[CVE-2026-74406][MODERATE REGULAR] vxlan: Fix potential null-ptr-deref in vxlan_gro_prepare_receive(). [ Upstream

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

CVE: CVE-2026-74406
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: vxlan: Fix potential null-ptr-deref in vxlan_gro_prepare_receive(). [ Upstream
Commit: 9c58c729d32e7cea5772cc44929c6cd61e5a31cd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c58c729d32e7cea5772cc44929c6cd61e5a31cd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74406
Analysis date: Sat, 15 Aug 2026 07:07:10 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A race in the VXLAN GRO receive path can leave `sk_user_data` NULL while packets are being processed, allowing a reachable VXLAN sender to contribute to a timing-dependent kernel NULL pointer dereference and denial of service during tunnel teardown.

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

The original Linux kernel CVE announcement for CVE-2026-74406 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74406

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-2026-74406	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-362;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'A race in the VXLAN/GRO receive path can leave sk_user_data as NULL while vxlan_gro_prepare_receive() is processing a packet. If udp_tunnel_sock_release() clears the socket user data concurrently, the receive path may dereference a missing vxlan_sock state and crash the kernel. For the CVSS the PR:N is used for the paranoid score because a remote sender may not need an account on the target when the VXLAN UDP endpoint is reachable and the tunnel is being torn down concurrently. The attack complexity is high because the crash depends on a narrow race with socket release. Impact is denial of service through kernel crash, with no concrete evidence of information disclosure or privilege escalation from the shown patch. VXLAN is usually deployed in data center or overlay networks rather than exposed to the public Internet, so practical exploitation normally requires reachability to the overlay or trusted internal network.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)	YES	NULLPTR SIMPLEFIX NETWORK  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Paranoid score signals:

* Remote reachable / network-triggerable: +2. VXLAN is a UDP based overlay protocol and the affected function is in the receive GRO path. A remote sender that can reach the VXLAN endpoint may contribute packets during the teardown race.
* Reliable kernel crash / strong DoS: +1. The patch fixes a potential NULL pointer dereference in `vxlan_gro_prepare_receive()`, which can crash the kernel.
* Common networking packet-processing path: +1. The bug is in a network receive path for VXLAN GRO processing.
* Broad/common deployed exposure: +1. VXLAN is common in data center and overlay networking deployments, though not usually exposed to the public Internet.
* Hard or unreliable race / special timing required: -1. Triggering requires overlap between packet receive processing and `udp_tunnel_sock_release()` clearing `sk_user_data`.

Conservative score does not fully count remote triggerability because reliable exploitation appears to require a concurrent tunnel/socket teardown event that is usually controlled by local network administration.

No memory corruption or LPE signals are awarded. The patch supports NULL dereference DoS, not UAF reclaim, stale object reuse, overwrite, type confusion, or privilege escalation.

## 3. Reachability analysis

The affected path is reachable through VXLAN packet reception when UDP socket GRO is used. A remote attacker may be able to send VXLAN traffic if they have access to the overlay or internal network where the VXLAN endpoint is exposed. However, the crash depends on a race with `udp_tunnel_sock_release()`, so the attacker may not be able to trigger it reliably unless tunnel teardown occurs concurrently.

Local reliable setup or teardown generally requires `CAP_NET_ADMIN`. Containers or delegated network namespaces could reduce the practical privilege barrier if untrusted workloads can create or destroy VXLAN devices, but this is not the default assumption for host networking.

VXLAN is not normally Internet-facing, but it is common in data center and virtualized networking environments.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class vulnerability. The realistic impact is kernel crash / DoS. Theoretical escalation is not supported by the shown patch because the fixed condition is a NULL check after `sk_user_data` may become NULL, with no evidence of writable stale object access or attacker-controlled memory reuse.

The main reason not to auto-close is that the bug is in a network packet-processing path and can potentially be influenced remotely in VXLAN deployments, even though the race makes exploitation unreliable.

## 5. One-sentence report phrase

A race in the VXLAN GRO receive path can leave `sk_user_data` NULL while packets are being processed, allowing a reachable VXLAN sender to contribute to a timing-dependent kernel NULL pointer dereference and denial of service during tunnel teardown.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Network reachable packet-processing races with kernel crash impact should be reviewed manually, even when the demonstrated primitive is only NULL pointer dereference and the race appears difficult to trigger reliably.

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

Patch: vxlan: Fix potential null-ptr-deref in vxlan_gro_prepare_receive(). [ Upstream
Commit: 9c58c729d32e7cea5772cc44929c6cd61e5a31cd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c58c729d32e7cea5772cc44929c6cd61e5a31cd

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=9c58c729d32e7cea5772cc44929c6cd61e5a31cd

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

The original Linux kernel CVE announcement for CVE-2026-74406 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74406

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 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).