[CVE-2026-53266][IMPORTANT] netfilter: bridge: make ebt_snat ARP rewrite writable [ Upstream

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

CVE: CVE-2026-53266
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: bridge: make ebt_snat ARP rewrite writable [ Upstream
Commit: bf84ad7c7a9ede46e31afaa41a1ba06a159e8c87
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf84ad7c7a9ede46e31afaa41a1ba06a159e8c87
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53266
Analysis date: Thu, 25 Jun 2026 09:51:48 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
The ebtables SNAT ARP rewrite path may write a MAC address into a non writable splice backed skb fragment, creating a shared page corruption primitive that is configuration dependent but serious enough for manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53266 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53266

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: IMPORTANT
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-2026-53266	IMPORTANT	CHECK	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-787;CWE-284;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:L';BEST CVSS score: '7';DESCR 'An ARP sender hardware address rewrite in ebtables SNAT can write into a non linear skb fragment that was not made writable first. The write is performed by skb_store_bits() at an offset relative to packet data, while skb_header_pointer() only provides a safe read of the ARP header and does not make the later SHA range writable. If that range is backed by a splice imported file page, the MAC rewrite can modify the underlying page rather than a private writable packet buffer. For the CVSS the PR:L is used because reliable triggering requires a local or reduced capability context able to generate matching bridged ARP traffic and construct the needed skb layout, not full host administrator privileges. The issue is not a normal remote IP packet path, but it can be relevant on hosts using bridge netfilter ebtables SNAT rules. Impact is at least integrity impact through unintended page modification and may allow local privilege escalation or denial of service depending on what backing page is modified.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB NETFILTER SKB HARDWARE KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local or container adjacent actor may be able to generate bridged ARP traffic and influence skb construction if the vulnerable ebtables SNAT rule already exists.
* Shared or file backed page corruption primitive: +2. The commit explicitly says skb_store_bits() may copy the rewritten MAC directly into a nonlinear skb fragment backed by a splice imported file page.
* Integrity impact plausible: +1. The primitive can modify data that should not be writable through packet rewriting.
* Common packet processing path: +1. This is netfilter bridge packet handling, although the exact ebtables SNAT ARP rewrite configuration is not default.
* Dirty Pipe like ownership concern: +1. This resembles a page ownership or COW bypass pattern, but the conservative score avoids fully double counting it.
* Hard trigger conditions: -1. Practical triggering requires ARP rewrite rules, nonlinear skb fragments, and splice imported file page backing.

Paranoid additions:

* Privilege escalation plausible: +2. Page backed corruption is a historically dangerous primitive and could become LPE if the attacker can choose a sensitive backing page and useful offset.
* Dirty Pipe like ownership or COW violation treated strongly: +2 instead of the weaker conservative bonus. The kernel writes into externally owned page backed storage without first ensuring a private writable skb area.
* Availability impact realistic: +1. Corruption of shared file backed pages or packet buffers may also produce crashes or persistent data corruption depending on target page selection.

Not counted separately:

* Generic memory corruption is not stacked again on top of the page cache and Dirty Pipe like primitive, to avoid over counting the same underlying write.

## 3. Reachability analysis

This is not a normal remote IP reachable vulnerability. The affected path requires bridge netfilter ebtables SNAT with optional ARP sender hardware address rewriting. In typical deployments creating such rules requires CAP_NET_ADMIN, but the attacker may not need that capability if a vulnerable rule is already present on a bridge.

The most plausible attacker is local, container adjacent, VM adjacent, or L2 adjacent in an environment where bridged ARP traffic passes through ebtables SNAT and skb fragments can be backed by splice imported file pages. Namespace or container setups may reduce the practical privilege requirement because a container or reduced capability process may be able to generate the relevant traffic while the host or orchestration layer owns the bridge rules.

## 4. Severity interpretation

This is not an ordinary DoS only Moderate. The patch describes a concrete write into a non writable nonlinear skb fragment backed by an imported file page. That makes it a shared page or page cache corruption candidate rather than a simple NULL dereference or validation cleanup.

Realistic exploitation is configuration dependent and likely difficult, so the conservative score is Actionable Moderate. The paranoid score reaches Strong Important candidate because the primitive resembles Dirty Pipe style ownership or COW bypass behavior and may plausibly support local privilege escalation if sensitive file backed data can be targeted.

## 5. One-sentence report phrase

The ebtables SNAT ARP rewrite path may write a MAC address into a non writable splice backed skb fragment, creating a shared page corruption primitive that is configuration dependent but serious enough for manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes an unintended write into externally backed skb fragment memory, which is a high risk page ownership corruption pattern and should not be auto closed even though triggering depends on bridge ebtables SNAT ARP rewrite configuration.

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

Patch: netfilter: bridge: make ebt_snat ARP rewrite writable [ Upstream
Commit: bf84ad7c7a9ede46e31afaa41a1ba06a159e8c87
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf84ad7c7a9ede46e31afaa41a1ba06a159e8c87

Changed files:
  net/bridge/netfilter/ebt_snat.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=bf84ad7c7a9ede46e31afaa41a1ba06a159e8c87

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

The original Linux kernel CVE announcement for CVE-2026-53266 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53266

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:L/UI:N/S:U/C:L/I:H/A:L
  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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).