[CVE-2026-63924][IMPORTANT] ipv6: exthdrs: refresh nh pointer after ipv6_hop_jumbo()

[CVE-2026-63924][IMPORTANT] ipv6: exthdrs: refresh nh pointer after ipv6_hop_jumbo()

[AL-KERNEL]

CVE: CVE-2026-63924
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: exthdrs: refresh nh pointer after ipv6_hop_jumbo()
Commit: b3ac54e5c905f86d22b502eacb5686a282c5659f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3ac54e5c905f86d22b502eacb5686a282c5659f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63924
Analysis date: Sun, 19 Jul 2026 21:57:35 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted IPv6 Hop-by-Hop Jumbo Payload packet may cause IPv6 TLV parsing to continue with a stale skb network header pointer after `pskb_trim_rcsum()`, creating a remote DoS risk and a UAF-style memory-corruption candidate that requires manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63924 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63924

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-2026-63924	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-825;*CWE-416;CWE-125;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'An IPv6 Hop by Hop Jumbo Payload option path can keep a stale nh pointer because ipv6_hop_jumbo calls pskb_trim_rcsum and that helper can change skb internal pointers. A crafted IPv6 packet with a jumbo option may then continue TLV parsing through the old network header pointer, which is a stale pointer condition and a possible use after free read or crash in the receive path. For the CVSS the PR:N is used because the trigger is a network packet and no local account on the target is required. The issue is network reachable when IPv6 Hop by Hop Jumbo Payload processing is enabled and such packets can reach the host, but practical reachability is reduced by jumbo frame and Hop by Hop filtering requirements. Impact is at least denial of service via kernel crash and the paranoid score treats the stale skb pointer as a memory corruption candidate that may require manual review for confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE SIMPLEFIX NETWORK SKB IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  IPv6 Hop-by-Hop option parsing is packet-triggered, and the affected path can be reached by crafted IPv6 packets if such packets are delivered to the host.

* Memory corruption, weak/indirect corruption candidate: +1
  The bug is a stale `nh` pointer after `ipv6_hop_jumbo()` calls `pskb_trim_rcsum()`, which can change skb internal pointers. The patch does not show an attacker-controlled overwrite, but stale skb header pointers are a credible indirect corruption/UAF-read candidate.

* Real lifetime corruption: +1
  The stale pointer may refer to old skb storage after skb pointer changes. This is not proven as exploitable reclaim, but it is more than pure bookkeeping.

* Common networking core path or packet-processing path: +1
  This is in IPv6 extension header parsing in the receive path.

* Hard or special triggering conditions: -1
  Practical triggering depends on IPv6 Hop-by-Hop Jumbo Payload handling, jumbo packet conditions, and whether such packets are filtered or accepted.

Conservative total: 5

Paranoid additional interpretation:

* Treat stale skb pointer as strong memory-corruption review candidate: upgrade weak corruption from +1 to +2
  In the worst still-defensible interpretation, stale `nh` after skb pointer mutation could behave like UAF-style header access in a remote packet path.

* Confidentiality impact plausible: +1
  If stale header access reads from freed or unintended memory, limited disclosure cannot be ruled out without deeper source review.

* Integrity impact plausible: +1
  If later TLV parsing decisions are made from stale memory, integrity impact is theoretically possible, although not demonstrated by the patch.

Paranoid total: 7

## 3. Reachability analysis

The trigger is a crafted IPv6 packet containing a Hop-by-Hop Jumbo Payload option. No local account on the target is required, so remote reachability is plausible in CVSS/actionable triage terms. In practice, exposure is reduced because Hop-by-Hop options are often filtered or slow-pathed, and IPv6 jumbogram handling requires specific packet characteristics and network support.

Namespaces and containers do not materially reduce the remote packet-triggered nature if packets can reach the host stack. The path is part of core IPv6 parsing, but the specific Jumbo option path is less commonly exercised than ordinary IPv6 traffic.

Call-site confidence: medium. The changed code and immediate caller context are visible, but full downstream behavior after stale `nh` use is not included in the patch excerpt.

## 4. Severity interpretation

This should not be treated as an ordinary low-priority Moderate because it is network reachable and involves a stale skb pointer in the IPv6 receive path. The conservative view is Actionable Moderate: likely remote DoS or invalid parsing with memory-lifetime risk. The paranoid view is Strong Important candidate because a stale pointer after skb mutation can be a UAF-style primitive and deserves manual review for possible confidentiality or integrity impact.

The realistic evidence supports remote DoS more strongly than privilege escalation or arbitrary code execution. The higher score is for manual-review sensitivity, not because the patch demonstrates a controlled overwrite or reliable exploit primitive.

## 5. One-sentence report phrase

A crafted IPv6 Hop-by-Hop Jumbo Payload packet may cause IPv6 TLV parsing to continue with a stale skb network header pointer after `pskb_trim_rcsum()`, creating a remote DoS risk and a UAF-style memory-corruption candidate that requires manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network reachable, located in core IPv6 packet parsing, and involves a stale skb pointer after skb pointer mutation. Even though the demonstrated impact is most clearly DoS, the stale-pointer class is strong enough to avoid auto-closing without source-level review.

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

Patch: ipv6: exthdrs: refresh nh pointer after ipv6_hop_jumbo()
Commit: b3ac54e5c905f86d22b502eacb5686a282c5659f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3ac54e5c905f86d22b502eacb5686a282c5659f

Commit description:

ipv6_hop_jumbo() calls pskb_trim_rcsum(), which can change skb pointers.
Let's recompute nh pointer to make sure any change won't mess things up.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Justin Iurman <justin.iurman@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260522112013.12342-1-justin.iurman@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/exthdrs.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=b3ac54e5c905f86d22b502eacb5686a282c5659f

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

The original Linux kernel CVE announcement for CVE-2026-63924 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63924

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:N/I:N/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: 5
  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).