[CVE-2026-63919][MODERATE 7.0] xfrm: input: hold netns during deferred transport reinjection [ Upstream

[CVE-2026-63919][MODERATE 7.0] xfrm: input: hold netns during deferred transport reinjection [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63919
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: input: hold netns during deferred transport reinjection [ Upstream
Commit: 7ee59eda8820b758ed29e1cd3222359c7b97302c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7ee59eda8820b758ed29e1cd3222359c7b97302c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63919
Analysis date: Sun, 19 Jul 2026 19:31:25 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A deferred XFRM transport reinjection path stored a `struct net` pointer without holding a netns reference, allowing namespace teardown before callback execution and creating a local race-dependent UAF candidate with likely DoS impact and possible higher impact requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-63919 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63919

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
======================================================================

CVE-2026-63919	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'Deferred XFRM transport reinjection stores a struct net pointer in skb cb and uses it later from xfrm_trans_reinject. Without holding a netns reference, the namespace can be torn down before the deferred callback runs, leaving a stale kernel pointer and creating a possible use after free condition. For the CVSS the PR:L is used because reliable triggering likely requires local code able to create or tear down network namespaces and exercise XFRM paths, which may be available to an unprivileged local user on systems with user namespaces enabled. The issue is not directly network reachable as a standalone remote attack because namespace lifetime control is local, although packet processing can be part of the trigger path. Impact is at least a local denial of service via kernel crash and in the paranoid case may allow confidentiality or integrity impact due to the UAF class, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX SKB HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  Likely reachable by local code that can create or tear down network namespaces and exercise XFRM/IPsec transport paths. On systems with unprivileged user namespaces enabled, this may be available without full host root.
* Memory corruption, weak primitive: +1
  The bug is a stale `struct net` pointer kept in `skb->cb` across deferred work. This is a UAF candidate, but no controlled reclaim, overwrite, type confusion, or arbitrary write is shown.
* Real lifetime corruption: +1
  The patch explicitly fixes a missing netns reference across async deferred reinjection work.
* Reliable kernel crash / strong DoS: +1
  A stale `struct net` used in the reinjection callback can plausibly crash the kernel.
* XFRM packet-processing path exposure: +1
  XFRM/IPsec is a networking packet-processing path. It is configuration-dependent, but should not be treated as a rare debug-only subsystem.
* Hard race / teardown timing required: -1
  Triggering requires a netns lifetime race against deferred reinjection.

Paranoid adjustment:

* Do not subtract the race point in the paranoid score because namespace teardown plus queued deferred work can be a realistic lifetime pattern under stress or crafted local workloads.

No additional LPE bonus is awarded conservatively because the patch does not show attacker-controlled reclaim, object replacement, callback control, refcount takeover, or write-after-free.

## 3. Reachability analysis

The bug is not directly remote-network exploitable as a standalone issue. Packet processing is part of the affected path, but the key condition is local control over namespace lifetime and XFRM transport reinjection timing.

Typical reliable triggering likely requires local code with the ability to create or destroy network namespaces and configure or exercise XFRM/IPsec paths. In host configurations where unprivileged user namespaces are enabled, a local unprivileged user may obtain namespace-local capabilities sufficient to reach relevant networking setup paths. If user namespaces are disabled and XFRM setup requires host `CAP_NET_ADMIN`, practical reachability is significantly lower.

The affected subsystem is not enabled in every deployment, but XFRM/IPsec is a real production networking feature, not a debug-only or rare hardware path.

Call-site confidence: high. The provided patch shows both the queueing site and the deferred callback consuming the stored `struct net` pointer.

## 4. Severity interpretation

This behaves above an ordinary Moderate because it is a real async lifetime bug in networking code and creates a plausible UAF condition on `struct net`.

Realistic impact is most clearly local DoS through kernel crash. Theoretical impact could be higher because stale namespace pointers are security-sensitive kernel objects, but the patch does not demonstrate controlled reuse or a concrete privilege-escalation primitive. Therefore the conservative score stays borderline/manual-review, while the paranoid score reaches Actionable Moderate.

This should not be auto-closed solely as a DoS cleanup.

## 5. One-sentence report phrase

A deferred XFRM transport reinjection path stored a `struct net` pointer without holding a netns reference, allowing namespace teardown before callback execution and creating a local race-dependent UAF candidate with likely DoS impact and possible higher impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real lifetime/UAF-class bug in a networking packet-processing path, with namespace-mediated local reachability and plausible but unproven impact beyond DoS.

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

Patch: xfrm: input: hold netns during deferred transport reinjection [ Upstream
Commit: 7ee59eda8820b758ed29e1cd3222359c7b97302c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7ee59eda8820b758ed29e1cd3222359c7b97302c

Changed files:
  net/xfrm/xfrm_input.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=7ee59eda8820b758ed29e1cd3222359c7b97302c

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

The original Linux kernel CVE announcement for CVE-2026-63919 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63919

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).