[CVE-2026-63992][MODERATE 7.0] tunnels: do not assume transport header in iptunnel_pmtud_check_icmp() [ Upstream

[CVE-2026-63992][MODERATE 7.0] tunnels: do not assume transport header in iptunnel_pmtud_check_icmp() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63992
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tunnels: do not assume transport header in iptunnel_pmtud_check_icmp() [ Upstream
Commit: 5a92cb45e34749865d03daf8d3500f77b5f6644c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5a92cb45e34749865d03daf8d3500f77b5f6644c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63992
Analysis date: Sun, 19 Jul 2026 14:08:27 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An unset skb transport header in IPv4 tunnel PMTU ICMP handling can cause an out-of-bounds read when processing crafted or unusual tunnel traffic, leading primarily to remote denial of service on affected tunnel configurations.

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

The original Linux kernel CVE announcement for CVE-2026-63992 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63992

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: MODERATE 7.0
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-2026-63992	MODERATE	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:N/A:H';CWE-125;*CWE-20;CWE-119;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'iptunnel_pmtud_check_icmp can be called when the skb transport header is not initialized, but the old code used that transport header to locate the ICMP header. This can turn the ICMP type access into an out-of-bounds read when tunnel PMTU handling processes a crafted or unusual IPv4 packet. For the CVSS the PR:N is used because a packet sender does not need local privileges on the target when the affected tunnel path is reachable. The attack is network reachable only on systems with the relevant IP tunnel or bridged tunnel configuration exposed to the attacker, so AC:H is used for the configuration and path dependency. Impact is primarily denial of service through a kernel fault or crash, with only limited confidentiality considered in the paranoid score because the primitive is an out-of-bounds read rather than a write or UAF.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE OOB SIMPLEFIX NETWORK SKB KPANIC PACKET  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The affected path is in IPv4 tunnel PMTU handling and can be reached by packet processing when the relevant tunnel or directly bridged tunnel configuration exists.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly says the unset skb transport header triggers an out-of-bound access. This is likely to produce a kernel fault, KASAN report, or crash in affected paths.

* Common networking core path or packet-processing path: +1
  This is in `net/ipv4/ip_tunnel_core.c`, not a purely local ioctl or admin-only control path.

Paranoid additional signal:

* Confidentiality impact plausible: +1
  The primitive is an out-of-bounds read of `icmph->type`. There is no clear user-visible disclosure channel, so this is only a limited paranoid confidentiality concern, not a strong info leak.

Not counted:

* No integrity impact: no write, overwrite, UAF, double-free, refcount misuse, or type confusion is shown.
* No privilege escalation signal: the patch shows an OOB read, not a reclaimable lifetime bug or write primitive.
* No extra availability point: crash impact is already counted as strong DoS.
* No rare-subsystem penalty: tunnel configuration is conditional, but this remains kernel networking packet-processing code.

## 3. Reachability analysis

A remote packet sender may trigger the bug if the target has an affected IPv4 tunnel or directly bridged tunnel PMTU path exposed to attacker-controlled traffic. No local account is required on the target in that configuration, so practical PR is none for the network-facing scenario. The path is configuration-dependent and not necessarily exposed on all systems, which supports high attack complexity in CVSS terms and prevents treating it as a broad Internet-default bug.

Namespaces and containers do not appear central to triggering this issue. The primary exposure depends on host networking configuration, tunnel use, and whether packets reaching the affected PMTU logic can be influenced by an attacker.

Call-site confidence: medium. The patch gives the vulnerable function and the bad access pattern, but not a full caller trace.

## 4. Severity interpretation

This behaves like an actionable Moderate networking memory-safety issue rather than a clear Important vulnerability. The theoretical primitive is an out-of-bounds read in the kernel network stack, and the realistic demonstrated impact is DoS. There is no evidence of attacker-controlled overwrite, object lifetime corruption, or privilege escalation. Because it is network-triggerable under plausible tunnel configurations, it should not be auto-closed as ordinary low-risk Moderate.

## 5. One-sentence report phrase

An unset skb transport header in IPv4 tunnel PMTU ICMP handling can cause an out-of-bounds read when processing crafted or unusual tunnel traffic, leading primarily to remote denial of service on affected tunnel configurations.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable kernel networking OOB read with likely DoS impact and limited paranoid confidentiality concern, even though no write primitive or LPE path is demonstrated.

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

Patch: tunnels: do not assume transport header in iptunnel_pmtud_check_icmp() [ Upstream
Commit: 5a92cb45e34749865d03daf8d3500f77b5f6644c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5a92cb45e34749865d03daf8d3500f77b5f6644c

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=5a92cb45e34749865d03daf8d3500f77b5f6644c

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

The original Linux kernel CVE announcement for CVE-2026-63992 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63992

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:L/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 4
  Paranoid ActionableScore: 5

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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).