[CVE-2026-63921][IMPORTANT] ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().

[CVE-2026-63921][IMPORTANT] ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().

[AL-KERNEL]

CVE: CVE-2026-63921
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().
Commit: 94ff740a7f9ef5c010784a325dca00cbf228f941
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94ff740a7f9ef5c010784a325dca00cbf228f941
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63921
Analysis date: Sun, 19 Jul 2026 20:17:38 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A namespace confusion flaw in the IPv6 VTI SIOCCHGTUNNEL path can let a local attacker in an unprivileged user and network namespace redirect creation netns VTI6 lookups to an attacker controlled migrated device, potentially exposing or disrupting matched XFRM traffic across container or tenant boundaries.

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

The original Linux kernel CVE announcement for CVE-2026-63921 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63921

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: IMPORTANT
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
======================================================================

CVE-2026-63921	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	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-284;CWE-863;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L';BEST CVSS score: '8.8';DESCR 'A namespace confusion bug in the IPv6 VTI SIOCCHGTUNNEL path can use dev_net(dev) for collision lookup even though the migrated tunnel is updated through its original t net namespace. A local attacker with access to an unprivileged user namespace and a network namespace can operate from the migrated netns and choose tunnel parameters that collide with a tunnel in the creation netns. This can make lookups in the creation netns resolve to an attacker controlled migrated device and can cause xfrm receive traffic to be delivered through that device. For the CVSS the PR:L is used because reliable triggering requires local code execution and namespace operations, but not full host administrative privileges. The issue is not directly reachable from the external network as a packet only attack, but it can cross tenant boundaries on container hosts where unprivileged user namespaces and VTI6 namespace migration are available. Impact can include confidentiality and integrity loss for matched tunnel traffic, with possible availability impact from misrouting or disruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE NETWORK HARDWARE IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  The commit explicitly says reachable from an unprivileged user namespace with `unshare --user --map-root-user --net`.

* Security-boundary bypass with practical sensitive-object access: +2
  The bug bypasses the intended namespace and capability boundary by allowing a caller in the migrated netns to affect lookup behavior in the creation netns.

* Cross-boundary isolation/policy bypass: +1
  The commit explicitly calls out cross tenant scope on container hosts.

* Practical exploit chain strongly implied: +1
  The commit describes a concrete chain: migrated tunnel lookup mismatch, hash insertion in the creation netns, later lookup resolving to the attacker controlled device, and xfrm receive delivering matched packets through it.

* Confidentiality impact plausible: +1
  Matched IPsec/VTI traffic may be delivered through an attacker controlled device, making traffic exposure plausible in affected deployments.

* Integrity impact plausible: +1
  Redirecting matched tunnel traffic can plausibly alter routing or delivery semantics across namespace boundaries.

* Availability impact realistic: +1 only in paranoid score
  Misrouting or tunnel lookup hijacking can disrupt traffic. I would count this in the paranoid score, but not necessarily in the conservative score because the main demonstrated impact is cross namespace traffic redirection rather than a guaranteed system-wide DoS.

* Requires CAP_NET_ADMIN penalty: 0
  Although tunnel ioctls normally involve CAP_NET_ADMIN, the vulnerable path is specifically reachable from an attacker controlled user namespace and bypasses the creation netns capability check. Applying the full -2 penalty would understate the namespace boundary bypass.

* Memory corruption signals: 0
  This is not a UAF, OOB write, double free, or type confusion issue. It is an access control and namespace confusion bug.

Call-site confidence: high. The commit message and diff directly show the mismatched `dev_net(dev)` lookup versus `self->net` update path.

## 3. Reachability analysis

The bug is locally triggerable by a user who can create an unprivileged user namespace and network namespace and operate on a migrated VTI6 tunnel. It is not a pure remote packet-triggered vulnerability, because the attacker needs local namespace and tunnel control rather than only sending packets from the network.

Namespaces materially reduce the required privilege level. The original check is against `dev_net(dev)->user_ns`, which after migration may be the attacker controlled namespace. The missing check against `self->net->user_ns` lets the attacker influence the original creation namespace without holding CAP_NET_ADMIN there.

The path is configuration dependent because it requires IPv6 VTI, namespace migration, and XFRM/VTI use. However, it should not be treated as a rare low-priority subsystem because the commit explicitly identifies unprivileged userns reachability and cross tenant impact on container hosts.

## 4. Severity interpretation

This behaves more like an Important-class namespace isolation bypass than an ordinary Moderate bug. There is no memory corruption primitive, but the security impact is still meaningful because the bug can cross namespace or tenant boundaries and redirect matched tunnel traffic to an attacker controlled device.

The realistic impact is confidentiality and integrity loss for affected VTI6/XFRM traffic, plus possible availability impact from traffic disruption. The paranoid score reaches 8 because the patch context supports cross tenant scope and practical sensitive traffic redirection, not because of speculative kernel code execution.

## 5. One-sentence report phrase

A namespace confusion flaw in the IPv6 VTI SIOCCHGTUNNEL path can let a local attacker in an unprivileged user and network namespace redirect creation netns VTI6 lookups to an attacker controlled migrated device, potentially exposing or disrupting matched XFRM traffic across container or tenant boundaries.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the patch explicitly describes unprivileged user namespace reachability, cross tenant scope, and a concrete traffic redirection primitive across namespace boundaries.

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

Patch: ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().
Commit: 94ff740a7f9ef5c010784a325dca00cbf228f941
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=94ff740a7f9ef5c010784a325dca00cbf228f941

Commit description:

After patch 1/2 in this series, vti6_update() unlinks and relinks
the tunnel through t->net. vti6_siocdevprivate() still uses
dev_net(dev) for the collision lookup. For a tunnel moved through
IFLA_NET_NS_FD, dev_net(dev) is the new netns, not t->net.

SIOCCHGTUNNEL on a migrated tunnel then runs:

  net = dev_net(dev)                    /* migrated netns */
  t   = vti6_locate(net, &p1, false)    /* misses target in t->net */
  ...
  t   = netdev_priv(dev)
  vti6_update(t, &p1, false)            /* mutates t->net's hash */

A caller in the migrated netns picks params that match a tunnel
in the creation netns. The lookup in dev_net(dev) finds nothing.
vti6_update() prepends the migrated tunnel at the head of the
creation netns hash bucket for those params. Later lookups in
the creation netns resolve to the migrated device. xfrm receive
delivers the matched packets through a device the caller controls.

Reachable from an unprivileged user namespace (unshare --user
--map-root-user --net). Cross tenant scope on container hosts.

Switch the SIOCCHGTUNNEL path on a non fallback device to use
t->net for the lookup. The lookup now matches the netns
vti6_update() operates on.

Also add ns_capable(self->net->user_ns, CAP_NET_ADMIN) before
the lookup. The check at the top of the case is against
dev_net(dev)->user_ns, which after migration is the attacker's
netns. A caller there can pick params absent from self->net,
the lookup returns NULL, t becomes self, and vti6_update()
inserts the device into the creation netns hash. The new check
requires CAP_NET_ADMIN in the creation netns user_ns too.

SIOCADDTUNNEL and SIOCCHGTUNNEL on the fallback device keep
dev_net(dev), which equals init_net there.

Fixes: 61220ab ("vti6: Enable namespace changing")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: Xiao Liang <shaw.leon@gmail.com>
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Link: https://patch.msgid.link/20260521130555.3421684-3-maoyixie.tju@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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=94ff740a7f9ef5c010784a325dca00cbf228f941

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

The original Linux kernel CVE announcement for CVE-2026-63921 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63921

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:H/I:H/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: 7
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).