[CVE-2026-80792][IMPORTANT] ipv6: fix use-after-free in ip6_finish_output2()

[CVE-2026-80792][IMPORTANT] ipv6: fix use-after-free in ip6_finish_output2()

[AL-KERNEL]

CVE: CVE-2026-80792
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: fix use-after-free in ip6_finish_output2()
Commit: 75e0a544ebe9af663ef53ca21e9e9185c51fb54a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75e0a544ebe9af663ef53ca21e9e9185c51fb54a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80792
Analysis date: Fri, 04 Sep 2026 12:10:05 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-reachable IPv6 LWT path can use a stale daddr pointer after skb head reallocation in ip6_finish_output2(), causing a UAF read that may crash the kernel, leak kernel data, pollute neighbour entries, or misdirect traffic.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80792 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80792

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-2026-80792	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80792 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-825;*CWE-200;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'ip6_finish_output2 caches a pointer to the IPv6 destination address before calling lwtunnel_xmit. LWT BPF or encapsulation processing can reallocate the skb head and free the memory backing the cached pointer, but execution may continue when LWTUNNEL_XMIT_CONTINUE is returned. Subsequent nexthop selection and neighbour lookup then use a stale daddr pointer, creating a UAF read condition that can crash the kernel, leak kernel memory through derived address data, pollute neighbour entries, or misdirect traffic. For the CVSS the PR:N is selected for the paranoid network scenario because a remote packet may trigger the vulnerable forwarding or output path once an affected LWT route is configured. The issue is network reachable only on systems using IPv6 LWT BPF or similar lwtunnel encapsulation and is more likely in router, overlay, or data center networking deployments than on default hosts.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE OOB LEAK UAF NETWORK SKB KERNEL_PANIC_PLUS_UAF IPV6 KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  A remote packet can plausibly traverse the affected IPv6 output or forwarding path when IPv6 LWT, LWT-BPF, or similar lwtunnel encapsulation is configured.

* Memory corruption, strong primitive: +2
  The patch and description explicitly identify a use-after-free read caused by using a cached pointer into an skb head after lwtunnel_xmit() may reallocate and free that memory.

* Real lifetime corruption: +1
  This is a stale pointer after free / skb lifetime bug, not just a validation or bookkeeping issue.

* Reliable kernel crash / strong DoS: +1
  The stale daddr pointer can lead to invalid neighbour lookup behavior and crash the system.

* Confidentiality impact plausible: +1
  The stale daddr value is derived from freed kernel memory and may leak sensitive kernel data through address-dependent behavior.

* Integrity impact plausible: +1
  The stale address can pollute neighbour table entries or misdirect traffic.

* Common networking core / packet-processing path: +1
  ip6_finish_output2() is in the IPv6 packet output path, although the vulnerable trigger depends on LWT behavior.

Paranoid-only interpretation:

* Weak LPE concern / broader impact sensitivity: +1
  Not because privilege escalation is demonstrated, but because this is a network-reachable kernel UAF read with both confidentiality and integrity consequences claimed by the bug context. Manual review should not auto-close it as DoS-only.

No subtraction applied for configuration rarity because this is still an IPv6 networking path, and the guidance says not to heavily penalize core networking paths merely due to runtime configuration requirements.

## 3. Reachability analysis

The bug can be triggered by traffic that enters an affected IPv6 route using LWT, LWT-BPF, or another lwtunnel encapsulation path capable of reallocating the skb head and returning LWTUNNEL_XMIT_CONTINUE. The attacker does not need local code execution if the affected route is already configured and reachable over the network.

The configuration itself normally requires administrative privileges to create. However, CVSS-style PR for the attacking packet can still be PR:N in the network scenario because the remote attacker is not required to configure the route, only to reach a preconfigured vulnerable path.

This is not a default exposure on ordinary hosts. It is more realistic on routers, overlay-network nodes, data-center networking systems, or hosts using IPv6 LWT/BPF encapsulation.

## 4. Severity interpretation

This behaves like an Important-class candidate rather than an ordinary Moderate issue. The realistic impact includes remote kernel crash, neighbour table corruption, traffic misdirection, and possible kernel data exposure.

Theoretical privilege escalation is not demonstrated by the patch. However, the combination of network reachability, explicit UAF read, stale skb memory use, confidentiality impact, and integrity impact makes this unsafe to auto-close as DoS-only.

## 5. One-sentence report phrase

A network-reachable IPv6 LWT path can use a stale daddr pointer after skb head reallocation in ip6_finish_output2(), causing a UAF read that may crash the kernel, leak kernel data, pollute neighbour entries, or misdirect traffic.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable kernel UAF read in a core IPv6 output path with plausible confidentiality, integrity, and availability impact. The actual exposure depends on LWT or LWT-BPF route configuration, but the primitive is strong enough that it should not be auto-closed.

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

Patch: ipv6: fix use-after-free in ip6_finish_output2()
Commit: 75e0a544ebe9af663ef53ca21e9e9185c51fb54a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75e0a544ebe9af663ef53ca21e9e9185c51fb54a

Commit description:

ip6_finish_output2() caches a pointer to the IPv6 destination
address (daddr) before invoking lwtunnel_xmit().  The LWT-BPF
transmit path or other encapsulation operations within
lwtunnel_xmit() can reallocate the skb head, freeing the memory
that daddr points to.  When lwtunnel_xmit() returns
LWTUNNEL_XMIT_CONTINUE, the function continues to use the stale
daddr pointer to compute the nexthop and to look up or create the
neighbour entry.  This results in a use-after-free read, which can
leak sensitive kernel data, pollute the neighbour table with
arbitrary values, misdirect traffic, or crash the system.

Fix this by re-fetching the IPv6 header and the destination
address pointer after lwtunnel_xmit() returns
LWTUNNEL_XMIT_CONTINUE, ensuring that the subsequent nexthop
computation and neighbour lookup operate on valid memory.

Fixes: e415ed3 ("ipv6: use skb_expand_head in ip6_finish_output2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/4aa3f53bc44e79572c6dd2340ec7b68ef1a3d87d.1786516730.git.rakukuip@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/ip6_output.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=75e0a544ebe9af663ef53ca21e9e9185c51fb54a

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

The original Linux kernel CVE announcement for CVE-2026-80792 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80792

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: 8
  Paranoid ActionableScore: 9

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).