[CVE-2026-80846][MODERATE 7.0] xfrm: drop ESP-in-TCP packets with no ingress device

[CVE-2026-80846][MODERATE 7.0] xfrm: drop ESP-in-TCP packets with no ingress device

[AL-KERNEL]

CVE: CVE-2026-80846
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: drop ESP-in-TCP packets with no ingress device
Commit: 239d0f71af09dc2029fd4d730cb24b8c83aaa43a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=239d0f71af09dc2029fd4d730cb24b8c83aaa43a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80846
Analysis date: Fri, 04 Sep 2026 13:25:57 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended. Actionable Moderate if ESP-in-TCP is exposed
Manual review required: YES

Summary:
Queued ESP-in-TCP records can be processed after the saved ingress device has been removed, causing handle_esp to pass a NULL skb dev into the XFRM receive path and triggering a network reachable kernel crash under device or namespace teardown races.

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

The original Linux kernel CVE announcement for CVE-2026-80846 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80846

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-2026-80846	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80846 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:N/I:N/A:H';**CWE-476;CWE-362;CWE-754;BEST CVSS score: '5.9';DESCR 'ESP-in-TCP can process queued TCP records after the original ingress device has been removed during veth or network namespace teardown. In handle_esp, dev_get_by_index_rcu can then return NULL and the XFRM IPv4 or IPv6 receive path may dereference skb dev while building route lookup data, causing a kernel crash. For the CVSS the PR:N is used because a network peer does not need a local user account on the victim when the ESP-in-TCP endpoint is reachable. The attack still has high complexity because it depends on an ESP-in-TCP deployment and a race with device or namespace teardown. The issue is network reachable in configured IPsec ESP-in-TCP environments, but practical exploitation is more likely inside VPN, storage, container, or data center networks than from the public Internet. Impact is denial of service via NULL pointer dereference and kernel crash. No concrete evidence of UAF, information disclosure, arbitrary write, or privilege escalation is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended. Actionable Moderate if ESP-in-TCP is exposed (with actual score 4)	YES	REMOTE SIMPLEFIX NETWORK SKB HARDWARE LINUS IPV6 KPANIC PACKET  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended. Actionable Moderate if ESP-in-TCP is exposed**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. ESP-in-TCP records arrive through TCP strparser from a network peer when an ESP-in-TCP endpoint is configured and reachable.
* Reliable kernel crash / strong DoS: +1. The patch prevents a NULL skb->dev from reaching xfrm4_rcv_encap or xfrm6_rcv_encap, where route lookup code expects a valid ingress device and can crash.
* Common networking core path or packet-processing path: +1. XFRM/IPsec receive handling is a kernel networking packet path, although not necessarily enabled by default.
* Hard or unreliable race / special timing required: -1. Triggering depends on queued TCP data being processed after ingress device or network namespace teardown.

Paranoid additional signal:

* Availability impact realistic: +1. In exposed ESP-in-TCP deployments, the failure mode is a host kernel crash, not merely a local warning or recoverable protocol error.

Not awarded:

* No generic memory corruption. The patch supports a NULL pointer dereference, not UAF, OOB write, double free, type confusion, or attacker-controlled overwrite.
* No privilege escalation plausible. There is no demonstrated reclaim, stale object reuse, callback control, refcount takeover, or arbitrary write.
* No confidentiality or integrity impact. The observed primitive is DoS only.

Call-site confidence: high. The patch directly shows handle_esp assigning skb->dev and then passing the skb into xfrm4_rcv_encap or xfrm6_rcv_encap.

## 3. Reachability analysis

A remote peer can influence ESP-in-TCP packet delivery if the host has an ESP-in-TCP IPsec endpoint configured and reachable. For the CVSS the PR:N interpretation is reasonable because the peer does not need a local account on the victim when the endpoint is exposed.

The practical trigger is still timing-dependent. Queued TCP data must be processed after the original ingress device is removed, for example during veth or network namespace teardown. If the attacker cannot influence teardown, exploitation may be opportunistic or require a local/container-side actor that can create traffic while a namespace or veth is being removed.

The path is not a default Internet-facing service on most systems. It is more plausible in VPN, container, cloud, or data-center environments using IPsec ESP-in-TCP.

## 4. Severity interpretation

This behaves like a network-reachable DoS with high attack complexity, not an Important-class memory corruption issue. Theoretical escalation is not supported by the patch because the device lookup returns NULL and the failure is a NULL dereference before route lookup, not stale pointer reuse.

Conservative handling is Borderline Moderate with manual review recommended. If ESP-in-TCP is exposed to untrusted peers in production, it becomes an Actionable Moderate because a network peer may crash the kernel during teardown races.

## 5. One-sentence report phrase

Queued ESP-in-TCP records can be processed after the saved ingress device has been removed, causing handle_esp to pass a NULL skb dev into the XFRM receive path and triggering a network reachable kernel crash under device or namespace teardown races.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the issue is network reachable in configured ESP-in-TCP deployments and can cause a kernel crash, but exposure depends strongly on IPsec configuration and teardown timing.

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

Patch: xfrm: drop ESP-in-TCP packets with no ingress device
Commit: 239d0f71af09dc2029fd4d730cb24b8c83aaa43a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=239d0f71af09dc2029fd4d730cb24b8c83aaa43a

Commit description:

ESP-in-TCP receives records through the TCP strparser. handle_esp()
restores skb->dev from the saved skb_iif before passing the packet into
the XFRM input path.

Queued TCP data can be processed after the original ingress device has
been removed, for example during veth or net namespace teardown. In that
case dev_get_by_index_rcu() returns NULL. The XFRM IPv4 and IPv6 input
paths both expect skb->dev to be valid while building the route lookup,
so queued ESP-in-TCP data can dereference a NULL device.

Drop the packet if the saved ingress device can no longer be resolved.
Such a packet can no longer be routed through the normal XFRM receive
path, and this preserves the existing behaviour for packets whose ingress
device still exists.

Fixes: e27cca9 ("xfrm: add espintcp (RFC 8229)")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <roxy520tt@gmail.com>
Assisted-by: Codex:gpt-5.4
Reviewed-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/xfrm/espintcp.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=239d0f71af09dc2029fd4d730cb24b8c83aaa43a

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

The original Linux kernel CVE announcement for CVE-2026-80846 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80846

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: Not available
  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: 3
  Paranoid ActionableScore: 4

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).