[CVE-2026-52998][MODERATE 7.0] netfilter: nfnetlink_osf: fix potential NULL dereference in ttl check [ Upstream

[CVE-2026-52998][MODERATE 7.0] netfilter: nfnetlink_osf: fix potential NULL dereference in ttl check [ Upstream

[AL-KERNEL]

CVE: CVE-2026-52998
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nfnetlink_osf: fix potential NULL dereference in ttl check [ Upstream
Commit: f4de0777e4554a7de19c920accde6319dd530782
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4de0777e4554a7de19c920accde6319dd530782
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52998
Analysis date: Wed, 24 Jun 2026 15:10:15 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A network packet reaching a configured nfnetlink_osf OS fingerprint rule can trigger a NULL pointer dereference in the TTL check when skb dev is missing, causing a kernel crash and remote DoS on affected netfilter configurations.

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

The original Linux kernel CVE announcement for CVE-2026-52998 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52998

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-2026-52998	MODERATE	CHECK	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-476;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'nfnetlink_osf can dereference a missing skb dev pointer while evaluating the OS fingerprint TTL check. A packet that reaches an OSF match rule on a path where skb dev is NULL can trigger a kernel NULL pointer dereference and crash the system. For the CVSS the PR:N is used because no attacker privileges are required when the vulnerable firewall rule is already reachable through network traffic. The issue is network reachable only on systems that load and use the passive OS fingerprint match in a suitable netfilter hook. Impact is denial of service via kernel crash. No confidentiality or integrity impact is assigned because this is a NULL pointer dereference without evidence of UAF, OOB access, or an arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE OOB NULLPTR SIMPLEFIX NETFILTER SKB HARDWARE KPANIC PACKET  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	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

Conservative signals:

* Remote reachable / network-triggerable: +2. A packet can reach the OSF match rule through the netfilter packet path if the rule is configured.
* Reliable kernel crash / strong DoS: +1. The bug is a NULL dereference of skb->dev and can plausibly crash the kernel.
* Common networking core path or packet-processing path: +1. This is netfilter packet processing, though the specific OSF match is not default.
* No memory corruption score. This is a NULL pointer dereference, not UAF, OOB write, double free, or type confusion.
* No C/I score. There is no evidence of information disclosure or integrity modification.

Paranoid additional signal:

* Availability impact realistic: +1. If a vulnerable OSF rule is deployed on a reachable hook where skb->dev can be NULL, repeated packets can cause a host-wide crash condition.

Not applied:

* Rare configuration penalty not applied. Netfilter is explicitly a packet-processing path that should not be heavily downgraded only because a specific rule is required.
* No privilege escalation signal. The patch does not show reclaim, overwrite, callback dispatch, refcount takeover, or attacker-controlled memory reuse.

## 3. Reachability analysis

An unauthenticated remote sender may trigger the issue if packets reach a configured passive OS fingerprinting rule using nfnetlink_osf / xt_osf in a hook where skb->dev is NULL. The attacker does not need local code execution once the vulnerable firewall rule is already present. Creating or changing such rules normally requires CAP_NET_ADMIN, but that is a deployment prerequisite rather than an attacker privilege requirement for packet-triggered exploitation. Containers and virtual switching increase plausibility because the commit explicitly mentions modern container and virtual networking environments as cases where the old local-subnet TTL assumption is unsafe.

## 4. Severity interpretation

This behaves like an actionable Moderate network-triggerable DoS, not an Important memory-corruption issue. The realistic impact is kernel crash through NULL pointer dereference. Theoretical escalation is not supported by the patch because there is no UAF, stale object reuse, writable corruption sink, or arbitrary write primitive. The issue should not be auto-closed because it is in a packet-processing path and can be triggered remotely when a vulnerable OSF rule is deployed.

## 5. One-sentence report phrase

A network packet reaching a configured nfnetlink_osf OS fingerprint rule can trigger a NULL pointer dereference in the TTL check when skb dev is missing, causing a kernel crash and remote DoS on affected netfilter configurations.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-triggerable packet-path DoS with configuration-dependent exposure. It is not memory corruption or LPE, but the remote reachability and netfilter context make it actionable enough for manual triage.

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

Patch: netfilter: nfnetlink_osf: fix potential NULL dereference in ttl check [ Upstream
Commit: f4de0777e4554a7de19c920accde6319dd530782
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4de0777e4554a7de19c920accde6319dd530782

Changed files:
  net/netfilter/nfnetlink_osf.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=f4de0777e4554a7de19c920accde6319dd530782

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

The original Linux kernel CVE announcement for CVE-2026-52998 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52998

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).