[CVE-2026-74660][MODERATE 7.0] netfilter: ebt_nflog: pin the NFLOG backend

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

CVE: CVE-2026-74660
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: ebt_nflog: pin the NFLOG backend
Commit: 3bcce49d617c593c7606083bfdb464a1761fa68d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3bcce49d617c593c7606083bfdb464a1761fa68d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74660
Analysis date: Sat, 22 Aug 2026 14:27:51 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
`ebt_nflog can keep calling the NFLOG backend without holding a module reference, allowing a race with nfnetlink_log unload that frees per-net state while the packet path still dereferences it, causing a slab use-after-free and likely kernel crash.`

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

The original Linux kernel CVE announcement for CVE-2026-74660 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74660

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-2026-74660	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'ebt_nflog can call the NFLOG backend without holding a module reference, so an ebtables NFLOG rule may remain callable while nfnetlink_log is being unloaded. During this race the per net nfnetlink_log state can be freed by module teardown while the packet path still reaches nfulnl_log_packet() and instance_lookup_get_rcu(), producing a slab use after free. For the CVSS the PR:L is used for the paranoid score because some environments delegate netfilter control or module lifecycle to service accounts or reduced capability root, while a stricter base score uses PR:H. The issue is not directly network reachable because packet traffic alone does not unload the backend, but network or local packets can hit the vulnerable rule during the local teardown window. Impact is at least local denial of service via kernel crash and in the worst defensible case may include confidentiality or integrity impact due to kernel use after free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	READ DANGER NETFILTER UAF LOG HARDWARE KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1. The patch and KASAN trace show a real slab use after free in `instance_lookup_get_rcu()`, but only a stale read is demonstrated.
* Real lifetime corruption: +1. The root cause is missing module/backend lifetime pinning while an ebtables NFLOG rule can still call into `nfnetlink_log`.
* Reliable kernel crash / strong DoS: +1. KASAN reports slab UAF in the packet path, and non-KASAN kernels may Oops or crash.
* Common packet-processing path: +1. The vulnerable call path is under netfilter bridge packet processing through `ebt_do_table()` and `ebt_nflog_tg()`.
* Broad/default/common subsystem: +1. Netfilter is common, although this specific ebtables NFLOG backend combination is more configuration dependent.
* Availability impact realistic: +1. The failure is in kernel packet processing and can become host-wide DoS once the race is hit.
* Hard race / special timing: -1. Triggering requires overlap between `nfnetlink_log` teardown and packet traversal of an existing ebtables NFLOG rule.
* Privilege requirement refinement: -1. Typical setup requires privileged netfilter rule control and module lifecycle influence, but delegated net admin or reduced-capability service contexts can make this less than full host-root risk.

Paranoid additions / interpretation:

* Confidentiality impact plausible: +1. Confirmed UAF can expose stale kernel memory in worst-case analysis, although no direct disclosure primitive is demonstrated.
* Integrity impact plausible: +1. Kernel UAF in a packet path can theoretically become more than DoS if object reuse is controllable, but this is not proven here.
* Paranoid score is capped at 5 under the race-UAF downgrade rule because the patch does not show attacker-controlled reclaim, write-after-free, callback dispatch, type confusion, or refcount takeover.

## 3. Reachability analysis

The most realistic trigger requires local control over ebtables or bridge netfilter configuration and a concurrent unload or teardown of `nfnetlink_log`. In ordinary host deployments this usually means root or CAP_NET_ADMIN plus module-management influence, so it is not a simple unprivileged local bug.

Namespaces and containers matter because some environments delegate netfilter control or run reduced-capability root/service accounts. In those cases, treating the issue as PR:L for paranoid triage is reasonable, even though full reliable triggering may still require module teardown coordination.

This is not directly remote reachable. Network or local packets can traverse the vulnerable rule, but packet traffic alone does not unload the NFLOG backend. Realistic exploitation requires a local control-plane race plus traffic hitting the rule.

Call-site confidence: high. The commit provides the concrete interleaving, the affected call path, and a KASAN UAF trace.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a confirmed kernel UAF in a packet-processing path, not merely a warning, leak, or validation cleanup. Realistically, the demonstrated impact is local privileged or semi-privileged denial of service through a teardown race.

Theoretical privilege escalation is not strongly demonstrated. There is no shown attacker-controlled reclaim, overwrite, callback control, or arbitrary read/write primitive. However, because the bug is a real lifetime UAF in netfilter packet processing, it should not be auto-closed without manual review.

## 5. One-sentence report phrase

`ebt_nflog can keep calling the NFLOG backend without holding a module reference, allowing a race with nfnetlink_log unload that frees per-net state while the packet path still dereferences it, causing a slab use-after-free and likely kernel crash.`

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: confirmed race-based UAF in a netfilter packet path with realistic DoS and a defensible but unproven worst-case memory-corruption concern.

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

Patch: netfilter: ebt_nflog: pin the NFLOG backend
Commit: 3bcce49d617c593c7606083bfdb464a1761fa68d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3bcce49d617c593c7606083bfdb464a1761fa68d

Commit description:

nf_log_unregister() runs after the per-net teardown so its final RCU
grace period also drains readers that obtained the logger from a per-net
binding.  However, ebt_nflog passes an explicit ULOG log type to
nf_log_packet() without holding a reference on the selected logger module,
unlike the xt_NFLOG and nft_log frontends.

An ebtables nflog rule can therefore remain callable while nfnetlink_log
is unloaded.  The resulting interleaving is:

  CPU 0                               CPU 1
  nfnetlink_log_fini()
    unregister_pernet_subsys()
      kfree(nfnl_log_pernet(net))
                                      ebt_nflog_tg()
                                        nf_log_packet()
                                          nfulnl_log_packet()
                                            instance_lookup_get_rcu()

The global ULOG logger is still registered at this point, so CPU 1
dereferences the per-net state after CPU 0 has freed it.  KASAN reported:

  BUG: KASAN: slab-use-after-free in instance_lookup_get_rcu
  Read of size 8 at addr ff110001052e6210 by task poc/92
  Call Trace:
   instance_lookup_get_rcu+0x1ce/0x1f0 [nfnetlink_log]
   nfulnl_log_packet+0x248/0x2fb0 [nfnetlink_log]
   nf_log_packet+0x204/0x300
   ebt_nflog_tg+0x351/0x550
   ebt_do_table+0xedf/0x22b0
  Allocated by task 90:
   __kmalloc_noprof+0x186/0x470
   ops_init+0x6d/0x420
   register_pernet_operations+0x2f6/0x670
   register_pernet_subsys+0x23/0x40
  Freed by task 93:
   kfree+0x131/0x3c0
   ops_undo_list+0x3e3/0x700
   unregister_pernet_operations+0x232/0x490
   unregister_pernet_subsys+0x1c/0x30
   nfnetlink_log_fini+0x34/0x450 [nfnetlink_log]

Acquire the ULOG logger module reference when an ebt_nflog rule is
validated and release it when the rule is destroyed.  Request the NFLOG
backend for legacy callers when needed, matching xt_NFLOG.  This prevents
module teardown until all ebt_nflog rules have stopped using the logger.

Fixes: c83fa19 ("netfilter: nf_log: don't call synchronize_rcu in nf_log_unset")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/netfilter/ebt_nflog.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=3bcce49d617c593c7606083bfdb464a1761fa68d

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

The original Linux kernel CVE announcement for CVE-2026-74660 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74660

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:H/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:H/I:H/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: 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).