[CVE-2026-53218][MODERATE 7.0] netfilter: nft_exthdr: fix register tracking for F_PRESENT flag [ Upstream

[CVE-2026-53218][MODERATE 7.0] netfilter: nft_exthdr: fix register tracking for F_PRESENT flag [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53218
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nft_exthdr: fix register tracking for F_PRESENT flag [ Upstream
Commit: 8738b1b6d0e639ca1fc0f61516afd3557ac4ecc6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8738b1b6d0e639ca1fc0f61516afd3557ac4ecc6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53218
Analysis date: Thu, 25 Jun 2026 07:58:14 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
`nft_exthdr F_PRESENT register tracking can mark more bytes as initialized than the eval path writes, allowing a CAP_NET_ADMIN or delegated netns attacker to expose uninitialized nft_regs stack data through later nftables expressions.`

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

The original Linux kernel CVE announcement for CVE-2026-53218 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53218

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-53218	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:H/I:L/A:N';*CWE-457;CWE-908;*CWE-200;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '6.1';DESCR 'nft_exthdr_init() allowed userspace to request F_PRESENT with a length larger than 1, and nft_parse_register_store() then marked that full length as initialized in the nft register tracking bitmap. The evaluation path for F_PRESENT only writes a boolean sized value, usually 1 byte and in one error path 4 bytes, so the remaining bytes can keep uninitialized nft_regs stack data and later expressions may treat them as valid. For the CVSS the PR:L is used because triggering requires a local process with nftables rule update capability such as CAP_NET_ADMIN in a network namespace or a delegated firewall management context, not full host root. The issue is not directly network reachable because packet traffic only evaluates rules after a local control plane rule has been installed. Impact is primarily kernel information disclosure through uninitialized register bytes, with limited integrity concern if those bytes influence rule logic or packet modification.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE SIMPLEFIX NETFILTER ERRORPATH  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	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

* Local privileged or delegated-trigger path: +1. Triggering requires nftables rule creation via CAP_NET_ADMIN, but this may be available to reduced-privilege users in network namespaces or delegated firewall-management contexts.
* Confidentiality impact plausible: +1. The bug can leave uninitialized `nft_regs` stack bytes marked as initialized, creating a plausible kernel stack information disclosure primitive.
* Integrity impact plausible in paranoid scoring: +1. Uninitialized register bytes may influence later nftables expressions or rule logic, but this is weaker than a controlled write primitive.
* Broad/default/common subsystem: +1. nf_tables/netfilter is widely deployed and commonly enabled.
* Common packet-processing path: +1. The malformed rule is installed via the control plane, but evaluation occurs in the netfilter packet path.
* Requires CAP_NET_ADMIN in typical deployments: -1 conservative adjustment. This is not treated as full host-root because namespace or delegated CAP_NET_ADMIN scenarios can reduce the practical privilege barrier.
* No generic memory corruption bonus. The patch indicates uninitialized data exposure, not UAF, OOB write, double-free, arbitrary write, or type confusion.
* No strong LPE bonus. There is no demonstrated reclaim, overwrite, callback dispatch, refcount takeover, or attacker-controlled object replacement.

## 3. Reachability analysis

The bug is triggered by a local actor able to install nftables rules using `NFT_EXTHDR_F_PRESENT` with an invalid length. In ordinary host deployments this requires CAP_NET_ADMIN, so it is not a remote packet-only attack. However, CAP_NET_ADMIN may be available inside network namespaces, containers, or delegated firewall-management services, making PR:L a defensible practical interpretation. Packet traffic can exercise the installed rule afterward, but the attacker must first control the nftables rule configuration.

## 4. Severity interpretation

This is stronger than an ordinary Low/Moderate cleanup because it creates a plausible uninitialized kernel stack data exposure path in nf_tables register handling. It does not look like an Important-class memory corruption issue because the patch does not show UAF, OOB write, arbitrary write, type confusion, or a concrete LPE primitive. The realistic impact is local information disclosure with possible limited rule-logic integrity effects, so Actionable Moderate handling and manual review are appropriate.

## 5. One-sentence report phrase

`nft_exthdr F_PRESENT register tracking can mark more bytes as initialized than the eval path writes, allowing a CAP_NET_ADMIN or delegated netns attacker to expose uninitialized nft_regs stack data through later nftables expressions.`

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is an nf_tables uninitialized kernel stack data exposure candidate with namespace-dependent privileges and possible limited integrity side effects, so it should not be auto-closed based only on a Moderate CVSS-style score.

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

Patch: netfilter: nft_exthdr: fix register tracking for F_PRESENT flag [ Upstream
Commit: 8738b1b6d0e639ca1fc0f61516afd3557ac4ecc6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8738b1b6d0e639ca1fc0f61516afd3557ac4ecc6

Changed files:
  net/netfilter/nft_exthdr.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=8738b1b6d0e639ca1fc0f61516afd3557ac4ecc6

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

The original Linux kernel CVE announcement for CVE-2026-53218 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53218

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:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N
  The Best / paranoid CVSS score: 6.1

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).