[CVE-2026-80664][MODERATE REGULAR] netfilter: xt_nat: reject unsupported target families

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

CVE: CVE-2026-80664
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: xt_nat: reject unsupported target families
Commit: 49abe564391411057a26a9a943c8e17867c3b9b4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=49abe564391411057a26a9a943c8e17867c3b9b4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80664
Analysis date: Fri, 28 Aug 2026 07:26:04 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / ordinary Moderate otherwise
Manual review required: YES

Summary:
xt_nat could accept unsupported nft_compat target families such as bridge even though SNAT and DNAT require IP family conntrack state, allowing a local CAP_NET_ADMIN capable actor to trigger a NULL pointer dereference and kernel DoS, with delegated network administration environments making manual exposure review advisable.

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

The original Linux kernel CVE announcement for CVE-2026-80664 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80664

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: MODERATE REGULAR
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-2026-80664	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'xt_nat SNAT and DNAT targets could be installed through nft_compat for an unsupported family such as bridge even though the handlers assume IP family conntrack and NAT state. This can make nf_nat_setup_info dereference a NULL pointer during rule setup or related NAT handling and cause a kernel crash. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires a local process that can install nftables or compat netfilter rules, often via CAP_NET_ADMIN or a delegated network administration context, but not necessarily full host root in all deployments. The issue is not directly network reachable because packet traffic alone does not install the invalid rule. Impact is denial of service via kernel crash, with no supported path to information disclosure or privilege escalation from this NULL pointer dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / ordinary Moderate otherwise (with actual score 2)	YES	REMOTE DANGER NULLPTR SIMPLEFIX NETFILTER LINUS IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / ordinary Moderate otherwise**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The commit explicitly describes a NULL dereference in nf_nat_setup_info() from an unsupported nft_compat family.
* Broad/default/common subsystem or broadly deployed exposure: +1
  netfilter and nftables are common, but this is a control plane rule installation path, not the packet fast path.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2
  Installing nftables or xt compat rules normally requires CAP_NET_ADMIN in the relevant network namespace.
* Weak corruption: +0
  This is a NULL pointer dereference, not UAF, OOB write, type confusion, refcount abuse, or attacker controlled memory corruption.
* Availability impact realistic: +1 not added separately
  The crash is already counted as strong DoS. No additional persistent corruption or broader impact is shown.

Paranoid signals:

* Reliable kernel crash / strong DoS: +1
* Broad/common netfilter exposure: +1
* Delegated CAP_NET_ADMIN / container or service context concern: +1
  In some deployments, CAP_NET_ADMIN can be delegated to containers, network management services, or reduced privilege root contexts. This makes PR:L a defensible paranoid interpretation for practical triage.
* No LPE signal added
  The patch supports a NULL dereference only. It does not show UAF, reclaim, overwrite, callback control, type confusion, or refcount takeover.
* No confidentiality or integrity signal added
  There is no supported read primitive, write primitive, or policy bypass beyond crashing the kernel.

## 3. Reachability analysis

The bug is triggered by installing an unsupported xt_nat SNAT or DNAT target family through nft_compat, for example bridge family where IP family NAT and conntrack state is assumed. Packet traffic alone does not install the invalid rule, so this is not directly network reachable.

In typical host deployments, reliable triggering requires CAP_NET_ADMIN and is therefore a privileged local control plane issue. In containerized or delegated network administration environments, the practical privilege requirement may be lower than full host root, so the paranoid score treats it as a delegated PR:L style exposure.

The path is in netfilter, but specifically in compatibility rule setup. The commit also notes that the crash was already fixed by an earlier nft_compat patch, so this patch is partly defense in depth on branches where that earlier fix is present.

## 4. Severity interpretation

This behaves like an ordinary Moderate issue in conservative scoring because it is a local CAP_NET_ADMIN gated NULL dereference with DoS only impact.

The paranoid score reaches borderline manual review territory because netfilter is widely deployed and CAP_NET_ADMIN is often delegated in containers or network management services. There is no evidence of memory corruption beyond invalid NULL dereference, and no plausible privilege escalation primitive is supported by the patch.

## 5. One-sentence report phrase

xt_nat could accept unsupported nft_compat target families such as bridge even though SNAT and DNAT require IP family conntrack state, allowing a local CAP_NET_ADMIN capable actor to trigger a NULL pointer dereference and kernel DoS, with delegated network administration environments making manual exposure review advisable.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended mainly for environments that delegate CAP_NET_ADMIN to containers, service accounts, or restricted network management components. If only full trusted host root can manage nftables rules and the earlier nft_compat fix 9dbba7e is already present, this can be treated as ordinary Moderate and lower urgency.

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

Patch: netfilter: xt_nat: reject unsupported target families
Commit: 49abe564391411057a26a9a943c8e17867c3b9b4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=49abe564391411057a26a9a943c8e17867c3b9b4

Commit description:

xt_nat SNAT and DNAT target handlers assume IP-family conntrack state
is present and can dereference a NULL pointer when instantiated from an
unsupported family through nft_compat. A bridge-family compat rule can
therefore trigger a NULL-dereference in nf_nat_setup_info().

Reject non-IP families in xt_nat_checkentry() so unsupported targets
cannot be installed. Keep NFPROTO_INET allowed for valid inet NAT
compat users and leave the runtime fast path unchanged.

[ The crash was fixed via
  9dbba7e ("netfilter: nft_compat: ebtables emulation must reject non-bridge targets"),
  so this patch is no longer critical.
  Nevertheless, NAT is only relevant for ipv4/ipv6, so this extra
  family check is a good idea in any case. ]

Fixes: c7232c9 ("netfilter: add protocol independent NAT core")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:GPT-5.4
Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/xt_nat.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=49abe564391411057a26a9a943c8e17867c3b9b4

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

The original Linux kernel CVE announcement for CVE-2026-80664 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80664

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:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 3

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).