[CVE-2026-80681][MODERATE 7.0] vxlan: re-fetch eth header after route_shortcircuit()

[CVE-2026-80681][MODERATE 7.0] vxlan: re-fetch eth header after route_shortcircuit()

[AL-KERNEL]

CVE: CVE-2026-80681
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vxlan: re-fetch eth header after route_shortcircuit()
Commit: 6375093eb45cd7d89f1945f939eeae3b29d79f56
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6375093eb45cd7d89f1945f939eeae3b29d79f56
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80681
Analysis date: Fri, 28 Aug 2026 04:31:18 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A use-after-free in the VXLAN transmit path occurs because vxlan_xmit() reuses a cached Ethernet header pointer after route_shortcircuit() may reallocate skb head, allowing network-adjacent VXLAN traffic to trigger a stale eth h_dest dereference and cause at least a kernel crash (DoS).

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80681 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80681

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-2026-80681	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-416;CWE-825;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A use-after-free in the VXLAN transmit path can occur because vxlan_xmit caches the Ethernet header pointer before route_shortcircuit(). route_shortcircuit() may call pskb_may_pull(), which can reallocate skb head and leave the cached eth pointer stale. A packet that reaches the affected VXLAN route shortcircuit path can then cause a dereference of eth h_dest from freed memory, leading at least to kernel crash or incorrect forwarding behavior. For the CVSS the PR:N because the attacker does not need a local account if they can inject traffic into the relevant VXLAN or attached tenant network path. The attack is best modeled as AV:A because VXLAN deployments are usually limited to adjacent overlay, cloud, or data center networks rather than the public Internet. Impact is at least denial of service, with limited confidentiality or integrity impact possible in the paranoid interpretation due to a stale UAF read that influences MAC lookup.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	UAF NETWORK SKB LINUS IPV6 KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	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:

* Remote reachable / network-triggerable: +2. The affected path is VXLAN packet transmission and can be reached by traffic entering the relevant VXLAN or tenant overlay path.
* Memory corruption, weak/indirect corruption candidate: +1. This is a stale pointer use-after-free read of a cached Ethernet header pointer, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or type confusion.
* Real lifetime corruption: +1. pskb_may_pull() may reallocate skb head, leaving the cached eth pointer referring to freed memory.
* Reliable kernel crash / strong DoS: +1. Dereferencing a freed Ethernet header pointer in the transmit path can plausibly crash the kernel.
* Common networking packet-processing path: +1. VXLAN is a commonly deployed overlay networking datapath in cloud and data-center environments.
* Hard or special triggering conditions: -1. Triggering requires the route_shortcircuit() path and an skb layout where pskb_may_pull() reallocates skb head.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Limited information exposure is theoretically possible if behavior depending on freed header bytes is externally observable, but no direct disclosure primitive is shown.
* Integrity impact plausible: +1. eth h_dest from stale memory may influence MAC lookup or forwarding decisions, giving a limited integrity concern in the paranoid interpretation.

Not awarded:

* Strong LPE plausibility: +0. No attacker-controlled reclaim, writable stale object access, function pointer control, type confusion, or refcount takeover is shown.
* Strong corruption primitive: +0. The demonstrated primitive is stale read/dereference, not an arbitrary write or controlled overwrite.
* Availability impact realistic: +0 additional. Crash/DoS was already counted and should not be double-counted.

## 3. Reachability analysis

The most realistic attacker is a network-adjacent actor that can inject traffic into the affected VXLAN or tenant overlay network. This should not be modeled as general Internet remote exposure in most deployments, because VXLAN is normally used inside data-center, virtualization, cloud, or overlay networks.

For the CVSS the PR:N because the attacker does not need a local account on the victim host if they can send packets into the relevant VXLAN path. Creating or configuring the VXLAN interface requires administrative privileges, but triggering the bug can be packet-driven once the vulnerable configuration exists.

Namespaces and containers matter mainly for deployment. A tenant or container workload may be able to generate traffic that reaches the VXLAN datapath, but the vulnerable route_shortcircuit configuration is typically host-controlled.

Call-site confidence: high. The provided diff shows the stale eth pointer being used immediately after route_shortcircuit() in vxlan_xmit().

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a real lifetime bug in a networking datapath and the trigger can be packet-driven in affected VXLAN deployments. It is not a clear Important LPE case because the patch only demonstrates stale pointer dereference/read, not controlled reuse or overwrite.

The conservative interpretation is Actionable Moderate. The paranoid interpretation reaches Strong Important candidate because it combines network-adjacent reachability, UAF lifetime corruption, likely DoS, and limited possible C/I impact from stale header data influencing forwarding behavior.

## 5. One-sentence report phrase

A use-after-free in the VXLAN transmit path occurs because vxlan_xmit() reuses a cached Ethernet header pointer after route_shortcircuit() may reallocate skb head, allowing network-adjacent VXLAN traffic to trigger a stale eth h_dest dereference and cause at least a kernel crash (DoS).

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: this is a real UAF in a networking packet path with network-adjacent trigger conditions. The available patch supports DoS strongly and limited C/I concern defensibly, but does not prove a practical privilege escalation primitive.

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

Patch: vxlan: re-fetch eth header after route_shortcircuit()
Commit: 6375093eb45cd7d89f1945f939eeae3b29d79f56
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6375093eb45cd7d89f1945f939eeae3b29d79f56

Commit description:

Before route_shortcircuit(), the eth header pointer is cached from eth_hdr(skb).

Inside route_shortcircuit(), pskb_may_pull() can be called, which may
reallocate skb->head.

In this case, returning to vxlan_xmit() leaves the cached eth pointer pointing to
freed memory, leading to a use-after-free when dereferencing eth->h_dest.

Fix this by updating eth = eth_hdr(skb) after calling route_shortcircuit().

Fixes: ae88408 ("VXLAN: Allow L2 redirection with L3 switching")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20260723144249.759100-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/vxlan/vxlan_core.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=6375093eb45cd7d89f1945f939eeae3b29d79f56

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

The original Linux kernel CVE announcement for CVE-2026-80681 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80681

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: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: 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: 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).