[CVE-2026-45841][MODERATE REGULAR] netfilter: nfnetlink_osf: fix divide-by-zero in OSF_WSS_MODULO [ Upstream

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

CVE: CVE-2026-45841
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nfnetlink_osf: fix divide-by-zero in OSF_WSS_MODULO [ Upstream
Commit: 8def8fbd23f40e945febe913d04b731012ce0082
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8def8fbd23f40e945febe913d04b731012ce0082
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45841
Analysis date: Wed, 27 May 2026 05:51:25 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A malformed nfnetlink OSF fingerprint with OSF_WSS_MODULO and zero WSS value can cause a divide-by-zero in the netfilter TCP SYN matching path, leading to a kernel panic after configuration by a CAP_NET_ADMIN capable actor.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-45841 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45841

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 REGULAR
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-2026-45841	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-369;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'nfnetlink_osf accepts an invalid OS fingerprint where OSF_WSS_MODULO is selected with a zero WSS value. When a later matching TCP SYN is processed, nf_osf_match_one computes the TCP window modulo that value and triggers a divide by zero in the kernel. For the CVSS the PR:L is used for the paranoid score because CAP_NET_ADMIN can be delegated to container root, reduced capability root, or a firewall management service account in some deployments. The bug is not a memory corruption primitive and does not indicate direct confidentiality or integrity impact. Impact is denial of service via kernel panic. The final crash can be triggered by packet processing after the bad fingerprint is installed, but practical exploitation still depends on access to the netfilter OSF control plane.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	REMOTE DANGER NETFILTER UAF KERNEL_PANIC_PLUS_UAF  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit shows a kernel divide-by-zero panic in `nf_osf_match_one()`.
* Availability impact realistic: +1. A matching TCP SYN can trigger a system-level kernel panic after the invalid OSF fingerprint is installed.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Creating the malicious OSF fingerprint via nfnetlink normally requires `CAP_NET_ADMIN`.
* Common packet-processing path: +1. The crash happens from the netfilter packet path during TCP SYN matching, although OSF itself is not a default/common match in many deployments.

Conservative total: 1.

Paranoid adjustment:

* Do not apply the full CAP penalty in delegated environments: effectively +2 compared with conservative. `CAP_NET_ADMIN` may be delegated to container root, reduced-capability root, or firewall management services, so the practical risk can be higher than full host-root-only.
* No memory corruption signal. This is divide-by-zero, not UAF, OOB write, double-free, or page-cache corruption.
* No confidentiality or integrity signal. The patch supports DoS only.

Paranoid total: 3.

## 3. Reachability analysis

The invalid fingerprint is installed through the nfnetlink OSF control plane, which normally requires `CAP_NET_ADMIN`. After that state exists, a matching TCP SYN packet reaches the netfilter OSF match path and can trigger the divide-by-zero panic. This is not independently remote exploitable by an unauthenticated network attacker unless the attacker can also influence OSF rule or fingerprint configuration, but the final crash is packet-triggered.

Namespaces and containers matter because `CAP_NET_ADMIN` is commonly delegated in containerized or service-managed firewall setups. In a strict host model this is PR:H-like and low priority. In a delegated network-administration model this becomes closer to PR:L practical risk.

The OSF xtables match is not a universal default path, but it is part of netfilter and can be reached through normal packet processing once configured.

## 4. Severity interpretation

This behaves like a DoS-focused netfilter control-plane validation bug, not an Important-class memory corruption issue. There is no evidence of privilege escalation, arbitrary write, UAF reclaim, sensitive data disclosure, or policy bypass. The reason it should not be completely ignored is that the resulting crash occurs in the packet path and CAP_NET_ADMIN delegation can make triggering practical in some environments.

Overall this is below Actionable Moderate by the custom score, but it is a reasonable manual-review borderline case because of delegated capability models and netfilter packet-path impact.

## 5. One-sentence report phrase

A malformed nfnetlink OSF fingerprint with OSF_WSS_MODULO and zero WSS value can cause a divide-by-zero in the netfilter TCP SYN matching path, leading to a kernel panic after configuration by a CAP_NET_ADMIN capable actor.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.

Reason: no memory corruption or privilege escalation is indicated, but the issue is in netfilter packet processing and practical severity depends on whether `CAP_NET_ADMIN` is delegated to less-trusted containers, services, or firewall-management components.

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

Patch: netfilter: nfnetlink_osf: fix divide-by-zero in OSF_WSS_MODULO [ Upstream
Commit: 8def8fbd23f40e945febe913d04b731012ce0082
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8def8fbd23f40e945febe913d04b731012ce0082

Changed files:
  net/netfilter/nfnetlink_osf.c
  net/netfilter/xt_osf.c
  net/ipv4/netfilter/ip_tables.c
  net/netfilter/core.c
  net/ipv4/ip_input.c
  include/linux/skbuff.h
  net/core/dev.c
  kernel/softirq.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=8def8fbd23f40e945febe913d04b731012ce0082

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

The original Linux kernel CVE announcement for CVE-2026-45841 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45841

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 3

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 REGULAR

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).