[CVE-2026-72502][MODERATE 7.0] tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) [ Upstream

[CVE-2026-72502][MODERATE 7.0] tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72502
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) [ Upstream
Commit: 560b33b434e922ef97f9ff23aa2e909ef7aacd5c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=560b33b434e922ef97f9ff23aa2e909ef7aacd5c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72502
Analysis date: Sat, 15 Aug 2026 05:42:36 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-72502 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72502

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-2026-72502	MODERATE	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:N/I:N/A:H';**CWE-476;*CWE-20;CWE-682;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'IPv6 TCP can advertise a default MSS of 65535 when the route MTU is very large. This value collides with the kernel GSO_BY_FRAGS magic value 0xFFFF, so skb_segment can treat a normal TCP packet as frag_list based GSO even though local TCP packets do not carry such a frag_list. A remote peer may be able to trigger the crash by causing the host to transmit TCP data over an IPv6 path with a very large MTU, but the practical setup is non-default and usually limited to jumbo frame, data center, lab, or special routing environments. For the CVSS the PR:N because the attacker does not need a local account if the vulnerable TCP path and service are reachable. Impact is denial of service via kernel crash. No confidentiality or integrity impact is assigned because the observed primitive is a NULL pointer dereference rather than UAF, OOB write, or another memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE DANGER NULLPTR SIMPLEFIX NETWORK IPV6 SYZBOT KPANIC PACKET 	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
* Remote reachable / network-triggerable: +2. A remote TCP peer may trigger host-side TCP transmit segmentation if an affected IPv6 service is reachable and the route MTU allows MSS 65535.
* Reliable kernel crash / strong DoS: +1. The described failure is a NULL pointer dereference in skb_segment().
* Common networking core path or packet-processing path: +1. The affected logic is in IPv6 TCP MSS selection and skb segmentation.
* Broad/default/common subsystem: +1 in paranoid scoring. IPv6 TCP is common, although the required very large MTU condition is not common.
* Rare or difficult-to-reach runtime condition: -1 in conservative scoring. Triggering depends on unusually large MTU or jumbo IPv6 conditions.
* No generic memory corruption bonus. The supported primitive is NULL pointer dereference, not UAF, OOB write, double free, type confusion, or controlled overwrite.
* No privilege escalation bonus. The patch and commit do not support an LPE primitive.

3. Reachability analysis
A remote peer can plausibly influence the vulnerable path by causing the host to send IPv6 TCP traffic over a route where ip6_default_advmss() returns 65535. No local account is needed if the affected TCP service is reachable. In practice, the trigger requires non-default networking conditions, mainly very large MTU or jumbo-payload style environments, so exposure is more likely in data-center, lab, or specialized networks than on ordinary Internet-facing hosts. Namespaces do not materially lower the requirement for the remote case, but local namespace-controlled networking could help reproduce the condition in test or containerized environments.

4. Severity interpretation
This behaves like an actionable Moderate network DoS issue, not an Important memory-corruption or privilege-escalation issue. The theoretical worst case is still bounded by the observed primitive: a NULL pointer dereference in a core networking path. There is no evidence of confidentiality impact, integrity impact, attacker-controlled memory reuse, or arbitrary write. Manual review is still justified because the bug is in TCP/IPv6 segmentation behavior and may be remotely triggerable under specific high-MTU conditions.

5. One-sentence report phrase
IPv6 TCP can select MSS 65535 on very large MTU routes, colliding with the GSO_BY_FRAGS magic value and causing skb_segment() to mis-handle a normal TCP skb as frag_list based GSO, leading to a remotely triggerable kernel crash under specific high-MTU conditions.

6. Manual review recommendation
MANUAL CHECK REQUIRED.
Reason: network-reachable kernel crash path in core TCP/IPv6 code, with non-default but realistic high-MTU deployment conditions.

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

Patch: tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) [ Upstream
Commit: 560b33b434e922ef97f9ff23aa2e909ef7aacd5c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=560b33b434e922ef97f9ff23aa2e909ef7aacd5c

Changed files:
  net/ipv6/route.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=560b33b434e922ef97f9ff23aa2e909ef7aacd5c

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

The original Linux kernel CVE announcement for CVE-2026-72502 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72502

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:A/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:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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).