[CVE-2026-72051][MODERATE 7.0] net: ip6_tunnel: require CAP_NET_ADMIN in the device netns for changelink [ Upstream

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

CVE: CVE-2026-72051
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: ip6_tunnel: require CAP_NET_ADMIN in the device netns for changelink [ Upstream
Commit: 2636d061bc237a2446a146e42dcc6563acfa7432
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2636d061bc237a2446a146e42dcc6563acfa7432
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72051
Analysis date: Sat, 15 Aug 2026 04:58:58 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
ip6_tnl_changelink lacked a capability check in the tunnel link netns, allowing a caller with CAP_NET_ADMIN only in the device netns to reconfigure an IPv6 tunnel across a namespace boundary.

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

The original Linux kernel CVE announcement for CVE-2026-72051 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72051

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-2026-72051	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H';CWE-863;CWE-284;CWE-269;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:L';BEST CVSS score: '8.8';DESCR 'ip6_tnl_changelink can modify an IPv6 tunnel whose link netns differs from dev_net(dev) because the existing rtnl changelink permission check only covered the device netns. A caller with CAP_NET_ADMIN in the device netns but not in the tunnel link netns can rewrite tunnel parameters across a namespace boundary. For the CVSS the PR:L is used because the attacker needs a delegated local network administration capability, for example in a container or reduced capability namespace, but not full host administrator privileges. The issue is not directly network reachable and is triggered through local rtnetlink control plane operations, not by packet traffic. Impact can include unauthorized tunnel reconfiguration, traffic redirection, policy bypass, or denial of service for traffic using the affected tunnel. In the paranoid score, the cross namespace authorization bypass is treated as potentially high impact for confidentiality, integrity, and availability if the tunnel carries sensitive traffic between tenants or namespaces. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE SIMPLEFIX NETWORK HARDWARE IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Security-boundary bypass with practical sensitive-object access: +2
  The bug bypasses the intended CAP_NET_ADMIN check for the tunnel link netns. A caller privileged only in dev_net(dev) can modify a tunnel whose link netns is different.
* Cross-boundary isolation/policy bypass: +1
  This is a network namespace authorization bypass and can affect objects outside the namespace where the caller is authorized.
* Integrity impact plausible: +1
  Unauthorized tunnel parameter changes can redirect, alter, or break tunnel traffic policy.
* Requires CAP_NET_ADMIN-like delegated privilege: 0 to -1
  This is not full host root in the relevant security model. A container or reduced-capability namespace user with CAP_NET_ADMIN in one netns may still lack authority in the tunnel link netns.

Paranoid additional signals:

* Confidentiality impact plausible: +1
  If the tunnel carries sensitive traffic, unauthorized reconfiguration can expose or redirect traffic.
* Availability impact realistic: +1
  Rewriting tunnel parameters can disrupt connectivity for traffic using the affected tunnel.
* No memory corruption signals applied.
  The patch is an authorization check, not UAF, OOB, refcount, or object lifetime corruption.

## 3. Reachability analysis

The issue is triggered locally through rtnetlink changelink operations, not by ordinary network packets. The attacker needs CAP_NET_ADMIN in the device netns, but not in the tunnel link netns, which makes this relevant for delegated network administration, containers, or multi-netns setups. It is not remotely reachable by default from the network. The path requires IPv6 tunnel devices with cross-netns state, so exposure is configuration-dependent, but the affected operation is a real kernel network control-plane path.

Call-site confidence: high, because the changed function and permission check location are visible in the patch.

## 4. Severity interpretation

This behaves like an authorization and namespace isolation bypass, not a memory-corruption vulnerability. Realistic exploitation impact depends on whether untrusted or semi-trusted users can manage network devices in one namespace while the tunnel link belongs to another. Conservative handling is Borderline to Actionable Moderate. Paranoid handling is Actionable Moderate because the affected object is a tunnel and unauthorized changes may impact confidentiality, integrity, and availability of cross-namespace traffic.

## 5. One-sentence report phrase

ip6_tnl_changelink lacked a capability check in the tunnel link netns, allowing a caller with CAP_NET_ADMIN only in the device netns to reconfigure an IPv6 tunnel across a namespace boundary.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is not a simple DoS or validation cleanup. It is a namespace capability bypass whose impact depends strongly on container, delegated CAP_NET_ADMIN, and tunnel deployment models.

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

Patch: net: ip6_tunnel: require CAP_NET_ADMIN in the device netns for changelink [ Upstream
Commit: 2636d061bc237a2446a146e42dcc6563acfa7432
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2636d061bc237a2446a146e42dcc6563acfa7432

Changed files:
  net/ipv6/ip6_tunnel.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=2636d061bc237a2446a146e42dcc6563acfa7432

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

The original Linux kernel CVE announcement for CVE-2026-72051 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72051

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:C/C:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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: 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: 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).