[CVE-2026-72436][MODERATE 7.0] netfilter: ipset: Don't use test_bit() in lockless RCU readers in hash types [ Upstream

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

CVE: CVE-2026-72436
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: ipset: Don't use test_bit() in lockless RCU readers in hash types [ Upstream
Commit: c107233d2ff4fd7cef5d02f9124b99194957a710
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c107233d2ff4fd7cef5d02f9124b99194957a710
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72436
Analysis date: Sun, 16 Aug 2026 06:01:42 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Lockless RCU readers in netfilter ipset hash types used relaxed test_bit() while add, delete, or gc could run in parallel, allowing packet matching to observe inconsistent element state and potentially cause limited filtering integrity impact under race conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72436 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72436

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-2026-72436	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';CWE-362;CWE-662;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.6';DESCR 'Lockless RCU readers in netfilter ipset hash types used relaxed test_bit() while the same hash data may be updated by add, delete, or garbage collection paths. This can allow a reader to observe the used bit without the required acquire ordering and then read hash element data in an inconsistent state. For the CVSS the PR:N is used in the paranoid score because packet matching can exercise the reader path remotely when an ipset backed firewall rule is reachable, while the required concurrent update or gc makes exploitation timing dependent. The issue is network reachable only in configurations where ipset is used in packet filtering and the set is modified or garbage collected in parallel. Impact is primarily a limited filtering integrity risk and possible denial of service rather than proven privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-72436 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE LOCK SIMPLEFIX NETFILTER TEST  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	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
  `mtype_test()` and `mtype_test_cidrs()` are used from netfilter/ipset packet matching paths when an ipset-backed firewall rule is active.
* Integrity impact plausible: +1
  Missing acquire ordering can allow packet-path readers to observe inconsistent hash element state, plausibly causing incorrect match decisions or limited filtering bypass.
* Common networking core path or packet-processing path: +1
  The affected reader path can run during netfilter packet processing.
* Hard or unreliable race / special timing required: -1
  Exploitation requires concurrent reader activity with add/delete/gc or resize/list activity, and depends on memory-ordering timing.

Paranoid additional signal:

* Memory corruption, weak/indirect corruption candidate: +1
  This is an RCU synchronization and memory-ordering bug where readers may consume element data without the intended acquire barrier. The patch does not show UAF, OOB write, arbitrary write, or attacker-controlled reclaim, so this remains weak and does not justify Important-class scoring by itself.

Not awarded:

* No strong generic memory corruption bonus.
  The patch only replaces `test_bit()` with `test_bit_acquire()` and does not demonstrate UAF, double-free, OOB write, type confusion, or arbitrary write.
* No privilege escalation bonus.
  There is no concrete LPE primitive.
* No confidentiality bonus in the conservative score.
  The likely consequence is inconsistent filtering state, not user-visible kernel data disclosure.
* No reliable kernel crash bonus.
  The commit does not describe panic, Oops, BUG, or reproducible host crash.

Call-site confidence: medium. The patch includes affected helper locations and the known packet-match reader functions, but not the full caller chain.

## 3. Reachability analysis

A remote host may trigger the reader side if packets traverse a firewall rule using an affected ipset hash type. However, the problematic condition also requires concurrent set mutation or garbage collection, such as add, delete, gc, resize, list, or related maintenance activity. Ordinary remote traffic alone is not enough unless the target system is actively modifying or expiring the set in parallel.

Local privileges for modifying ipsets are normally administrative, usually `CAP_NET_ADMIN`. But the packet matching reader path itself can be exercised by remote traffic without local credentials when the firewall rule is exposed. Namespaces or containers may matter if an untrusted tenant can manage ipsets in a delegated network namespace, but typical host firewall management remains privileged.

The path is configuration-dependent because ipset hash types must be used by netfilter rules. It is still a meaningful networking exposure and should not receive a rare-subsystem penalty.

## 4. Severity interpretation

This behaves like an actionable Moderate rather than ordinary Low/Moderate because it is an RCU memory-ordering bug in a packet filtering path and may cause incorrect security decisions under concurrent updates. It is not a strong Important candidate because the patch does not establish a concrete UAF, write primitive, type confusion, information leak, or privilege escalation path.

The theoretical concern is inconsistent or partially ordered element reads. The realistic evidence supports limited integrity impact and possible availability degradation, not proven kernel compromise.

## 5. One-sentence report phrase

Lockless RCU readers in netfilter ipset hash types used relaxed test_bit() while add, delete, or gc could run in parallel, allowing packet matching to observe inconsistent element state and potentially cause limited filtering integrity impact under race conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a network-adjacent RCU synchronization bug in a packet-filtering path. Even though no strong memory corruption primitive is shown, the combination of remote packet reachability, concurrent updates, and possible firewall decision inconsistency makes it unsuitable for automatic closure.

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

Patch: netfilter: ipset: Don't use test_bit() in lockless RCU readers in hash types [ Upstream
Commit: c107233d2ff4fd7cef5d02f9124b99194957a710
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c107233d2ff4fd7cef5d02f9124b99194957a710

Changed files:
  net/netfilter/ipset/ip_set_hash_gen.h

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=c107233d2ff4fd7cef5d02f9124b99194957a710

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

The original Linux kernel CVE announcement for CVE-2026-72436 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72436

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:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.6

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: YES
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).