[CVE-2026-63868][MODERATE REGULAR] net: garp: fix unsigned integer underflow in garp_pdu_parse_attr [ Upstream

[CVE-2026-63868][MODERATE REGULAR] net: garp: fix unsigned integer underflow in garp_pdu_parse_attr [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63868
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: garp: fix unsigned integer underflow in garp_pdu_parse_attr [ Upstream
Commit: 29f28172afb2ae7b31e9bf3e978396f20b381688
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=29f28172afb2ae7b31e9bf3e978396f20b381688
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63868
Analysis date: Sun, 19 Jul 2026 10:51:42 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A GARP receive parser unsigned integer underflow can cause received Join/Leave events to be ignored, breaking GVRP VLAN registration from an adjacent L2 attacker without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63868 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63868

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.

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

CVE-2026-63868	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:L/A:L';CWE-191;CWE-682;CWE-681;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.4';DESCR 'A GARP receive-side parser bug computes the attribute data length with reversed operands, causing unsigned integer underflow for normal attributes where the on-wire length is larger than the GARP attribute header. The truncated length passed into lookup usually does not match locally registered attributes, so received Join/Leave events can be ignored and GVRP VLAN registration can fail. For the CVSS the PR:N is used because a sender on the same L2 segment can transmit GARP frames without local code execution on the target. The issue is adjacent-network reachable rather than Internet routable because GARP is a Layer 2 control protocol. Impact is mainly availability loss for dynamic registration behavior and possible low integrity impact for stale or incorrect VLAN registration state, with no evidence of memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	OOB SIMPLEFIX LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	guess

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  GARP/GVRP frames can be supplied by a sender on the same L2 segment. This is adjacent-network reachable, not Internet-routable.

* Availability impact realistic: +1
  The bug can cause received Join/Leave events to be ignored, breaking GARP/GVRP registration behavior.

* Integrity impact plausible: +1 paranoid only
  In the paranoid interpretation, ignored Join/Leave events may leave VLAN registration state stale or incorrect, which is a low integrity impact on network policy/state.

Negative signal:

* Rare AND difficult-to-reach subsystem/configuration: -1 conservative
  GARP/GVRP is not a universal default path on most hosts and requires the relevant L2 registration functionality to be in use.

Not triggered:

* No generic memory corruption.
  The underflowed value is truncated and used for attribute lookup length comparison. The patch does not show OOB write, UAF, double-free, type confusion, or arbitrary write.

* No privilege escalation signal.
  There is no demonstrated memory corruption primitive or security-boundary bypass.

* No strong kernel crash signal.
  The described failure mode is broken protocol handling, not kernel panic.

## 3. Reachability analysis

An attacker on the same Layer 2 network segment may send crafted or normal GARP/GVRP traffic that reaches the receive-side GARP parser. No local user account on the target is required, so this is PR:N in CVSS-style reasoning, but the exposure is adjacent-network rather than broad Internet remote.

Namespaces and containers do not materially change the main attack model, because the trigger is packet delivery on the local L2 domain. The affected path is only relevant when GARP/GVRP functionality is enabled and used. Realistic exploitation conditions require access to a network segment where GARP/GVRP VLAN registration is active.

Call-site confidence: high. The patch and commit message identify the exact parser, the incorrect length calculation, and the downstream garp_attr_lookup length mismatch consequence.

## 4. Severity interpretation

This behaves like an actionable networking correctness and availability issue rather than an Important-class memory corruption vulnerability. The theoretical risk is limited to L2 protocol state disruption and possible stale or incorrect VLAN registration state. Realistic evidence points to broken Join/Leave processing, not kernel memory corruption or privilege escalation.

Conservative handling is Borderline because the subsystem is configuration-dependent. Paranoid handling reaches Actionable Moderate only because the bug is network-triggerable on the local L2 segment and may affect VLAN registration state.

## 5. One-sentence report phrase

A GARP receive parser unsigned integer underflow can cause received Join/Leave events to be ignored, breaking GVRP VLAN registration from an adjacent L2 attacker without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue is adjacent-network reachable and affects VLAN registration state, but it should not be treated as Important without additional evidence of memory corruption, kernel crash, or a practical security-boundary bypass.

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

Patch: net: garp: fix unsigned integer underflow in garp_pdu_parse_attr [ Upstream
Commit: 29f28172afb2ae7b31e9bf3e978396f20b381688
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=29f28172afb2ae7b31e9bf3e978396f20b381688

Changed files:
  net/802/garp.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=29f28172afb2ae7b31e9bf3e978396f20b381688

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

The original Linux kernel CVE announcement for CVE-2026-63868 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63868

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 5.4

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: 3
  Paranoid ActionableScore: 4

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).