[CVE-2026-68426][IMPORTANT] xfrm: fix stale skb->prev after async crypto steals a GSO segment [ Upstream

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

CVE: CVE-2026-68426
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: xfrm: fix stale skb->prev after async crypto steals a GSO segment [ Upstream
Commit: 33e1b0d25ca0d2818c635ff80e6aa0d295e08a98
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=33e1b0d25ca0d2818c635ff80e6aa0d295e08a98
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68426
Analysis date: Mon, 10 Aug 2026 14:37:30 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale `skb->prev` pointer in the XFRM GSO transmit path can survive async crypto segment stealing and later cause `validate_xmit_skb_list()` to write through a freed skb pointer, creating a network-influenced UAF write with likely DoS and possible memory corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68426 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68426

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
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CVE-2026-68426	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-787;CWE-362;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free write can occur in the XFRM transmit path when async crypto steals a GSO segment but the list head still keeps skb prev pointing to the stolen last segment. Later validate_xmit_skb_list can treat this stale pointer as the list tail and write tail next, which may touch memory already owned by the crypto engine or already freed. For the CVSS the PR:N is used for the paranoid score because traffic may be supplied by a remote peer or forwarded flow when the target is configured with XFRM async crypto and GSO. The attack complexity is high because it depends on a specific XFRM configuration, GSO segmentation, async crypto ownership transfer, and the stolen segment being the tail segment. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption. This is network stack memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	UAF SKB LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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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

Conservative signals:

* Remote reachable / network-triggerable: +2. The bug is in the XFRM transmit path and may be reachable via traffic processed by an IPsec/XFRM endpoint or gateway, especially for forwarded or peer-supplied flows.
* Memory corruption, strong corruption primitive: +2. The commit explicitly describes a stale `skb->prev` pointer and a later `tail->next = skb` write through that stale pointer.
* Real lifetime corruption: +1. The stolen segment is owned by async crypto and may be freed while the list head still points to it.
* Reliable kernel crash / strong DoS: +1. A write through a freed or foreign skb list element is a credible kernel crash path.
* Weak LPE concern: +1. This is not just a NULL dereference. It is a write-after-free style primitive, but target and payload control are not demonstrated.
* Common packet-processing path: +1. XFRM/IPsec is a network packet-processing path and should not be treated as a purely obscure local-only subsystem.
* Hard or unreliable triggering conditions: -1. Triggering requires GSO segmentation, async crypto stealing, and specifically the stolen segment being the tail segment.
* No attacker-controlled payload or overwrite target shown: -1. The write is a kernel-derived pointer assignment to `tail->next`, not a demonstrated arbitrary write.

Paranoid adjustment:

* Confidentiality impact plausible: +1 and integrity impact plausible: +1 are defensible only for paranoid triage because this is a UAF write in networking memory. However these are not strongly demonstrated primitives, so they support manual review rather than assuming reliable code execution.

## 3. Reachability analysis

A remote peer may be able to influence packet flows that pass through an affected XFRM/IPsec transmit path, especially on gateways or hosts using XFRM offload or async crypto with GSO. This is not a generic Internet-wide bug unless the target is configured to process attacker-influenced traffic through XFRM with async crypto. Local users may also trigger it by generating suitable outbound traffic if they can use the configured XFRM path.

Namespaces and containers may matter if container workloads can generate traffic through host-managed XFRM/IPsec policy, but the patch does not prove a container escape primitive. The path is configuration-dependent, but XFRM/IPsec is a real production networking subsystem, so I would not apply a rare-subsystem downgrade.

Call-site confidence: high. The commit directly states that `validate_xmit_skb_list()` later relies on `skb->prev` as the tail and performs `tail->next = skb`.

## 4. Severity interpretation

This behaves above ordinary Moderate because the patch fixes a concrete stale pointer lifetime bug with a write through the stale pointer. Realistic exploitation is likely difficult and configuration-dependent, so the conservative view is not Critical. Theoretical impact includes memory corruption and possible privilege escalation or stronger kernel compromise, but the demonstrated evidence is primarily UAF write leading to crash or corruption.

Practical classification: Strong Important candidate, or at minimum actionable Moderate that should not be auto-closed.

## 5. One-sentence report phrase

A stale `skb->prev` pointer in the XFRM GSO transmit path can survive async crypto segment stealing and later cause `validate_xmit_skb_list()` to write through a freed skb pointer, creating a network-influenced UAF write with likely DoS and possible memory corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network packet-processing lifetime bug with an explicit use-after-free write primitive. Even though reliable exploitation is complex and configuration-dependent, it is not safe to auto-close as ordinary DoS.

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

Patch: xfrm: fix stale skb->prev after async crypto steals a GSO segment [ Upstream
Commit: 33e1b0d25ca0d2818c635ff80e6aa0d295e08a98
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=33e1b0d25ca0d2818c635ff80e6aa0d295e08a98

Changed files:
  net/xfrm/xfrm_device.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=33e1b0d25ca0d2818c635ff80e6aa0d295e08a98

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

The original Linux kernel CVE announcement for CVE-2026-68426 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68426

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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).