[CVE-2026-53267][IMPORTANT] netfilter: nft_ct: bail out on template ct in get eval [ Upstream

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

CVE: CVE-2026-53267
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nft_ct: bail out on template ct in get eval [ Upstream
Commit: af80f78ce984649e1698b841cd33f4fa505ad828
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af80f78ce984649e1698b841cd33f4fa505ad828
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53267
Analysis date: Thu, 25 Jun 2026 11:06:13 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted nf_tables conntrack rule can cause nft_ct_get_eval() to treat a template conntrack object as a real entry and perform an out-of-bounds stack write past struct nft_regs, making this a local CAP_NET_ADMIN or namespace-delegated memory corruption issue with plausible privilege escalation impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53267 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53267

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-2026-53267	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-121;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'nft_ct_get_eval() can treat a per CPU template conntrack object as a real conntrack entry after a rule first attaches a template ct and then reads ct original address fields from the same skb. In that state nf_ct_l3num(ct) can be 0 and the old code may take the 16 byte address copy path even when the destination register was validated for a shorter value. With dreg set near the end of struct nft_regs, this becomes an out of bounds write past the nft_regs area on the kernel stack. For the CVSS the PR:L is used because reliable triggering requires local ability to install crafted nftables rules, typically CAP_NET_ADMIN or delegated network administration in a namespace, but not full host root in all deployments. The issue is not scored as AV:N because packet traffic only triggers the already installed local rule. Impact is at least local denial of service and the memory corruption primitive makes privilege escalation plausible, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE OOB DANGER NETFILTER SYZBOT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged or delegated trigger: +1
  Reliable triggering requires installing crafted nftables rules. This usually needs CAP_NET_ADMIN, but that capability can be available in user or network namespace scenarios and does not always mean full host root.

* Privilege escalation plausible: +2 in paranoid score, +1 in conservative score
  The patch describes a stack out-of-bounds write past struct nft_regs. The write content is constrained, but kernel stack corruption is a credible LPE concern and should not be auto-closed.

* Generic memory corruption / OOB write: +2 paranoid, +1 conservative
  The bug is an out-of-bounds memcpy into or past nft register storage on the kernel stack. Conservative scoring treats it as constrained because the copied tuple bytes may be zeroed or kernel-derived.

* Reliable kernel crash / strong DoS: +1
  Stack corruption in the packet evaluation path can realistically crash the kernel.

* Common packet-processing / netfilter path: +1
  The vulnerable code is in nf_tables conntrack expression evaluation, a common firewall packet-processing path.

* Broad subsystem exposure: +1
  nf_tables and conntrack are widely deployed, even though the crafted rule setup is control-plane gated.

* Capability requirement: -1 conservative adjustment
  CAP_NET_ADMIN is normally required, but full -2 is too harsh because namespace delegation and containerized network administration can reduce the practical privilege barrier.

## 3. Reachability analysis

The bug is not directly remotely reachable as AV:N because a remote packet alone cannot create the vulnerable nftables rule. A local actor must first install a crafted nf_tables rule that attaches a template conntrack object and then reads conntrack address fields from the same skb. After that, packet traffic can exercise the vulnerable evaluation path.

In typical host deployments this requires CAP_NET_ADMIN. In container or namespace deployments, CAP_NET_ADMIN may be delegated to a less trusted local actor, so PR:L is the more useful practical interpretation. The affected subsystem is common and not a rare hardware path.

## 4. Severity interpretation

This is not an ordinary Moderate DoS-only issue. The patch and commit message explicitly describe a 16-byte memcpy path that can overflow past struct nft_regs on the kernel stack. The realistic exploitation evidence is still weaker than an arbitrary write because the copied data appears constrained, but the primitive is real kernel stack memory corruption in a common netfilter path.

Conservatively this is Actionable Moderate and should not be auto-closed. Paranoid triage places it as a Strong Important candidate because privilege escalation is plausible from stack OOB corruption even if not demonstrated.

## 5. One-sentence report phrase

A crafted nf_tables conntrack rule can cause nft_ct_get_eval() to treat a template conntrack object as a real entry and perform an out-of-bounds stack write past struct nft_regs, making this a local CAP_NET_ADMIN or namespace-delegated memory corruption issue with plausible privilege escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch shows a concrete stack OOB write in kernel packet evaluation, not merely a warning or validation cleanup. The constrained payload lowers confidence in direct exploitation, but the memory corruption primitive is strong enough to require manual security review.

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

Patch: netfilter: nft_ct: bail out on template ct in get eval [ Upstream
Commit: af80f78ce984649e1698b841cd33f4fa505ad828
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af80f78ce984649e1698b841cd33f4fa505ad828

Changed files:
  net/netfilter/nft_ct.c
  net/netfilter/nft_ct_fast.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=af80f78ce984649e1698b841cd33f4fa505ad828

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

The original Linux kernel CVE announcement for CVE-2026-53267 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53267

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  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).