[CVE-2026-52909][MODERATE 7.0] ip6_vti: set netns_immutable on the fallback device. [ Upstream

[CVE-2026-52909][MODERATE 7.0] ip6_vti: set netns_immutable on the fallback device. [ Upstream

[AL-KERNEL]

CVE: CVE-2026-52909
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ip6_vti: set netns_immutable on the fallback device. [ Upstream
Commit: ecf8904067dcba0dad86ece80874841e60317885
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ecf8904067dcba0dad86ece80874841e60317885
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52909
Analysis date: Fri, 19 Jun 2026 10:53:12 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
The ip6_vti0 per netns fallback tunnel device was not marked netns immutable, allowing a local actor with delegated network namespace control to move an internal tunnel device across namespaces and potentially break namespace isolation or tunnel lifecycle assumptions.

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

The original Linux kernel CVE announcement for CVE-2026-52909 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52909

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-52909	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H';CWE-284;CWE-668;CWE-693;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.1';DESCR 'The ip6_vti per netns fallback tunnel device ip6_vti0 was not marked as netns immutable, unlike similar tunnel fallback devices. A local actor with the ability to manage links in a network namespace could move this internal device to another namespace and break the expected ownership model for per netns tunnel state. For the CVSS the PR:L is used because reliable triggering requires local namespace or link management capability such as CAP_NET_ADMIN, which can be delegated in container or user namespace setups and is not necessarily full host root. The issue is not directly network reachable and is triggered through control plane netdevice operations rather than packet traffic. Impact is mainly namespace isolation or policy bypass with possible availability impact if the fallback device lifecycle is disrupted.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE SIMPLEFIX NETWORK HARDWARE LINUS IPV6  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	guess

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local reduced privilege trigger: +1. Triggering requires local netdevice or namespace control, typically CAP_NET_ADMIN, but this capability can be delegated inside containers or user namespaces and is not always equivalent to full host root.
* Cross-boundary isolation or policy bypass: +1. The fallback ip6_vti0 device is a per netns internal object and moving it to another namespace violates the intended network namespace ownership model.
* Availability impact realistic: +1. Moving a per netns fallback tunnel device can plausibly disrupt tunnel lifecycle assumptions, cleanup, or namespace teardown behavior, although no concrete crash or UAF is shown.

Paranoid additional interpretation:

* Security-boundary bypass with practical sensitive kernel object access: +2. In the paranoid view, the movable fallback device is treated as a sensitive per namespace kernel object whose migration may break container or namespace isolation assumptions.
* No memory corruption signal is added. The patch only sets netns_immutable and does not show UAF, OOB access, double free, refcount misuse, type confusion, or arbitrary write.
* No remote/network-triggerable signal is added. The bug is triggered through rtnetlink/netdevice control operations, not packet processing.

## 3. Reachability analysis

The realistic trigger is local. An attacker needs the ability to manipulate network links and network namespaces, usually CAP_NET_ADMIN in the relevant namespace. In traditional host setups this is privileged and would lower urgency. In containerized or user namespace enabled environments, CAP_NET_ADMIN may be available to a reduced privilege actor, so this should not be treated as full host-root-only in all deployments.

The affected device is the per-netns ip6_vti0 fallback tunnel device. The issue is not remotely reachable by sending IPv6 or IPsec traffic. It requires control-plane operations that move a netdevice between namespaces. The path is configuration dependent on ip6_vti availability, but it is still a normal Linux networking namespace/tunnel path and should not be treated as a rare hardware-only condition.

## 4. Severity interpretation

This behaves more like an actionable Moderate namespace isolation issue than an ordinary low-risk cleanup. The patch does not support a memory corruption or direct LPE classification. However, the missing netns_immutable flag permits movement of an internal per namespace fallback tunnel device, which can violate kernel namespace lifecycle and ownership assumptions.

Theoretical worst case is namespace or container isolation disruption plus availability impact. Realistic evidence supports policy/isolation bypass and possible DoS, not arbitrary code execution or proven privilege escalation.

## 5. One-sentence report phrase

The ip6_vti0 per netns fallback tunnel device was not marked netns immutable, allowing a local actor with delegated network namespace control to move an internal tunnel device across namespaces and potentially break namespace isolation or tunnel lifecycle assumptions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not memory corruption, but it affects a namespace isolation invariant and CAP_NET_ADMIN may be delegated in container or user namespace deployments, so auto-closing as a low-risk privileged-only issue could miss practical containment impact.

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

Patch: ip6_vti: set netns_immutable on the fallback device. [ Upstream
Commit: ecf8904067dcba0dad86ece80874841e60317885
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ecf8904067dcba0dad86ece80874841e60317885

Changed files:
  net/ipv6/ip6_vti.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=ecf8904067dcba0dad86ece80874841e60317885

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

The original Linux kernel CVE announcement for CVE-2026-52909 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52909

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:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 6.1

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: 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: 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).