[CVE-2026-52988][MODERATE 7.0] netfilter: nf_tables: join hook list via splice_list_rcu() in commit phase [ Upstream

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

CVE: CVE-2026-52988
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nf_tables: join hook list via splice_list_rcu() in commit phase [ Upstream
Commit: 1346be9379639c30877083b12747d4eacb83c24f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1346be9379639c30877083b12747d4eacb83c24f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52988
Analysis date: Thu, 25 Jun 2026 04:57:26 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
nf_tables used non-RCU list splicing when publishing new basechain or flowtable hooks during commit, allowing concurrent RCU netlink dump traversal to observe an inconsistent hook list and potentially cause a local kernel crash or limited memory-safety impact.

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

The original Linux kernel CVE announcement for CVE-2026-52988 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52988

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: 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-2026-52988	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'nf_tables publishes newly added basechain or flowtable hooks during the commit phase while netlink dump code may traverse the hook list under RCU at the same time. Using a non RCU list splice can expose an inconsistent list state to concurrent RCU readers and may lead to a crash or other memory safety symptoms during ruleset update and dump races. For the CVSS the PR:L is used because reliable triggering requires a local process with nf_tables control plane access such as CAP_NET_ADMIN in a network namespace, but not necessarily full host root in all deployments. The issue is not directly network reachable through packet traffic. Impact is at least local denial of service via kernel crash or stalled traversal. For the paranoid score, limited confidentiality or integrity impact is kept because RCU list publication bugs can become stale pointer or invalid traversal candidates, but the patch does not show a concrete arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4) 	MAYBE	REMOTE SIMPLEFIX NETFILTER HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Real lifetime corruption / RCU lifetime issue: +1
  The patch changes `list_splice()` to `list_splice_rcu()` because the hook list can be traversed by RCU readers during concurrent netlink dump.
* Reliable kernel crash / strong DoS: +1
  Unsafe list publication during concurrent traversal can plausibly cause invalid traversal, warning, crash, or stalled traversal.
* Broad/common subsystem exposure: +1
  nf_tables is a widely deployed firewall subsystem.
* Hard or unreliable race / special timing required: -1
  Triggering requires overlap between ruleset commit and netlink dump traversal.
* Requires CAP_NET_ADMIN in typical deployments: -2
  Updating nf_tables chains or flowtables normally requires CAP_NET_ADMIN.

Paranoid signals:

* Weak/indirect corruption candidate: +1
  Non-RCU list publication can expose inconsistent list state to RCU readers, but the patch does not show a controlled overwrite or strong reclaim primitive.
* Real lifetime corruption / RCU lifetime issue: +1
  The bug is specifically about RCU-safe publication of list nodes.
* Reliable kernel crash / strong DoS: +1
  Invalid RCU traversal can realistically produce a kernel crash or stuck traversal.
* Broad/common subsystem exposure: +1
  nf_tables is common and security-sensitive.
* Confidentiality impact plausible: +1
  Limited C impact is defensible only as a paranoid consequence of invalid/stale traversal, not as a proven info leak.
* Integrity impact plausible: +1
  Limited I impact is defensible only as a paranoid memory-safety concern, not as a demonstrated write primitive.
* Hard or unreliable race / special timing required: -1
  The attack depends on a concurrent update/dump race.
* Reduced privilege model instead of full host-root: -1
  CAP_NET_ADMIN may be available inside network namespaces or delegated environments, so the full -2 admin penalty may be too strong for paranoid triage.

## 3. Reachability analysis

The bug is locally reachable through nf_tables control-plane operations, not through ordinary packet traffic. A realistic trigger requires one actor performing ruleset updates that add hooks to an existing netdev chain or flowtable while another path performs a netlink dump that traverses the hook list under RCU.

In typical host deployments this requires CAP_NET_ADMIN, so it is not a normal unprivileged local bug. However, namespace and container setups can reduce the practical privilege barrier when a process has CAP_NET_ADMIN inside a network namespace. The affected subsystem is common, but the exact vulnerable path is a race in commit/list publication rather than a default packet-processing path.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because the patch fixes an RCU publication bug in nf_tables hook lists. The conservative interpretation is local DoS from an unsafe concurrent list traversal. The paranoid interpretation treats it as an actionable memory-safety candidate because RCU list races can produce stale or inconsistent pointer traversal, but there is no demonstrated arbitrary write, UAF reclaim control, or privilege-escalation primitive in the patch.

Overall this fits Actionable Moderate rather than clear Important. Manual review is justified because nf_tables RCU/list lifetime bugs have historically been security-sensitive and can be underestimated from small patches.

## 5. One-sentence report phrase

nf_tables used non-RCU list splicing when publishing new basechain or flowtable hooks during commit, allowing concurrent RCU netlink dump traversal to observe an inconsistent hook list and potentially cause a local kernel crash or limited memory-safety impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an nf_tables RCU race involving list publication and concurrent traversal. Even though the demonstrated impact is most likely DoS and the trigger usually requires CAP_NET_ADMIN, the memory-lifetime pattern is security-sensitive enough to avoid auto-close.

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

Patch: netfilter: nf_tables: join hook list via splice_list_rcu() in commit phase [ Upstream
Commit: 1346be9379639c30877083b12747d4eacb83c24f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1346be9379639c30877083b12747d4eacb83c24f

Changed files:
  net/netfilter/nf_tables_api.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=1346be9379639c30877083b12747d4eacb83c24f

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

The original Linux kernel CVE announcement for CVE-2026-52988 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52988

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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  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).