[CVE-2026-68476][IMPORTANT] ipvs: reload ip header after head reallocation

[CVE-2026-68476][IMPORTANT] ipvs: reload ip header after head reallocation

[AL-KERNEL]

CVE: CVE-2026-68476
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipvs: reload ip header after head reallocation
Commit: ac6ac3d35bfc0ade9d17d354c84e503a946ebdab
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac6ac3d35bfc0ade9d17d354c84e503a946ebdab
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68476
Analysis date: Sat, 15 Aug 2026 09:46:39 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote IPVS packet-processing path may reuse a stale IP header pointer after skb head reallocation, allowing a constrained write-after-free-style kernel memory corruption that can cause DoS and warrants manual review for possible higher impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68476 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68476

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-2026-68476	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-825;*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 'An IPVS transmit path can keep a stale iphdr pointer across __ip_vs_get_out_rt because that helper can call skb_ensure_writable and reallocate skb head. A later ip_send_check on the stale pointer can write through an invalid header pointer, creating a use after free style memory corruption condition in the kernel networking path. For the CVSS the PR:N is used because a remote sender normally needs only network reachability to an enabled IPVS service or forwarding path, not local access on the target. The issue is network reachable when IPVS is configured and the affected bypass transmit path processes attacker controlled packets. Attack complexity is high because exploitation depends on skb head reallocation or copy on write conditions, but impact is at least remote denial of service and may include confidentiality or integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE SIMPLEFIX NETFILTER SKB LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  IPVS is a packet-processing path. A remote packet sender can plausibly reach the affected transmit path when IPVS is enabled and configured.

* Constrained kernel buffer overwrite: +1
  `ip_send_check()` writes through a cached `iph` pointer after `skb_ensure_writable()` may have reallocated `skb->head`. The write is constrained, likely checksum-related and not an arbitrary attacker-chosen payload.

* Real lifetime corruption: +1
  The cached IP header pointer may become stale after skb head reallocation, creating a stale-pointer / use-after-free-style lifetime bug.

* Reliable kernel crash / strong DoS: +1
  Writing through a stale skb header pointer can plausibly crash the kernel.

* Common networking packet-processing path: +1
  This is in IPVS/netfilter packet transmission logic, not a purely local control-plane ioctl.

* Hard or unreliable trigger condition: -1
  Exploitation depends on making `skb_ensure_writable()` actually reallocate or copy the skb head in the affected path.

Paranoid additions / interpretation:

* Memory corruption, strong corruption primitive: +2 instead of constrained +1
  In the paranoid view, a stale pointer followed by a write after skb head reallocation is treated as a real write-after-free candidate.

* Weak LPE concern: +1
  Privilege escalation is not demonstrated, but writable stale memory in a network packet path is enough to require manual review.

* Integrity impact plausible: +1
  The stale write may corrupt kernel memory, even if payload and target control are limited.

## 3. Reachability analysis

The likely attacker is a remote sender whose packets are processed by an enabled IPVS setup. No local account on the target is required if the affected IPVS service or forwarding path is reachable, so the practical PR interpretation is PR:N. The path is not universally default-enabled because IPVS must be configured, but it is a real networking data path used in load-balancing deployments. Namespaces may affect IPVS availability, but the main concern is host-level packet processing once IPVS is active. The main exploitation constraint is forcing the skb writable operation to reallocate `skb->head`, which makes the attack less reliable and supports AC:H style reasoning.

Call-site confidence: high. The provided patch shows the stale `iph` use around `__ip_vs_get_out_rt()`, and the commit message explicitly states that this helper may call `skb_ensure_writable()` and reallocate `skb->head`.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is network-reachable in a packet-processing subsystem and involves a stale pointer followed by a write. The conservative view treats it as actionable Moderate due to constrained write-after-free-style corruption and high trigger complexity. The paranoid view treats it as a Strong Important candidate because remote kernel memory corruption in a networking path can be underestimated, even when the write primitive is limited and no exploit chain is shown.

Theoretical memory corruption potential is real because the pointer can become invalid after skb head reallocation. Realistic exploitation evidence is limited because the write appears constrained and depends on specific skb reallocation conditions.

## 5. One-sentence report phrase

A remote IPVS packet-processing path may reuse a stale IP header pointer after skb head reallocation, allowing a constrained write-after-free-style kernel memory corruption that can cause DoS and warrants manual review for possible higher impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network-reachable when IPVS is configured, involves stale pointer lifetime corruption, and performs a write through the stale pointer. Even though the write is constrained and exploitation depends on skb reallocation, this class should not be auto-closed.

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

Patch: ipvs: reload ip header after head reallocation
Commit: ac6ac3d35bfc0ade9d17d354c84e503a946ebdab
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac6ac3d35bfc0ade9d17d354c84e503a946ebdab

Commit description:

__ip_vs_get_out_rt() calls skb_ensure_writable() which may
reallocate skb->head.

Fixes: 8d8e20e ("ipvs: Decrement ttl")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-sonnet-4-6
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/ipvs/ip_vs_xmit.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=ac6ac3d35bfc0ade9d17d354c84e503a946ebdab

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

The original Linux kernel CVE announcement for CVE-2026-68476 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68476

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: 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).