[CVE-2026-63994][IMPORTANT] tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]() [ Upstream

[CVE-2026-63994][IMPORTANT] tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63994
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]() [ Upstream
Commit: 95b6d772bfe788331d9742d73eaa12e113b2adc4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95b6d772bfe788331d9742d73eaa12e113b2adc4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63994
Analysis date: Sun, 19 Jul 2026 23:24:31 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A possible UAF in tunnel PMTU ICMP and ICMPv6 response construction occurs because network header pointers are cached before `skb_cow()`, allowing a remote packet on an affected tunnel path to trigger stale skb header use after skb head reallocation.

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

The original Linux kernel CVE announcement for CVE-2026-63994 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63994

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
======================================================================

CVE-2026-63994	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:L/I:L/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7';DESCR 'A possible UAF exists in the tunnel PMTU ICMP and ICMPv6 build paths because ip_hdr() and ipv6_hdr() were cached before skb_cow(). If skb_cow() reallocates skb head, the cached header pointer can become stale and later be used while constructing the ICMP response. For the CVSS the PR:N is used because a remote sender does not need an account on the target if the affected tunnel PMTU path is reachable. The attack is not a generic Internet wide trigger and depends on a tunnel or bridged tunnel configuration plus skb_cow() reallocation conditions, so AC:H is used. Impact is at least denial of service and possible limited confidentiality or integrity impact from stale header reads, but no arbitrary write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE UAF NETWORK SKB LINUS IPV6 KPANIC PACKET 	NO	NO	checked

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

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. A packet reaching the affected tunnel PMTU path can trigger ICMP or ICMPv6 construction without a local account on the target.
* Generic memory corruption: +2. The commit explicitly describes a possible UAF caused by caching `ip_hdr()` or `ipv6_hdr()` before `skb_cow()`, which may reallocate `skb->head`.
* Real lifetime corruption: +1. The stale header pointer can survive an skb head reallocation and be used after the old storage is no longer valid.
* Common networking packet-processing path: +1. This is in core tunnel PMTU handling, not an obscure debug-only path.
* Special configuration and trigger conditions: -1. Exploitation depends on a tunnel or bridged-tunnel PMTU path and on `skb_cow()` actually reallocating the skb head.

Paranoid additional signals:

* Confidentiality impact plausible: +1. The stale header read may consume freed skb-head contents while constructing a network response, although no broad info-leak primitive is demonstrated.
* Integrity impact plausible: +1. Stale header fields may influence generated ICMP or ICMPv6 response metadata, but there is no arbitrary write or controlled overwrite primitive.

Not counted:

* No strong LPE bonus. The patch shows stale pointer use after skb head reallocation, but not attacker-controlled reclaim, object replacement, writable UAF, type confusion, callback control, or refcount takeover.
* No Dirty-Pipe-like bonus. This does not show writes into externally owned file-backed or page-cache memory.
* No reliable strong DoS bonus in the conservative score. A crash is plausible for UAF, but the patch does not demonstrate a reliable panic path.

## 3. Reachability analysis

A remote sender may trigger the bug if packets can reach the affected tunnel PMTU path and cause ICMP or ICMPv6 PMTU response construction. The attacker does not need local credentials once the vulnerable tunnel configuration is reachable, so PR is effectively none for the network trigger.

This is not a generic Internet-wide condition. It depends on deployment of the relevant tunnel or directly bridged tunnel setup, PMTU handling, and skb conditions that make `skb_cow()` reallocate `skb->head`. Namespaces mostly affect who can create the tunnel configuration, typically requiring `CAP_NET_ADMIN`, but the runtime trigger can be remote after the configuration exists.

Call-site confidence: medium. The affected helper body and commit message clearly identify the tunnel PMTU path, but full caller context is not included in the provided patch.

## 4. Severity interpretation

This behaves above an ordinary Moderate because it is a network-reachable lifetime bug in the networking stack. The realistic primitive is stale pointer read or dereference after skb head reallocation, not a demonstrated arbitrary write or reliable privilege escalation path.

Conservatively, this is an actionable Moderate because it combines remote reachability with a real UAF class. Paranoid handling can justify Strong Important candidate review because network-reachable skb lifetime bugs have historically been underestimated, even when the first visible primitive is only stale header use.

## 5. One-sentence report phrase

A possible UAF in tunnel PMTU ICMP and ICMPv6 response construction occurs because network header pointers are cached before `skb_cow()`, allowing a remote packet on an affected tunnel path to trigger stale skb header use after skb head reallocation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

This should not be auto-closed because it is a network-reachable UAF candidate in skb/tunnel handling, even though the currently visible primitive is limited and no direct privilege-escalation path is demonstrated.

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

Patch: tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]() [ Upstream
Commit: 95b6d772bfe788331d9742d73eaa12e113b2adc4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=95b6d772bfe788331d9742d73eaa12e113b2adc4

Changed files:
  net/ipv4/ip_tunnel_core.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=95b6d772bfe788331d9742d73eaa12e113b2adc4

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

The original Linux kernel CVE announcement for CVE-2026-63994 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63994

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:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7

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).