[CVE-2026-72055][MODERATE 7.0] net: ip6_vti: require CAP_NET_ADMIN in the device netns for changelink

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

CVE: CVE-2026-72055
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: ip6_vti: require CAP_NET_ADMIN in the device netns for changelink
Commit: f5254e7766b4ac02b66b2ccef896cd278503cb11
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5254e7766b4ac02b66b2ccef896cd278503cb11
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72055
Analysis date: Sat, 15 Aug 2026 21:34:46 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
ip6_vti changelink lacked a CAP_NET_ADMIN check for the tunnel link namespace, allowing a caller with delegated CAP_NET_ADMIN in another netns to rewrite VTI6 tunnel configuration across a namespace boundary and potentially cause traffic redirection or denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72055 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72055

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-2026-72055	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:U/C:L/I:H/A:H';CWE-863;CWE-862;CWE-284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H';BEST CVSS score: '7.3';DESCR 'ip6_vti changelink authorization was checked only against dev_net(dev), while vti6_changelink can also modify a tunnel whose link namespace is t net. A caller with CAP_NET_ADMIN in the device namespace but not in the tunnel link namespace can therefore rewrite tunnel configuration across a namespace boundary when such a device is moved or created across netns. For the CVSS the PR:L is used because reliable triggering requires a local process with CAP_NET_ADMIN in one namespace, such as container root or a delegated network administration context, but not full privileges in the affected tunnel namespace. The issue is not directly network reachable because it is triggered through rtnetlink control plane operations, not by packets. Impact can include unauthorized tunnel reconfiguration, traffic disruption, and possible traffic redirection or policy bypass depending on the deployment.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 7)	YES	REMOTE SIMPLEFIX NETWORK HARDWARE IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

* Security-boundary bypass with practical sensitive-object access: +2
  The bug bypasses the intended CAP_NET_ADMIN check for the tunnel link namespace and allows a caller privileged only in dev_net(dev) to modify a tunnel associated with another netns.

* Cross-boundary isolation/policy bypass: +1
  This is a namespace/capability isolation failure. It can matter for containers, delegated network administration, or reduced-capability service accounts.

* Practical exploit chain demonstrated or strongly implied: +1
  The commit directly describes the chain: device netns and tunnel link netns differ, rtnl checks only dev_net(dev), and the caller can rewrite a tunnel in t->net without the needed capability there.

* Integrity impact plausible: +1
  Unauthorized tunnel reconfiguration can alter routing, tunnel endpoints, keys/options, or policy-relevant tunnel behavior.

* Availability impact realistic: +1
  Malicious or accidental tunnel rewriting can disrupt connectivity or break IPsec/VTI routing paths.

* Requires CAP_NET_ADMIN in one namespace: -1 conservative only
  This is not ordinary unprivileged local access, but the full -2 penalty is too harsh because the bug specifically bypasses a capability boundary between namespaces.

## 3. Reachability analysis

The trigger is local rtnetlink changelink control-plane access, not packet traffic, so it is not directly network reachable. Reliable triggering requires CAP_NET_ADMIN in the device namespace, but not in the tunnel link namespace being affected. This makes the issue relevant for containers, network namespaces, and delegated network-management services where CAP_NET_ADMIN is intentionally scoped. The affected VTI6 path is configuration-dependent, but it is a real kernel networking feature and should not be treated as a rare hardware-only issue.

Call-site confidence: high. The commit message and patch directly identify vti6_changelink(), the missing namespace capability check, and the exact authorization gap.

## 4. Severity interpretation

This is not memory corruption and does not imply kernel code execution. Realistic impact is an authorization and namespace isolation bypass that can allow unauthorized tunnel reconfiguration, traffic disruption, or policy bypass. It behaves stronger than an ordinary Moderate because it crosses a Linux capability namespace boundary, but it is below a strong Important memory-corruption case because exploitation requires a specific VTI6 cross-netns setup and scoped CAP_NET_ADMIN access.

## 5. One-sentence report phrase

ip6_vti changelink lacked a CAP_NET_ADMIN check for the tunnel link namespace, allowing a caller with delegated CAP_NET_ADMIN in another netns to rewrite VTI6 tunnel configuration across a namespace boundary and potentially cause traffic redirection or denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a practical capability and namespace boundary bypass, and the real severity depends on whether the product uses containers, delegated CAP_NET_ADMIN, or VTI6 tunnels across network namespaces.

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

Patch: net: ip6_vti: require CAP_NET_ADMIN in the device netns for changelink
Commit: f5254e7766b4ac02b66b2ccef896cd278503cb11
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5254e7766b4ac02b66b2ccef896cd278503cb11

Commit description:

vti6_changelink() operates on at most two netns, dev_net(dev) and the
tunnel link netns t->net. They differ once the device is created in or
moved to a netns other than the one the request runs in. The rtnl
changelink path checks CAP_NET_ADMIN only against dev_net(dev), so a
caller privileged there but not in t->net can rewrite a tunnel that
lives in t->net.

Gate vti6_changelink() on rtnl_dev_link_net_capable() at its top,
before any attribute is parsed.

Reported-by: Xiao Liang <shaw.leon@gmail.com>
Closes: https://lore.kernel.org/netdev/CABAhCOSzP1vaThGV35_VnsRCb=87_CPjPVsTHbq905k8A+BuUg@mail.gmail.com/
Fixes: 61220ab ("vti6: Enable namespace changing")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260612085941.3158249-7-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=f5254e7766b4ac02b66b2ccef896cd278503cb11

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

The original Linux kernel CVE announcement for CVE-2026-72055 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72055

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:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
  The Best / paranoid CVSS score: 7.3

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: 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).