[CVE-2026-46172][MODERATE 7.0] ipv6: xfrm6: release dst on error in xfrm6_rcv_encap()

[CVE-2026-46172][MODERATE 7.0] ipv6: xfrm6: release dst on error in xfrm6_rcv_encap()

[AL-KERNEL]

CVE: CVE-2026-46172
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: xfrm6: release dst on error in xfrm6_rcv_encap()
Commit: c2efc4956981066df2fef1cc77391b523db6d8e4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c2efc4956981066df2fef1cc77391b523db6d8e4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46172
Analysis date: Thu, 28 May 2026 11:03:45 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
`xfrm6_rcv_encap()` leaked referenced IPv6 `dst` entries on error routes, allowing repeated ESP UDPv6 encapsulated packets to cause resource-exhaustion DoS in configured XFRM deployments.

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

The original Linux kernel CVE announcement for CVE-2026-46172 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46172

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: NO

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-2026-46172	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:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'xfrm6_rcv_encap leaked a referenced dst entry when IPv6 route lookup returned an error route for an encapsulated XFRM packet. A remote peer can contribute to the leak by sending packets that hit the XFRM UDPv6 encapsulation path and resolve to an error route, causing dropped packets to retain dst references. For the CVSS the PR:N is used because the network peer does not need local privileges on the victim once the affected XFRM path is configured. The issue is network reachable only for systems using IPv6 XFRM or ESP UDP encapsulation and matching error route conditions. Impact is availability only through resource exhaustion, with no supported memory corruption, information disclosure, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	REMOTE SIMPLEFIX LEAK NETWORK SKB LINUS IPV6 KPANIC PACKET  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A remote peer can send IPv6 ESP UDP encapsulated packets that reach `xfrm6_rcv_encap()`.
* Availability impact realistic: +1. Repeated packets can leak referenced `dst` entries and consume kernel routing resources.
* Common networking / XFRM packet path: +1. XFRM/IPsec is a network packet-processing path, and remote resource leaks there deserve triage.

Paranoid additional signal:

* Reliable resource-exhaustion DoS: +1. If the error-route condition is repeatable, sustained traffic can exhaust `dst` references or memory and degrade the network namespace or host.

Not counted:

* No memory corruption.
* No UAF, double free, OOB write, or arbitrary write.
* No information disclosure.
* No privilege escalation.
* No authentication or policy bypass.

## 3. Reachability analysis

The issue is reachable through the IPv6 XFRM UDP encapsulation receive path. The system must have relevant XFRM/ESP UDPv6 encapsulation configuration, and packets must resolve to an error route after `ip6_route_input_lookup()`.

The attacker does not need local privileges on the victim if they can send matching packets to the configured path. This is not a generic IPv6 bug affecting all systems, but it is network-triggerable in configured IPsec/XFRM deployments. Namespaces matter if XFRM policies are configured inside network namespaces.

## 4. Severity interpretation

This behaves like an actionable Moderate availability issue. The realistic impact is resource exhaustion from leaked `dst` references.

It does not behave like an Important memory-corruption vulnerability. The primitive is a reference leak on an error path, and the patch supports DoS only.

## 5. One-sentence report phrase

`xfrm6_rcv_encap()` leaked referenced IPv6 `dst` entries on error routes, allowing repeated ESP UDPv6 encapsulated packets to cause resource-exhaustion DoS in configured XFRM deployments.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review is recommended for products using IPv6 XFRM or ESP UDP encapsulation, because the issue is network-triggerable in that configuration. The supported impact is availability-only resource exhaustion.

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

Patch: ipv6: xfrm6: release dst on error in xfrm6_rcv_encap()
Commit: c2efc4956981066df2fef1cc77391b523db6d8e4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c2efc4956981066df2fef1cc77391b523db6d8e4

Commit description:

xfrm6_rcv_encap() performs an IPv6 route lookup when the skb does not
already have a dst attached. ip6_route_input_lookup() returns a
referenced dst entry even when the lookup resolves to an error route.

If dst->error is set, xfrm6_rcv_encap() drops the skb without attaching
the dst to the skb and without releasing the reference returned by the
lookup. Repeated packets hitting this path therefore leak dst entries.

Release the dst before jumping to the drop path.

Fixes: 0146dca ("xfrm: add support for UDPv6 encapsulation of ESP")
Cc: stable@kernel.org
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Suggested-by: Xin Liu <bird@lzu.edu.cn>
Tested-by: Ruide Cao <caoruide123@gmail.com>
Signed-off-by: Yilin Zhu <zylzyl2333@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/xfrm6_protocol.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=c2efc4956981066df2fef1cc77391b523db6d8e4

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

The original Linux kernel CVE announcement for CVE-2026-46172 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46172

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:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 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: 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).