[CVE-2026-53003][MODERATE REGULAR] pppoe: drop PFC frames [ Upstream

[CVE-2026-53003][MODERATE REGULAR] pppoe: drop PFC frames [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53003
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: pppoe: drop PFC frames [ Upstream
Commit: cb3beef35ab5e0c1afca9fd7648c6ae499786377
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cb3beef35ab5e0c1afca9fd7648c6ae499786377
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53003
Analysis date: Thu, 25 Jun 2026 13:36:04 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended / Ordinary Moderate
Manual review required: YES

Summary:
A PPPoE peer on the same L2 segment can send a PFC frame that shifts the PPP payload by one byte and may trigger unaligned access exceptions on strict-alignment systems, causing a kernel DoS without evidence of confidentiality, integrity, or privilege-escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-53003 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53003

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-2026-53003	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-20;CWE-754;CWE-755;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'A PPPoE receiver can accept a PPP frame with Protocol Field Compression even though PPPoE normally expects an uncompressed 2 byte PPP protocol field. A peer or attacker on the same L2 PPPoE segment can send a compressed 1 byte protocol field, shifting the following PPP payload by one byte. This can make later network headers 4 byte misaligned and may trigger unaligned access exceptions on strict alignment architectures, causing a kernel crash or service disruption. For the CVSS the PR:N is used for the paranoid score because a malicious PPPoE peer or adjacent L2 attacker does not need a local account on the victim. The issue is not Internet routable in the usual IP sense and requires access to a PPPoE access segment or peer position. Impact is denial of service only, with no supported confidentiality or integrity primitive from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / Ordinary Moderate (with actual score 3) 	MAYBE	SIMPLEFIX NETWORK SKB HARDWARE LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended / Ordinary Moderate**::

## 2. Signal breakdown

Conservative scoring:

* Remote reachable / network-triggerable: +2
  PPPoE frames can be supplied by a peer or attacker on the same L2 PPPoE segment. This is network-adjacent, not Internet-routable IP remote.
* Reliable kernel crash / strong DoS: +1
  The malformed PFC frame can shift the PPP payload and cause 4-byte misalignment, which may trigger unaligned access exceptions and kernel crash on strict-alignment architectures.
* Rare AND difficult-to-reach subsystem/configuration: -1
  Requires PPPoE exposure plus an architecture or configuration where misaligned network header access faults rather than being tolerated.

Paranoid additional interpretation:

* Packet-processing receive path concern: +1
  The bug is in a protocol receive path and can be triggered by crafted L2 input from an adjacent peer, so manual review is reasonable even though the impact is DoS-only.

Not counted:

* No generic memory corruption: no UAF, double-free, OOB write, heap overwrite, or type confusion is shown.
* No privilege escalation plausible signal: the patch context supports crash via misalignment, not a privilege escalation primitive.
* No confidentiality or integrity impact: no leak, write primitive, or ownership bypass is indicated.
* Availability impact is not double-counted separately because the crash/DoS signal already captures it.

## 3. Reachability analysis

A malicious PPPoE peer or attacker with access to the same L2 PPPoE segment can send a compressed PPP protocol field that PPPoE did not negotiate and does not expect. No local account on the victim is required, so PR:N is reasonable for CVSS-style reasoning, but the attacker must be adjacent to the PPPoE segment. This is not broadly Internet-routable and is mainly relevant to PPPoE servers, access concentrators, routers, CPE, or systems using PPPoE sessions.

Namespaces and containers do not materially lower the trigger requirement because the input is external L2 traffic, not a local syscall interface. Practical exploitation depends on PPPoE being enabled and reachable and on the CPU architecture handling the resulting misaligned access as an exception.

## 4. Severity interpretation

This behaves like an ordinary Moderate issue with manual-review value, not an Important-class vulnerability. The realistic impact is network-adjacent DoS on affected strict-alignment systems. Theoretical memory corruption is not supported by the patch: the described failure mode is payload misalignment leading to unaligned access exceptions, not attacker-controlled overwrite or lifetime corruption.

## 5. One-sentence report phrase

A PPPoE peer on the same L2 segment can send a PFC frame that shifts the PPP payload by one byte and may trigger unaligned access exceptions on strict-alignment systems, causing a kernel DoS without evidence of confidentiality, integrity, or privilege-escalation impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network-adjacent kernel DoS in a packet receive path should not be blindly auto-closed, but the absence of memory corruption, privilege escalation, or broad Internet reachability keeps it below Actionable Moderate or Important.

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

Patch: pppoe: drop PFC frames [ Upstream
Commit: cb3beef35ab5e0c1afca9fd7648c6ae499786377
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cb3beef35ab5e0c1afca9fd7648c6ae499786377

Changed files:
  drivers/net/ppp/ppp_generic.c
  drivers/net/ppp/pppoe.c
  include/linux/ppp_defs.h

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=cb3beef35ab5e0c1afca9fd7648c6ae499786377

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

The original Linux kernel CVE announcement for CVE-2026-53003 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53003

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 2
  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).