[CVE-2026-74625][MODERATE 7.0] netfilter: bridge: release template ct on non-IP path

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

CVE: CVE-2026-74625
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: bridge: release template ct on non-IP path
Commit: fc90df37540627d092af770215fb4b7befe9409b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc90df37540627d092af770215fb4b7befe9409b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74625
Analysis date: Sat, 22 Aug 2026 17:26:23 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A bridge netfilter conntrack template reference leak on the non-IP path can be triggered by network-adjacent L2 traffic in affected nftables bridge configurations, potentially causing slab exhaustion and host DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74625 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74625

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 7.0
Manual review required: NO

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-2026-74625	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'Bridge netfilter can leak conntrack template references when a bridge nftables ct zone set rule attaches a template to an skb and nf_ct_bridge_pre later handles a non IP or non IPv6 EtherType. The non IP path overwrites skb _nfct with IP_CT_UNTRACKED without dropping the existing reference, making per CPU and temporary templates unreachable and allowing slab memory exhaustion. For the CVSS the PR:N is used for the paranoid score because an attacker only needs L2 reachability to send non IP frames to an affected bridge and does not need a local account on the victim. The issue is network adjacent rather than Internet routable because non IP bridge frames are normally limited to the local broadcast domain. Impact is denial of service through resource exhaustion. No UAF, OOB access, or privilege escalation primitive is indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX NETFILTER LEAK SKB IPV6  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The trigger is packet based and can be exercised by sending non IPv4 / non IPv6 Ethernet frames into an affected bridge path. This is network adjacent rather than Internet routable.
* Reliable kernel crash / strong DoS: +1
  The commit explicitly says leaked conntrack templates can accumulate until the host runs out of slab.
* Common networking core path or packet-processing path: +1
  The bug is in bridge netfilter packet processing, not in an obscure one-shot control path.
* Rare AND difficult-to-reach subsystem/configuration: -1
  Triggering requires a bridge nftables conntrack zone rule that attaches a template before `nf_ct_bridge_pre()` handles a non-IP frame.

Paranoid additional signal:

* Availability impact realistic: +1
  Sustained traffic can plausibly create system-wide slab exhaustion, so the availability impact can exceed a minor leak.

Not awarded:

* Generic memory corruption: +0
  The patch shows a leaked reference, not UAF, OOB write, double free, type confusion, or arbitrary write.
* Privilege escalation plausible: +0
  No reclaim, overwrite, callback control, or object confusion primitive is supported.
* Confidentiality / Integrity impact: +0
  The demonstrated impact is resource exhaustion only.

## 3. Reachability analysis

An attacker who can send L2 frames into the affected bridge can trigger the vulnerable path if the host has bridge netfilter conntrack configured with an nftables `ct zone set` rule that attaches a conntrack template. No local account on the victim is required for the packet sender, so the trigger side is effectively PR:N, but exposure is limited to the local broadcast domain or adjacent virtual network. The path is not generally Internet-routable and depends on a specific firewall or bridge configuration. Containers or VMs connected to the same bridge may be relevant if they can emit arbitrary non-IP EtherTypes.

Call-site confidence: high. The patch directly shows `nf_ct_bridge_pre()` changing the skb conntrack state on the non-IP path.

## 4. Severity interpretation

This behaves like an actionable resource-exhaustion Moderate rather than an Important memory-corruption issue. The practical risk is stronger than an ordinary tiny leak because the leak is in a packet-processing path and can be driven by network-adjacent traffic until slab exhaustion. However, the patch context supports DoS only and does not justify treating it as LPE, information disclosure, or memory corruption.

## 5. One-sentence report phrase

A bridge netfilter conntrack template reference leak on the non-IP path can be triggered by network-adjacent L2 traffic in affected nftables bridge configurations, potentially causing slab exhaustion and host DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network-adjacent with PR:N and can cause host-wide slab exhaustion, but it remains a resource leak without evidence of memory corruption or privilege escalation.

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

Patch: netfilter: bridge: release template ct on non-IP path
Commit: fc90df37540627d092af770215fb4b7befe9409b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc90df37540627d092af770215fb4b7befe9409b

Commit description:

A bridge nftables ct zone set rule can attach a conntrack template to
an skb before nf_ct_bridge_pre() sees it. For non-IPv4 and non-IPv6
EtherTypes, nf_ct_bridge_pre() currently overwrites skb->_nfct with
IP_CT_UNTRACKED without releasing the existing template reference.

That makes the per-cpu template, and any temporary templates allocated
for concurrent use, unreachable and leaks memory until the host runs out
of slab.

Reset the skb conntrack state before marking the frame untracked so the
existing template reference is dropped on the non-IP path.

Fixes: 3c171f4 ("netfilter: bridge: add connection tracking system")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/netfilter/nf_conntrack_bridge.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=fc90df37540627d092af770215fb4b7befe9409b

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

The original Linux kernel CVE announcement for CVE-2026-74625 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74625

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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  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: 4
  Paranoid ActionableScore: 5

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 7.0

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).