[CVE-2026-98621][IMPORTANT] net: gue: reject invalid REMCSUM offsets [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-98621
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: net: gue: reject invalid REMCSUM offsets [ Upstream
Commit: 4b9bc21b2396c396d88dd909caaf73d5be6ff593
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4b9bc21b2396c396d88dd909caaf73d5be6ff593
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98621
Analysis date: Sat, 24 Oct 2026 08:54:57 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A remote GUE REMCSUM tuple can underflow checksum metadata and cause a later hardware-checksum path to write two bytes approximately 64 KiB beyond a kernel destination buffer.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98621 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98621

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-2026-98621	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98621 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	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-191;*CWE-787;*CWE-20;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '8.1';DESCR 'An invalid GUE REMCSUM tuple with offset less than start underflows when offset minus start is stored in the u16 skb csum_offset field. A forwarded CHECKSUM_PARTIAL packet can carry the resulting value to a NETIF_F_HW_CSUM path where skb_copy_and_csum_dev writes two checksum bytes about 64 KiB beyond the destination buffer. For the CVSS the PR:N value is used because a remote sender does not need an authenticated local user context when it can reach a configured GUE endpoint. Exploitation has AC:H because it also depends on GUE REMCSUM processing, packet forwarding, retention of CHECKSUM_PARTIAL metadata, and a compatible egress checksum path. The demonstrated primitive is a two byte kernel out of bounds write with likely crash and integrity impact. Reliable code execution or information disclosure is not proven, but HHH is a defensible paranoid boundary because adjacent kernel memory can be corrupted. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 8.1 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 4.8 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	REMOTE OOB SIMPLEFIX LINUS PACKET INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no additional impact is sufficiently proven.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Strong corruption primitive: reversed attacker-supplied REMCSUM offsets underflow `skb->csum_offset`, causing a confirmed two-byte heap OOB write roughly 64 KiB beyond the checksum destination buffer.
* +2 — Remote reachability: supplied evidence grounds triggering by an unauthenticated sender against an already configured GUE endpoint.
* +1 — Strong DoS plausible: a far-OOB kernel write can credibly fault or corrupt host state.
* No timing penalty: the multi-stage forwarding and checksum prerequisites increase complexity, but no narrow race is shown.
* No privilege penalty: host administration may configure the endpoint, but the subsequent vulnerable data-path trigger is remotely invocable.
* No separate integrity, confidentiality, arbitrary-write, LPE, or controlled-target bonus. The payload is only two bytes, and no adjacent victim or exploit chain is identified.
* B5-R does not apply: no retry-accumulated index is involved. This is a direct checksum-offset underflow.
* No extra Availability point: it would duplicate the same possible crash.

3. Reachability analysis

A remote sender can supply the invalid GUE REMCSUM tuple to an existing GUE endpoint. Processing creates malformed `CHECKSUM_PARTIAL` metadata; forwarding can preserve it until a `NETIF_F_HW_CSUM` egress path invokes `skb_copy_and_csum_dev()`. Endpoint and compatible egress configuration are prerequisites, but the triggering packet itself requires neither host UID 0 nor a namespace capability. No race, special hardware, or unrelated vulnerability is shown.

4. Primitive and severity reading

The primitive is an attacker-influenced, fixed-width heap OOB write, not a UAF or in-SKB-only edit. The active checksum metadata is first corrupted and then consumed by a privileged kernel/driver checksum operation, crossing the remote-sender-to-host-kernel trust boundary. The OOB write is proven; arbitrary target selection, controlled payload, disclosure, LPE, and reliable code execution remain unproven. This is CLASS_A and an Important candidate because auto-downgrading would discard a concrete remotely influenced kernel write primitive.

5. One-sentence report phrase

A remote GUE REMCSUM tuple can underflow checksum metadata and cause a later hardware-checksum path to write two bytes approximately 64 KiB beyond a kernel destination buffer.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm affected GUE deployments, `CHECKSUM_PARTIAL` preservation, egress-driver reachability, allocation layout, and whether the two-byte write can strike an exploitable adjacent object.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: gue: reject invalid REMCSUM offsets [ Upstream
Commit: 4b9bc21b2396c396d88dd909caaf73d5be6ff593
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4b9bc21b2396c396d88dd909caaf73d5be6ff593

Changed files:
  include/net/gue.h

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=4b9bc21b2396c396d88dd909caaf73d5be6ff593

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

The original Linux kernel CVE announcement for CVE-2026-98621 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98621

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: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: unknown
  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: IMPORTANT
KPANIC detected for this report: NO
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).