[CVE-2026-97984][LOW] net: ipv6: Fix UDP length overflow with PMTU discover and big MTU [ Upstream

[CVE-2026-97984][LOW] net: ipv6: Fix UDP length overflow with PMTU discover and big MTU [ Upstream

[AL-KERNEL]

CVE: CVE-2026-97984
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net: ipv6: Fix UDP length overflow with PMTU discover and big MTU [ Upstream
Commit: 1d56a00ec91f2885c64ea5b8cea18cb746b91bbb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d56a00ec91f2885c64ea5b8cea18cb746b91bbb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97984
Analysis date: Fri, 25 Sep 2026 13:26:59 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97984 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97984

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-97984	LOW	https://www.kernelcve.org/list/?q=CVE-2026-97984 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-190;*CWE-20;CWE-1284;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An integer overflow in the IPv6 UDP transmit path can occur when the device MTU exceeds IP6_MAX_MTU and IPV6_MTU_DISCOVER is set to IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE. Sending 65528 bytes of UDP payload then produces a UDP length of 65536, which cannot be represented in the 16 bit length field and triggers a WARN in udp_set_len_short. For the CVSS the PR:L value is used because a local user or process can create the UDP socket and send the crafted payload after an administrator has configured the unusually large device MTU. The issue is not remotely triggerable by received network traffic because packet construction occurs on the local transmitting host. Impact is normally limited to a warning and failed or incorrect packet processing, but it can become a kernel panic and system wide denial of service when panic_on_warn is enabled. There is no supported UAF, out of bounds access, information leak, or write primitive, so confidentiality and integrity remain unaffected.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)  SKIP CVE-2026-97984 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE DANGER NETWORK HARDWARE IPV6 SYZBOT PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +0 Integer overflow. The overflow affects the 16 bit UDP length field and produces a WARN, but no kernel memory access violation or corruption primitive is established.
* +0 WARN-only behavior. WARN_ON alone does not qualify as reliable kernel crash or strong DoS.
* +0 Common networking path not awarded. IPv6 UDP is common, but the vulnerable condition requires an unusually large MTU above IP6_MAX_MTU and specific PMTU discovery modes.
* +0 Local unprivileged trigger not awarded. Socket creation and transmission are unprivileged, but the complete trigger also requires the oversized MTU prerequisite, whose attacker-accessible setup is not established.
* +0 No configuration penalty. The oversized MTU is unusual, but the evidence does not establish that it is both rare and inaccessible through an attacker-created namespace.

Paranoid signals:

* +1 Reliable kernel crash under panic_on_warn. On a system configured to panic on this WARN, the crafted local UDP transmission can cause a host-wide denial of service.

Important non-awarded signals:

* No UAF, OOB access, stale pointer, double release, active-state retirement bypass, or ownership violation is established.
* No attacker-controlled kernel write, controlled reclaim, disclosure, or security-boundary bypass is demonstrated.
* Remote reachability is not established because the overflow occurs while constructing a locally transmitted packet.
* Confidentiality and integrity points are not awarded.
* No extra Availability point is awarded beyond the paranoid panic consequence.

3. Reachability analysis

A local process can create an IPv6 UDP socket, select IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE, and transmit the triggering payload. The relevant network device must already have an MTU greater than IP6_MAX_MTU.

Setting that MTU normally requires CAP_NET_ADMIN for the governing network namespace. The supplied evidence does not establish whether the complete setup can be performed inside an attacker-created user and network namespace, so both host-admin and namespace-scoped reachability remain unresolved. A separately configured oversized-MTU device is a prior privileged setup when the attacker cannot create that configuration.

Received network traffic does not directly reach the vulnerable operation. No special physical hardware or race is required once the oversized MTU configuration exists.

4. Primitive and severity interpretation

The exact primitive is a scalar integer overflow when the UDP header length reaches 65536 and cannot be represented in its 16 bit field. The demonstrated result is a WARN in udp_set_len_short, not memory corruption.

There is no stale storage, active structured-object UAF, repeated release, or lifetime mutation. PrimitiveClass is CLASS_C because the behavior is an invalid protocol state with a potential configuration-dependent crash consequence.

ImportantCandidate is NO because no reusable corruption, ownership, authorization, or disclosure primitive exists. AutoDowngradeAllowed is YES because the practical impact is WARN-only in ordinary configurations and panic-based DoS only when panic_on_warn is enabled. The likely operational severity is Low-like to ordinary Moderate.

5. One-sentence report phrase

An oversized IPv6 device MTU combined with UDP PMTU discovery can overflow the 16 bit UDP length field and trigger a WARN, with kernel panic possible only on systems configured with panic_on_warn.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a bounded protocol-length overflow and WARN without memory corruption, remote reachability, confidentiality impact, integrity impact, or unresolved security-boundary leverage.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: ipv6: Fix UDP length overflow with PMTU discover and big MTU [ Upstream
Commit: 1d56a00ec91f2885c64ea5b8cea18cb746b91bbb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d56a00ec91f2885c64ea5b8cea18cb746b91bbb

Changed files:
  net/ipv6/ip6_output.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=1d56a00ec91f2885c64ea5b8cea18cb746b91bbb

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

The original Linux kernel CVE announcement for CVE-2026-97984 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97984

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:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 0
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).