[CVE-2026-46244][IMPORTANT] netfilter: nft_inner: Fix IPv6 inner_thoff desync

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

CVE: CVE-2026-46244
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nft_inner: Fix IPv6 inner_thoff desync
Commit: c161ad9157f5a0429b5ff94d9770faf3bf48d273
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c161ad9157f5a0429b5ff94d9770faf3bf48d273
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46244
Analysis date: Wed, 03 Jun 2026 15:47:27 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An IPv6 inner packet parsing desynchronization in nft_inner can make nftables use a correct L4 protocol value with an incorrect transport header offset, allowing crafted packets with extension headers to forge L4 matches and potentially bypass firewall policy.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46244 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46244

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-2026-46244	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:L';CWE-693;CWE-436;*CWE-20;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N';BEST CVSS score: '8.9';DESCR 'nft_inner IPv6 parsing can desynchronize inner_thoff from l4proto because ipv6_find_hdr computes the real transport header offset but the result is overwritten with the IPv6 base header size. A remote attacker can craft an inner IPv6 packet with extension headers so nftables inner matching reads transport fields from the wrong offset, enabling forged L4 matches or firewall policy bypass when affected rules are deployed. For the CVSS the PR:N is used for the paranoid score because the attacker does not need local code execution or an account on the target, only network reachability to a path where nft_inner rules process the packet. The issue is network reachable but configuration dependent, so AC:H is used because it requires nft_inner rules, inner IPv6 traffic, and extension headers. Impact is a protection bypass rather than memory corruption, with possible confidentiality and integrity impact if the firewall rule was intended to protect sensitive services. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	REMOTE NETFILTER IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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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

Conservative score signals:

* Remote reachable / network-triggerable: +2
  A crafted packet can trigger the affected parsing path if nft_inner rules process inner IPv6 traffic.

* Cross-boundary isolation/policy bypass: +1
  The bug can make netfilter match L4 fields at the wrong offset, creating a firewall policy bypass condition.

* Common networking core path or packet-processing path: +1
  This is in netfilter packet processing, even though it depends on nft_inner configuration.

* Confidentiality impact plausible: +1
  Firewall bypass can expose services or traffic that rules were intended to block.

* Integrity impact plausible: +1
  Bypassing filtering rules can allow prohibited traffic or interaction with protected services.

Paranoid additional interpretation:

* Security-boundary bypass with practical sensitive-object access: +2 instead of the weaker +1 policy-bypass signal
  If the affected nft_inner rule protects sensitive internal services, the wrong transport-header offset can enable forged L4 matches and unauthorized access.

No memory-corruption signals are applied. This is a parser logic and policy-enforcement bug, not UAF, OOB write, type confusion, or arbitrary write.

## 3. Reachability analysis

A remote attacker can plausibly trigger the issue by sending inner IPv6 packets with extension headers through a path where nft_inner rules inspect encapsulated traffic. No local account on the target is required, so PR is effectively none for the network attacker model. The issue is configuration dependent because nft_inner rules and relevant inner IPv6 traffic must be present. This is not a default universal exposure, but it is still a netfilter packet-processing path and should not be treated as low priority solely due to configuration dependence.

Namespaces and containers mainly matter on the rule-management side, not the packet-trigger side. An attacker does not need CAP_NET_ADMIN to exploit an already deployed vulnerable rule path, but CAP_NET_ADMIN would usually be required to create such rules.

## 4. Severity interpretation

This behaves like an actionable network firewall bypass, not ordinary DoS and not memory corruption. The realistic impact is bypassing nftables policy for inner IPv6 traffic when extension headers are used. Theoretical impact depends on what the affected firewall rule protects. If it guards sensitive services or tenant boundaries, confidentiality and integrity impact can be meaningful. Because it is network reachable in affected configurations and affects packet filtering semantics, it should not be auto-closed as an ordinary Moderate.

## 5. One-sentence report phrase

An IPv6 inner packet parsing desynchronization in nft_inner can make nftables use a correct L4 protocol value with an incorrect transport header offset, allowing crafted packets with extension headers to forge L4 matches and potentially bypass firewall policy.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Network-reachable firewall bypasses require manual review because real severity depends on deployed nft_inner rules, tunnel topology, and whether the affected rule protects sensitive services or isolation boundaries.

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UPSTREAM PATCH SUMMARY
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Patch: netfilter: nft_inner: Fix IPv6 inner_thoff desync
Commit: c161ad9157f5a0429b5ff94d9770faf3bf48d273
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c161ad9157f5a0429b5ff94d9770faf3bf48d273

Commit description:

In nft_inner_parse_l2l3(), when processing inner IPv6 packets,
ipv6_find_hdr() correctly computes the transport header offset
traversing all extension headers, but the result is immediately
overwritten with nhoff + sizeof(_ip6h) (40 bytes), which only
accounts for the IPv6 base header. This creates a desync between
inner_thoff (wrong — points to extension header start) and l4proto
(correct — e.g., IPPROTO_TCP), enabling transport header forgery
and potential firewall bypass. This issue affects stable versions
from Linux 6.2.

For comparison, the normal (non-inner) IPv6 path correctly
preserves ipv6_find_hdr()'s result. Removing the incorrect overwrite
ensures that ipv6_find_hdr()'s calculated transport header offset is
preserved, thereby fixing the desynchronization.

Fixes: 3a07327 ("netfilter: nft_inner: support for inner tunnel header matching")
Cc: stable@vger.kernel.org
Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Assisted-by: GLM:5.1 Z.ai
Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nft_inner.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=c161ad9157f5a0429b5ff94d9770faf3bf48d273

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

The original Linux kernel CVE announcement for CVE-2026-46244 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46244

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:C/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:L
  The Best / paranoid CVSS score: 8.9

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).