[CVE-2026-53221][MODERATE 7.0] ip6_vti: fix incorrect tunnel matching in vti6_tnl_lookup() [ Upstream

[CVE-2026-53221][MODERATE 7.0] ip6_vti: fix incorrect tunnel matching in vti6_tnl_lookup() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53221
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ip6_vti: fix incorrect tunnel matching in vti6_tnl_lookup() [ Upstream
Commit: c327fa4fca31415431202e063767a7ae342e19c6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c327fa4fca31415431202e063767a7ae342e19c6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53221
Analysis date: Thu, 25 Jun 2026 06:32:57 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Missing wildcard endpoint validation in vti6_tnl_lookup() can cause IPv6 VTI traffic to match the wrong tunnel under hash collision conditions, potentially bypassing intended tunnel routing or isolation policy in exposed IPsec/VTI deployments.

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

The original Linux kernel CVE announcement for CVE-2026-53221 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53221

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-2026-53221	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';CWE-697;CWE-284;CWE-693;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.6';DESCR 'ip6_vti can select the wrong VTI6 tunnel because fallback lookup for wildcard endpoints did not verify that the candidate tunnel actually has a wildcard remote or wildcard local address. A remote packet can reach this lookup path when IPv6 VTI or IPsec tunnel endpoints are exposed, but reliable impact depends on the target tunnel configuration and on hash collision conditions. For the CVSS the PR:N is used because the attacker does not need local code execution on the victim when the affected tunnel endpoint is reachable over the network. AC:H is used because exploitation depends on non trivial tunnel layout, wildcard fallback behavior, and hash bucket collision. Impact is mainly a network policy or routing integrity issue, with possible limited confidentiality impact in multi tunnel deployments, but there is no evidence of kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE OOB SIMPLEFIX NETWORK LINUS IPV6 PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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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:

* Remote reachable / network-triggerable: +2. The lookup can be reached by packets hitting an exposed IPv6 VTI/IPsec tunnel endpoint.
* Cross-boundary isolation/policy bypass: +1. Incorrect tunnel selection can violate intended tunnel separation or routing policy.
* Integrity impact plausible: +1. Packets may be classified into the wrong tunnel path or policy domain.
* Hard configuration / collision-dependent condition: -1. Reliable impact depends on specific VTI6 tunnel configuration, wildcard fallback behavior, and hash collision conditions.

Conservative total: 3.

Paranoid additional signals:

* Confidentiality impact plausible: +1. In multi-tunnel deployments, wrong tunnel matching may expose traffic or metadata to the wrong logical tunnel context.
* Common networking packet-processing path: +1. This is in the IPv6 VTI tunnel receive/lookup path, not a purely local admin-only control path.

Paranoid total: 5.

Not counted:

* No generic memory corruption. The patch adds missing address wildcard checks and does not show UAF, OOB write, double-free, type confusion, or refcount misuse.
* No privilege escalation signal. The bug is a tunnel matching logic error, not a demonstrated kernel memory corruption primitive.
* No strong DoS signal. Misrouting or policy mismatch may disrupt traffic, but the patch does not show a reliable crash, panic, or resource exhaustion path.

## 3. Reachability analysis

A remote peer can potentially trigger the lookup path by sending traffic to a reachable IPv6 VTI/IPsec endpoint. No local account on the victim is required if the tunnel endpoint is exposed, so the practical PR interpretation is PR:N for the network-facing case.

The issue is configuration-dependent. It requires VTI6/IPsec tunnels, a wildcard fallback lookup path, and a colliding or otherwise co-located candidate tunnel in the same hash table. Namespaces or containers mainly matter if they can create or influence tunnel configuration, but packet-triggering itself can be remote once the tunnel exists.

VTI6 is not a universal default Internet-facing service, but XFRM/IPsec/VTI is security-sensitive networking infrastructure and should not be treated as a low-priority obscure subsystem when exposed.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than ordinary Low/Moderate because it is a network-reachable tunnel matching bug with possible policy-boundary impact.

It is not an Important-class memory corruption issue on the provided evidence. The theoretical worst case is limited confidentiality and integrity impact through incorrect tunnel selection in a multi-tunnel deployment. Realistic exploitation depends strongly on deployment layout and collision conditions.

## 5. One-sentence report phrase

Missing wildcard endpoint validation in vti6_tnl_lookup() can cause IPv6 VTI traffic to match the wrong tunnel under hash collision conditions, potentially bypassing intended tunnel routing or isolation policy in exposed IPsec/VTI deployments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

This is network-reachable in configured VTI6/IPsec deployments and affects tunnel selection semantics, so it should not be auto-closed even though there is no evidence of kernel memory corruption or privilege escalation.

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

Patch: ip6_vti: fix incorrect tunnel matching in vti6_tnl_lookup() [ Upstream
Commit: c327fa4fca31415431202e063767a7ae342e19c6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c327fa4fca31415431202e063767a7ae342e19c6

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=c327fa4fca31415431202e063767a7ae342e19c6

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

The original Linux kernel CVE announcement for CVE-2026-53221 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53221

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.6

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).