[CVE-2026-64414][MODERATE 7.0] netfilter: handle unreadable frags

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

CVE: CVE-2026-64414
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: handle unreadable frags
Commit: 3b13e7635795394705920cca1e1db7e4ca2e334b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b13e7635795394705920cca1e1db7e4ca2e334b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64414
Analysis date: Sat, 25 Jul 2026 08:45:05 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-64414 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64414

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-2026-64414	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-617;CWE-754;*CWE-20;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'Unreadable skb fragments can reach netfilter paths that still assume skb_copy_bits can access the full packet data. When such a packet is processed by xt_u32, NFQUEUE, NFLOG, or IPv6 conntrack reassembly, skb_copy_bits may fail and the old code can hit a reachable BUG path, causing a kernel panic. For the CVSS the PR:N is used because a remote sender does not need a local account when the affected packet path is exposed and the system is configured to produce unreadable skb fragments. The issue is network reachable in deployments using devmem TCP or similar unreadable fragment support together with affected netfilter features. Impact is denial of service via kernel crash. No concrete UAF, OOB write, information leak, or privilege escalation primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64414 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 65249feb6b3df9e17bab5911ee56fa7b0971e231 YES  NO NO unknown 	MAYBE	REMOTE NETFILTER NETWORK SKB LOG LINUS TEST IPV6 KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

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

2. Signal breakdown
* Remote reachable / network-triggerable: +2. A remote sender can plausibly supply packets that enter affected netfilter paths when the target is configured with unreadable skb fragments such as devmem TCP.
* Reliable kernel crash / strong DoS: +1. The old code can reach BUG() after skb_copy_bits() fails, causing a kernel panic.
* Common networking core path or packet-processing path: +1. The affected code is in netfilter, NFQUEUE, NFLOG, xt_u32, and IPv6 conntrack reassembly packet processing.
* Availability impact realistic: +1 in paranoid scoring only. On an affected configured system, the impact is host-wide kernel crash, but this overlaps with the strong DoS signal, so it is not counted in the conservative score.
* No memory corruption score. The patch shows safe failure from skb_copy_bits() and reachable BUG(), not UAF, OOB write, double free, type confusion, or attacker-controlled overwrite.
* No confidentiality or integrity score. There is no concrete leak or write primitive.
* No rare configuration penalty in the main score. devmem TCP and unreadable frags are special, but the vulnerable paths are netfilter packet paths, and the guidance says not to heavily penalize core networking or netfilter solely for runtime configuration dependence.

3. Reachability analysis
A remote peer may trigger the crash if packets are processed as skbs with unreadable fragments and traverse one of the affected netfilter paths. Practical exposure depends on devmem TCP or similar unreadable-fragment support being enabled and on rules or logging or queueing paths that inspect packet data beyond the linear area. No local account is required for the packet sender in that deployment, so PR:N is reasonable for CVSS-style reachability. Containers and namespaces mainly affect who can configure NFQUEUE, NFLOG, xt_u32, or devmem TCP, but the actual triggering packet can be remote once the path is configured.

4. Severity interpretation
This behaves like an actionable Moderate network-triggerable DoS, not an Important memory corruption issue. The realistic impact is kernel panic through BUG() on a configured system. Theoretical escalation is not supported because the patch does not expose a stale pointer, writable corruption sink, object reuse, or disclosure path. The reason it should not be auto-closed is the combination of network packet reachability, netfilter involvement, and host-wide crash potential.

5. One-sentence report phrase
Unreadable skb fragments in netfilter packet-processing paths can make skb_copy_bits() fail and reach BUG() in xt_u32, NFQUEUE, NFLOG, or IPv6 conntrack reassembly, allowing a remote DoS on systems using devmem TCP or similar unreadable-fragment configurations.

6. Manual review recommendation
MANUAL CHECK REQUIRED. The bug is likely DoS-only, but it is network reachable in affected configurations and touches common netfilter packet paths, so exposure and deployment assumptions should be verified manually.

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

Patch: netfilter: handle unreadable frags
Commit: 3b13e7635795394705920cca1e1db7e4ca2e334b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b13e7635795394705920cca1e1db7e4ca2e334b

Commit description:

sashiko reports:
 When an skb with unreadable fragments (such as from devmem TCP, where
 skb_frags_readable(skb) returns false) is processed by the u32 module,
 skb_copy_bits() will safely return a negative error code [..]

xt_u32: bail out with hotdrop in this case.
gather_frags: return -1, just as if we had no fragment header.
nfnetlink_queue: restrict to the linear part.
nfnetlink_log: restrict to the linear part.

v2:
 - skb_zerocopy helpers don't copy readable flag, i.e. nfnetlink_queue
 is broken too
 xt_u32 shouldn't return true if hotdrop was set.

Fixes: 65249fe ("net: add support for skbs with unreadable frags")
Cc: stable@vger.kernel.org
Acked-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/netfilter/nf_conntrack_reasm.c
  net/netfilter/nfnetlink_log.c
  net/netfilter/nfnetlink_queue.c
  net/netfilter/xt_u32.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=3b13e7635795394705920cca1e1db7e4ca2e334b

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

The original Linux kernel CVE announcement for CVE-2026-64414 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64414

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: 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).