[CVE-2026-74597][IMPORTANT] ip6_tunnel: clear skb2->cb[] in ip6ip6_err()

[CVE-2026-74597][IMPORTANT] ip6_tunnel: clear skb2->cb[] in ip6ip6_err()

[AL-KERNEL]

CVE: CVE-2026-74597
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ip6_tunnel: clear skb2->cb[] in ip6ip6_err()
Commit: 44fe898df302e91c5ee5acbc71ffa74e78e6c183
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=44fe898df302e91c5ee5acbc71ffa74e78e6c183
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74597
Analysis date: Sat, 22 Aug 2026 14:18:40 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale IPv6 skb control block in `ip6ip6_err()` can cause the ICMPv6 reply path to apply an outer Home Address Option offset to a quoted inner packet, leading to out-of-bounds access and `skb_shared_info` corruption on reachable IPv6-in-IPv6 tunnel endpoints.

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

The original Linux kernel CVE announcement for CVE-2026-74597 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74597

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74597	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-787;CWE-119;*CWE-664;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 stale IPv6 skb control block in ip6ip6_err can make the ICMPv6 reply path reuse outer packet metadata after the skb is pulled to the quoted inner IPv6 packet. If the outer packet carried a Home Address Option, the stale dsthao offset can be applied to attacker controlled inner destination options and mip6_addr_swap can run past the quoted packet and corrupt skb_shared_info. For the CVSS the PR:N is used because a remote attacker does not need local code execution or an authenticated user context when the affected IPv6 tunnel endpoint is reachable. The attack is network reachable but requires a specific IPv6 in IPv6 tunnel and ICMPv6 error handling path, so AC:H is used. Impact is at least remote denial of service via kernel memory corruption and in the paranoid case may include confidentiality and integrity impact because skb_shared_info corruption is a plausible kernel memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE OOB SIMPLEFIX NETWORK SKB IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  The affected path is in IPv6 tunnel ICMPv6 error handling. A remote packet sequence can plausibly reach the vulnerable path when the IPv6-in-IPv6 tunnel endpoint is reachable.

* Generic memory corruption, strong corruption primitive: +2
  The commit explicitly states that stale `dsthao` metadata can make `mip6_addr_swap()` run past the end of the quoted packet and corrupt `skb_shared_info`. This is stronger than a NULL dereference or warning.

* Weak-to-strong LPE concern: +1 conservative, +2 paranoid
  Conservative scoring uses +1 because no reliable exploit primitive is demonstrated. Paranoid scoring uses +2 because corruption of `skb_shared_info` is a known high-value networking memory corruption sink and may plausibly affect kernel object lifetime or skb fragment metadata.

* Reliable kernel crash / strong DoS: +1
  Out-of-bounds access and `skb_shared_info` corruption can realistically crash the kernel.

* Common networking / packet-processing relevance: +1
  Although the exact trigger requires IPv6 tunnel and ICMPv6 error handling, the bug is in the network packet-processing path and affects skb metadata handling.

Subtracted signals:

* Hard or special triggering conditions: -1
  Exploitation requires a specific IPv6-in-IPv6 tunnel error path, an outer IPv6 packet with Home Address Option metadata, and a crafted quoted inner destination-options header.

Not applied:

* No admin privilege penalty. The plausible attacker is remote when the tunnel endpoint is exposed.
* No Dirty-Pipe-like or page-cache signal. This is skb metadata corruption, not file-backed or COW/shared-page corruption.
* No firmware/device-management signal.

## 3. Reachability analysis

A remote attacker may trigger the bug if they can send traffic that reaches an affected IPv6-in-IPv6 tunnel endpoint and can cause the ICMPv6 error handling path in `ip6ip6_err()` to process a cloned skb. The attack requires specific packet structure involving stale outer IPv6 control-block metadata and malformed inner IPv6 destination options.

No local account is inherently required in the vulnerable host context. Namespaces or containers are not central to reachability unless they expose or configure the tunnel endpoint. The path is not universally default-enabled because IPv6-in-IPv6 tunnel configuration is required, but it is still a network-facing kernel path when deployed.

Realistic exploitation conditions are non-trivial, so the complexity is high. However, the primitive is serious because the patch and commit message explicitly identify out-of-bounds behavior and `skb_shared_info` corruption.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate issue. The realistic impact is at least remote DoS under specific tunnel conditions. The theoretical impact is higher because `skb_shared_info` corruption is a concrete kernel memory corruption primitive rather than a simple validation failure or NULL pointer dereference.

The conservative score places it as Actionable Moderate. The paranoid score places it as a Strong Important candidate because the bug is remotely reachable in affected deployments and corrupts high-value skb metadata.

## 5. One-sentence report phrase

A stale IPv6 skb control block in `ip6ip6_err()` can cause the ICMPv6 reply path to apply an outer Home Address Option offset to a quoted inner packet, leading to out-of-bounds access and `skb_shared_info` corruption on reachable IPv6-in-IPv6 tunnel endpoints.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network reachable in affected tunnel deployments and the patch explicitly describes corruption of `skb_shared_info`, which is a concrete kernel memory corruption sink with plausible impact beyond a simple crash.

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

Patch: ip6_tunnel: clear skb2->cb[] in ip6ip6_err()
Commit: 44fe898df302e91c5ee5acbc71ffa74e78e6c183
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=44fe898df302e91c5ee5acbc71ffa74e78e6c183

Commit description:

ip6ip6_err() clones an outer IPv6 ICMP error skb, pulls it to the
quoted inner IPv6 packet, and then passes the clone to icmpv6_send().
The clone still carries the outer packet's inet6_skb_parm in skb->cb.

If the outer packet had a Home Address Option, IP6CB(skb2)->dsthao
remains non-zero after skb_pull(). icmpv6_send() later calls
mip6_addr_swap(), which uses that stale dsthao offset against the quoted
inner packet. A malformed inner destination-options header can then make
the HAO lookup and address swap run past the end of the quoted packet
and corrupt skb_shared_info.

Clear skb2->cb[] before pulling the quoted inner IPv6 packet so the
reply path does not reuse metadata left by the outer IPv6 stack.

Fixes: e490d1d ("[IPV6] IP6TUNNEL: Split out generic routine in ip6ip6_err().")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/fe1a5e765fbca88d69391887f0ed26a19e3e4d39.1785736562.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/ip6_tunnel.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=44fe898df302e91c5ee5acbc71ffa74e78e6c183

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

The original Linux kernel CVE announcement for CVE-2026-74597 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74597

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: 6
  Paranoid ActionableScore: 7

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).