[CVE-2026-72405][MODERATE 7.0] net: udp_tunnel: prevent double queueing in udp_tunnel_nic_device_sync [ Upstream

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

CVE: CVE-2026-72405
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: udp_tunnel: prevent double queueing in udp_tunnel_nic_device_sync [ Upstream
Commit: cee6688e5731c0591643521716d1a1a5c1a98bf8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cee6688e5731c0591643521716d1a1a5c1a98bf8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72405
Analysis date: Sun, 16 Aug 2026 00:05:17 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Strong Important only if follow-up confirms attacker-controlled reclaim, stale object reuse, callback control, or another concrete LPE primitive.
Manual review required: YES

Summary:
A race in `udp_tunnel_nic_device_sync()` can double queue sync work and allow `udp_tunnel_nic_unregister()` to free the structure while queued work still references it, causing a kernel UAF and at least local DoS under concurrent network device operations.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72405 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72405

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-2026-72405	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in udp_tunnel_nic_device_sync can queue the same work item twice while an earlier worker is already running. The first worker can clear the work_pending flag for a later queued instance, allowing udp_tunnel_nic_unregister to observe work_pending as 0 and free the udp_tunnel_nic structure while queued work still references it. This is a kernel use after free in a workqueue path and can cause at least a local denial of service via crash. For the CVSS the PR:L is used for the paranoid score because reliable triggering normally needs local control of network device or tunnel configuration, but that control can be delegated through CAP_NET_ADMIN, container root, or a network management service account. The issue is not directly network reachable through ordinary UDP tunnel packet traffic. Impact may extend beyond DoS in worst case because UAF on a freed kernel object can be a memory corruption primitive, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE UAF NETWORK HARDWARE KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Strong Important only if follow-up confirms attacker-controlled reclaim, stale object reuse, callback control, or another concrete LPE primitive.

## 2. Signal breakdown

* Local low-privilege or delegated trigger: +1
  Reliable triggering likely needs local control over network device or tunnel configuration. This is often CAP_NET_ADMIN, but may be delegated to container root, a network-management service, or reduced-capability root.

* Memory corruption, weak race-UAF primitive: +1
  The commit explicitly describes UAF, but it is race-dependent and does not show attacker-controlled reclaim, overwrite, type confusion, callback hijack, or refcount takeover.

* Real lifetime corruption: +1
  `udp_tunnel_nic_unregister()` may free `udp_tunnel_nic` while a second queued work item still references it.

* Reliable kernel crash / strong DoS: +1
  Reported UAF and debugobjects warning indicate a realistic kernel failure mode during concurrent device operations.

* Broad networking infrastructure exposure: +1
  This is in kernel networking UDP tunnel NIC offload infrastructure, not a rare debug-only path.

* Conservative race penalty: -1
  The bug depends on timing between workqueue execution, requeueing, and unregister. For the paranoid score this penalty is not applied because concurrent device operations may naturally reproduce it under load.

## 3. Reachability analysis

This is not directly remotely reachable through ordinary UDP tunnel packet traffic. The likely trigger is local control-plane activity involving UDP tunnel NIC synchronization and device unregister or concurrent device operations. In typical host deployments this requires CAP_NET_ADMIN or equivalent network-device management rights. In containerized or delegated environments, the practical privilege level may be lower than full host root, especially where network management is exposed to service accounts or container root.

The path is not a generic packet receive fast path, but it is part of normal networking infrastructure and can be present on systems using UDP tunnel offload support. Call-site confidence: medium, because the commit message describes the lifetime sequence clearly, but the full caller graph is not included in the supplied patch.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is an explicit kernel UAF in a workqueue lifetime path. However, the provided patch does not demonstrate a strong exploitation primitive such as controlled reclaim, write-after-free, callback control, or type confusion. Realistic impact is at least local DoS via kernel crash. Theoretical LPE concern exists because UAF is a memory corruption class, but current evidence supports Actionable Moderate rather than definite Important.

## 5. One-sentence report phrase

A race in `udp_tunnel_nic_device_sync()` can double queue sync work and allow `udp_tunnel_nic_unregister()` to free the structure while queued work still references it, causing a kernel UAF and at least local DoS under concurrent network device operations.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit race-based UAF in kernel networking workqueue lifetime handling. It should not be auto-closed unless manual review confirms there is no practical reclaim, stale reuse, callback, or write primitive beyond crash.

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

Patch: net: udp_tunnel: prevent double queueing in udp_tunnel_nic_device_sync [ Upstream
Commit: cee6688e5731c0591643521716d1a1a5c1a98bf8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cee6688e5731c0591643521716d1a1a5c1a98bf8

Changed files:
  net/ipv4/udp_tunnel_nic.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=cee6688e5731c0591643521716d1a1a5c1a98bf8

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

The original Linux kernel CVE announcement for CVE-2026-72405 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72405

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 4
  Paranoid ActionableScore: 5

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).