill path. This does not make the attacker local, but it makes practical exposure environment-dependent.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation-only cleanup. The patch prevents a concrete remote heap buffer overflow caused by trusting packet-controlled `trace->nodelen` without checking consistency with `trace->type`.

Realistically, the directly demonstrated impact is a remote kernel crash / DoS in IOAM-enabled deployments. The Important-class concern comes from the primitive: an out-of-bounds kernel heap write into adjacent `skb_shared_info` from a network packet parsing path. There is no demonstrated arbitrary write or full code execution path in the patch, so the conservative score stays below Critical handling, but the bug should not be auto-closed as a routine Moderate.

## 5. One-sentence report phrase

A crafted IPv6 IOAM packet can provide an inconsistent trace `nodelen` and `type`, causing `__ioam6_fill_trace_data()` to write beyond the allocated trace area into adjacent skb memory, leading to remote DoS and plausible kernel heap corruption impact in IOAM-enabled deployments.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a network-reachable heap buffer overflow in IPv6 packet processing with an explicit out-of-bounds write into adjacent kernel heap memory. Even though real-world exposure is limited by IOAM deployment and filtering, the primitive is strong enough to require manual security review.

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

Patch: ipv6: ioam: fix heap buffer overflow in __ioam6_fill_trace_data() [ Upstream
Commit: f4d9d4b8fd839719d564651671e24c62c545c23b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4d9d4b8fd839719d564651671e24c62c545c23b

Changed files:
  include/net/ioam6.h
  net/ipv6/exthdrs.c
  net/ipv6/ioam6.c
  net/ipv6/ioam6_iptunnel.c
  net/lwtunnel.h
  net/ioam6.h

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=f4d9d4b8fd839719d564651671e24c62c545c23b

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

The original Linux kernel CVE announcement for CVE-2026-43186 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43186

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: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: 6
  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).