[CVE-2026-80634][MODERATE 7.0] netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag [ Upstream

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

CVE: CVE-2026-80634
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag [ Upstream
Commit: 2f55fa28011c97d6495d5787808db10a8c2d690d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f55fa28011c97d6495d5787808db10a8c2d690d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80634
Analysis date: Fri, 28 Aug 2026 04:57:03 -0400
ActionableScore: 6
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A u8 underflow in netfilter flowtable bridge VLAN untag handling can wrap num_encaps to 255, causing an OOB stack read from info.encap and bogus route descriptor metadata, with typical exploitation requiring privileged flowtable setup but paranoid adjacent traffic triggering possible when a vulnerable path is already configured.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80634 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80634

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
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CVE-2026-80634	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-191;CWE-125;CWE-129;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '6.4';DESCR 'An 8 bit underflow in netfilter flowtable path handling can occur when the bridge VLAN untag case decrements info.num_encaps while it is already zero. The value can wrap to 255 and later code may walk info.encap beyond its size of 2 entries, causing an out-of-bounds stack read and a bogus encapsulation count in the route descriptor. For the CVSS the PR:N is used in the paranoid case because a remote or adjacent host may only need to send traffic through an already configured vulnerable bridge flowtable path, while creating such configuration normally requires administrator rights. The issue is not a public Internet remote vector in the usual deployment model and depends on bridge VLAN and flowtable offload path conditions. Impact is at least denial of service and in worst case may allow limited confidentiality or integrity impact due to the out-of-bounds read and corrupted route metadata, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE OOB SIMPLEFIX NETFILTER HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1. The patch explicitly describes a u8 underflow that makes num_encaps wrap from 0 to 255, followed by an OOB stack read from info.encap, which has size 2.
* Reliable crash or strong DoS concern: +1. Walking far beyond a small stack array can plausibly crash or destabilize the kernel, although the commit does not show a demonstrated panic trace.
* Common networking / netfilter path: +1. The affected code is in netfilter flowtable bridge VLAN path handling.
* Requires CAP_NET_ADMIN or admin setup in typical deployments: -2. Creating and managing nftables flowtables, bridge VLAN setup, and forwarding configuration normally requires administrative privileges.
* Hard or malformed path condition: -1. The commit says this should only happen on a malformed bridge path stack, so reliable triggering is not clearly demonstrated.

Conservative total: 3

Paranoid additional interpretation:

* Adjacent or traffic-mediated trigger possibility: +2. If a vulnerable bridge VLAN flowtable path is already configured, an adjacent host may only need to send traffic through that path to exercise the forwarding logic.
* Confidentiality impact plausible: +1. The bug is an OOB stack read, although no direct user-visible disclosure path is shown.
* Integrity impact plausible: +1. A bogus encapsulation count and route descriptor metadata may influence forwarding/offload state, but no arbitrary write primitive is shown.
* Availability impact realistic: +1. In the paranoid case, the oversized count can make the kernel process invalid stack-derived encap metadata and plausibly cause host-visible DoS.

Paranoid total after grounding and avoiding double counting: 6

## 3. Reachability analysis

Typical triggering requires a configured netfilter flowtable with bridge VLAN handling, which normally means CAP_NET_ADMIN or equivalent administrative control. The malformed bridge path stack condition suggests the bug is not trivially reachable from ordinary local users.

The paranoid case treats the forwarding path as potentially traffic-mediated once the vulnerable configuration already exists. In that model, an adjacent network sender may be able to exercise the affected path without having privileges on the target host. This is not a general Internet remote vector and depends on bridge VLAN flowtable deployment.

Namespaces can matter if CAP_NET_ADMIN is delegated in a container or network namespace with access to relevant bridge and flowtable objects. In normal host deployments, this remains privileged configuration.

## 4. Severity interpretation

This is stronger than an ordinary warning or pure validation cleanup because the commit explicitly identifies an OOB stack read and bogus route metadata. It is not strong memory corruption: there is no OOB write, UAF, controlled overwrite, callback control, or demonstrated privilege escalation primitive.

Realistically, this behaves like a Moderate netfilter DoS or metadata-corruption issue under special configuration. Paranoid handling should not auto-close it because the affected path is networking-related and the bug includes an explicit OOB stack read.

## 5. One-sentence report phrase

A u8 underflow in netfilter flowtable bridge VLAN untag handling can wrap num_encaps to 255, causing an OOB stack read from info.encap and bogus route descriptor metadata, with typical exploitation requiring privileged flowtable setup but paranoid adjacent traffic triggering possible when a vulnerable path is already configured.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the patch explicitly describes an OOB stack read in a netfilter flowtable path, and the realistic reachability depends on bridge VLAN and flowtable configuration details that should be checked manually.

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

Patch: netfilter: flowtable: avoid num_encaps underflow on bridge VLAN untag [ Upstream
Commit: 2f55fa28011c97d6495d5787808db10a8c2d690d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f55fa28011c97d6495d5787808db10a8c2d690d

Changed files:
  net/netfilter/nf_flow_table_path.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=2f55fa28011c97d6495d5787808db10a8c2d690d

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

The original Linux kernel CVE announcement for CVE-2026-80634 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80634

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:H/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.4

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: 6

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 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).