[CVE-2026-72348][IMPORTANT] netfilter: ip6tables: mark malformed IPv6 extension headers for hotdrop [ Upstream

[CVE-2026-72348][IMPORTANT] netfilter: ip6tables: mark malformed IPv6 extension headers for hotdrop [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72348
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: ip6tables: mark malformed IPv6 extension headers for hotdrop [ Upstream
Commit: f16d856b6af5fd0e7cb0b0212f70825b599ef72e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f16d856b6af5fd0e7cb0b0212f70825b599ef72e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72348
Analysis date: Sun, 16 Aug 2026 09:03:39 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate. Network reachable firewall policy bypass that should not be auto closed.
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-72348 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72348

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-2026-72348	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N';CWE-693;*CWE-20;CWE-284;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '6.5';DESCR 'Malformed IPv6 AH, HBH, or RT extension headers can be treated as a rule mismatch in ip6tables instead of being dropped when the advertised extension header length is larger than the available packet data. A remote attacker can send such malformed IPv6 packets to bypass rules that were intended to match and drop packets with these extension headers. For the CVSS the PR:N is used because no local account or code execution is needed when the filtered IPv6 traffic path is reachable. The issue is network reachable but only affects systems with relevant ip6tables IPv6 extension header match rules. Impact is primarily firewall policy bypass. In the paranoid score this is treated as possible limited confidentiality and integrity impact because traffic that should have been blocked may reach protected services.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE SIMPLEFIX NETFILTER NETWORK SKB LINUS IPV6 KPANIC PACKET 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

1. ActionableScore
* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate. Network reachable firewall policy bypass that should not be auto closed.

2. Signal breakdown
* Remote reachable / network-triggerable: +2. A remote sender can craft malformed IPv6 packets with AH, HBH, or RT extension headers.
* Cross-boundary isolation/policy bypass: +1. The bug can bypass ip6tables rules intended to drop packets with these IPv6 extension headers.
* Common networking core path or packet-processing path: +1. This is netfilter IPv6 packet filtering logic.
* Integrity impact plausible: +1. Traffic that should have been blocked by firewall policy may be accepted.
* Confidentiality impact plausible, paranoid only: +1. If bypassed rules protect internal services or restricted traffic paths, limited confidentiality impact is plausible.
* No memory corruption signal. The patch adds length validation and hotdrop behavior, not lifetime, UAF, OOB write, or object corruption handling.
* No strong DoS signal. The main impact is policy bypass, not crash or resource exhaustion.

3. Reachability analysis
A remote attacker can trigger the affected path by sending malformed IPv6 packets if the target routes the traffic through ip6tables rules using the affected AH, HBH, or RT matches. No local account is required, so PR:N is appropriate. The issue is configuration dependent because it matters only when such ip6tables IPv6 extension header rules are deployed. However, netfilter packet filtering is a common security boundary, so this should not be treated as a rare unreachable subsystem. Namespaces or containers do not reduce the remote trigger requirement, but they may affect where such filtering rules are installed.

4. Severity interpretation
This behaves like an actionable Moderate issue rather than ordinary Low or pure correctness cleanup. It is not an Important memory corruption candidate because there is no UAF, OOB write, type confusion, refcount bug, or arbitrary write. The realistic impact is remote firewall rule bypass for malformed IPv6 extension header packets. The paranoid score reaches 6 because limited confidentiality and integrity impact are defensible when bypassed firewall policy protects access to sensitive services.

5. One-sentence report phrase
Malformed IPv6 AH, HBH, or RT extension headers may be treated as ip6tables rule mismatches instead of hotdrops, allowing a remote attacker to bypass rules intended to drop such packets when the relevant IPv6 extension header matches are configured.

6. Manual review recommendation
MANUAL CHECK REQUIRED. The issue is network reachable and affects firewall policy enforcement, so it should not be auto-closed even though the patch does not indicate memory corruption.

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

Patch: netfilter: ip6tables: mark malformed IPv6 extension headers for hotdrop [ Upstream
Commit: f16d856b6af5fd0e7cb0b0212f70825b599ef72e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f16d856b6af5fd0e7cb0b0212f70825b599ef72e

Changed files:
  net/ipv6/netfilter/ip6t_ah.c
  net/ipv6/netfilter/ip6t_hbh.c
  net/ipv6/netfilter/ip6t_rt.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=f16d856b6af5fd0e7cb0b0212f70825b599ef72e

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

The original Linux kernel CVE announcement for CVE-2026-72348 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72348

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:L/PR:N/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS score: 6.5

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: 6

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).