[CVE-2026-74492][MODERATE 7.0] netfilter: ipset: do not update comments from kernel-side hash adds

[CVE-2026-74492][MODERATE 7.0] netfilter: ipset: do not update comments from kernel-side hash adds

[AL-KERNEL]

CVE: CVE-2026-74492
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: ipset: do not update comments from kernel-side hash adds
Commit: 661ff9c0cfbe07f8eed920dde9f7781491738207
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=661ff9c0cfbe07f8eed920dde9f7781491738207
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74492
Analysis date: Sat, 15 Aug 2026 09:23:32 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A packet-triggerable ipset comment lifetime race during hash resize can leave a replacement-table entry with a stale comment pointer, causing a UAF dereference during backlog replay and a kernel crash, with limited memory-corruption concern requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-74492 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74492

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
======================================================================

CVE-2026-74492	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:H';*CWE-416;CWE-362;CWE-825;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An ipset comment UAF can occur during hash resize because mtype_resize copies ip_set_comment_rcu pointers rather than comment objects. During the race window, packet-side xt_SET add-set exist updates may hit the old published table and free comment storage that is also referenced by the replacement table. A later backlog replay on the new table can call ip_set_init_comment and strlen on the stale pointer, causing a kernel crash and possible limited memory corruption impact. For the CVSS the PR:N because a remote packet sender can trigger packet-side set updates once a vulnerable ipset with comments and an xt_SET add-set exist rule is already configured. The issue is network reachable through packet processing, but exploitation requires a specific firewall configuration and a resize race, so AC:H is appropriate. Impact is at least denial of service via kernel crash. In the paranoid score, limited confidentiality or integrity impact is kept for manual-review sensitivity because the primitive is a UAF stale pointer dereference, but no strong arbitrary write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-74492 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE SIMPLEFIX NETFILTER KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

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

Conservative signals:

* Remote reachable / network-triggerable: +2
  Packet-side `xt_SET --add-set --exist` updates can be triggered by network traffic once the vulnerable ipset and rule are configured.

* Memory corruption, weak/indirect corruption candidate: +1
  The bug creates a stale `ip_set_comment_rcu` pointer and later dereferences it through `strlen()`. This is a UAF dereference, but no arbitrary write, callback control, type confusion, or attacker-controlled reclaim is shown.

* Real lifetime corruption: +1
  The old and replacement hash-table entries temporarily share the same comment pointer. Packet-side update can free the shared object while the replacement entry still references it.

* Reliable kernel crash / strong DoS: +1
  The stale pointer is later dereferenced during backlog replay, which can cause a kernel crash.

* Common networking core path or packet-processing path: +1
  The affected path is netfilter/ipset packet-side rule processing.

* Hard or unreliable race / special timing required: -1
  Exploitation requires hitting the resize window before table swap and backlog replay.

Conservative total: 5

Paranoid additional signals:

* Confidentiality impact plausible: +1
  A stale pointer dereference in `strlen()` is primarily an invalid read, but limited disclosure or secondary effects cannot be fully excluded without deeper allocator and reclaim analysis.

* Integrity impact plausible: +1
  Integrity impact is not demonstrated, but limited memory corruption concern is defensible for manual triage because this is a real UAF lifetime bug in packet processing.

Paranoid total: 7

Not awarded:

* Strong LPE plausibility: +0
  No controlled reclaim, overwrite, callback dispatch, type confusion, refcount takeover, or arbitrary write is shown.

* Dirty-Pipe-like or COW violation: +0
  No shared page, page cache, file-backed ownership, or COW boundary violation is involved.

* Requires admin/root/CAP_NET_ADMIN: 0
  Admin privileges are needed to create the vulnerable ipset/rule configuration, but the trigger can be a remote packet sender once that configuration exists.

## 3. Reachability analysis

The bug is reachable through netfilter ipset packet-side updates when a system uses an ipset with comments and an `xt_SET --add-set --exist` rule. A remote sender can potentially trigger the packet-side add path if traffic reaches a rule that performs set updates. The vulnerable condition also requires a concurrent hash resize and the narrow window where the old table is still published while the replacement table already shares copied comment pointers.

Namespaces and containers matter mainly for configuration. Creating or modifying ipsets normally requires `CAP_NET_ADMIN`, but a remote attacker does not need that capability if the firewall is already configured in the vulnerable way. This is not a default exposure for every Linux host, but it is still a realistic packet-processing path in deployments using ipset-based dynamic firewall rules.

## 4. Severity interpretation

This is more than an ordinary Moderate issue because it is a real lifetime bug in a network packet-processing path and can be remotely triggered under a plausible firewall configuration. The conservative interpretation is Actionable Moderate because the demonstrated primitive is UAF dereference leading to DoS with a hard race.

The paranoid interpretation reaches Strong Important candidate because the bug class is UAF in netfilter packet processing and the trigger may be remote. However, the evidence supports only stale pointer dereference and crash with possible limited C/I concern, not a strong privilege-escalation primitive.

## 5. One-sentence report phrase

A packet-triggerable ipset comment lifetime race during hash resize can leave a replacement-table entry with a stale comment pointer, causing a UAF dereference during backlog replay and a kernel crash, with limited memory-corruption concern requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable netfilter packet-processing UAF candidate with a real stale pointer lifetime bug. Even though the demonstrated impact is mainly DoS and the race is hard, the combination of packet-side trigger, UAF dereference, and netfilter exposure should not be auto-closed.

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

Patch: netfilter: ipset: do not update comments from kernel-side hash adds
Commit: 661ff9c0cfbe07f8eed920dde9f7781491738207
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=661ff9c0cfbe07f8eed920dde9f7781491738207

Commit description:

mtype_resize() copies comment pointers with memcpy(), not the comment
objects themselves. During the window after an entry has been copied but
before the table swap and backlog replay, the old table is still
published for packet-side updates while the replacement-table entry
already holds the same ip_set_comment_rcu pointer.

If xt_SET --add-set ... --exist hits that old entry in this window,
mtype_add() calls ip_set_init_comment() even though packet-side adds
carry no comment payload. That call frees the shared comment through the
old entry, so the replacement-table entry now holds a stale pointer.
When the queued add is replayed on the new table, mtype_add() calls
ip_set_init_comment() again and strlen() dereferences the stale pointer.

Fix this in mtype_add() by skipping ip_set_init_comment() when
ext->target marks a packet-side add. Userspace adds still update
comments, while packet-side adds can no longer free comment storage
shared with a resize copy.

Fixes: f66ee04 ("netfilter: ipset: Fix "INFO: rcu detected stall in hash_xxx" reports")
Cc: stable@vger.kernel.org
Signed-off-by: David Lee <david.lee@trailofbits.com>
Assisted-by: Codex:gpt-5.5
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=661ff9c0cfbe07f8eed920dde9f7781491738207

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

The original Linux kernel CVE announcement for CVE-2026-74492 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74492

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:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7

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: 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).