[CVE-2026-99128][MODERATE 7.0] sctp: validate chunk length in the inqueue parser

[CVE-2026-99128][MODERATE 7.0] sctp: validate chunk length in the inqueue parser

[AL-KERNEL]

CVE: CVE-2026-99128
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: sctp: validate chunk length in the inqueue parser
Commit: 40507f59f230dcf60ae13cb48d34f8c6e9f40659
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40507f59f230dcf60ae13cb48d34f8c6e9f40659
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99128
Analysis date: Fri, 30 Oct 2026 12:22:39 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A remote SCTP peer can submit a zero-length ASCONF chunk that prevents inqueue progress and locks the receive softirq on specially configured hosts.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99128 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99128

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99128	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99128 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-20;*CWE-835;**CWE-400;BEST CVSS score: '7.5';DESCR 'The SCTP inqueue parser accepts a declared chunk length below the mandatory four byte generic header. A zero length ASCONF chunk can leave chunk_end at the current header and cause the same malformed chunk to be returned repeatedly when SCTP AUTH policy allows processing to continue before the state machine length check. This can lock the receive softirq and cause a remotely triggered denial of service with severe loss of network availability. For the CVSS the PR:N value is selected because a remote SCTP peer can establish the association and trigger the malformed ASCONF path without application credentials or a shared SCTP AUTH key. Exploitation requires CONFIG_IP_SCTP with addip_enable and auth_enable enabled. No UAF, OOB access, information leak, or memory corruption primitive is supported by the patch evidence. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 7.5 and ActionableScore 4 with primitive class CLASS_C 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 MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	DEADLOCK LEAK NETWORK SKB HARDWARE LINUS IPV6 PACKET  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; no additional technically supported primitive.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Remote reachability: an unauthenticated remote SCTP peer can establish the association and submit the zero-length ASCONF chunk.
* +1 Reliable kernel DoS: repeated return of the same malformed chunk locks the receive softirq; watchdog soft-lockups and 36/37 failed health probes support substantial availability impact.
* +1 Network packet-processing path: the defect is in the shared SCTP receive inqueue parser.
* -1 Rare and attacker-unconfigurable prerequisites: exploitation requires CONFIG_IP_SCTP plus administrator-enabled `addip_enable=1` and `auth_enable=1`.
* No second availability point: probe failures and soft lockups are the same DoS consequence.
* No OOB access, write, UAF, disclosure, authorization bypass, or integrity primitive is shown. NEW52/B5-R does not apply because there is no heap OOB write or retry-accumulated write state.
* The AUTH-policy early continuation is a control-flow condition, not evidence that a mandatory authorization DENY becomes ALLOW.

3. Reachability analysis

The packet trigger is remotely reachable without application credentials, a shared SCTP AUTH key, host UID 0, or host-administrator powers. A separate administrator must previously enable the SCTP feature and both sysctls, but that does not make the later packet sender privileged. No attacker namespace capability is involved, and no race or special hardware is required.

4. Primitive and severity reading

The exact defect is malformed-length parser non-progress: length zero leaves `chunk_end` at the current header, allowing the same chunk to be returned repeatedly. The patch rejects lengths below the four-byte generic header and marks the packet for discard. This is CLASS_C CPU/softirq livelock with demonstrated DoS, not memory corruption. No non-crash leverage or protected-state modification remains, so it is not an Important candidate and automatic Moderate-class downgrade is allowed.

5. One-sentence report phrase

A remote SCTP peer can submit a zero-length ASCONF chunk that prevents inqueue progress and locks the receive softirq on specially configured hosts.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm affected product kernels expose SCTP with both required sysctls enabled and assess whether soft-lockup recovery behavior makes the service or host unavailable.

7. Call-site confidence

Call-site confidence: high

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

Patch: sctp: validate chunk length in the inqueue parser
Commit: 40507f59f230dcf60ae13cb48d34f8c6e9f40659
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40507f59f230dcf60ae13cb48d34f8c6e9f40659

Commit description:

SCTP chunks always include a four-byte generic header, but
sctp_inq_pop() currently accepts shorter declared lengths. A zero-length
chunk leaves chunk_end at the current header.

When ASCONF is covered by the association's SCTP-AUTH policy,
sctp_assoc_bh_rcv() can continue before the state machine performs its
normal chunk-length check. sctp_inq_pop() then returns the same malformed
chunk repeatedly and the receive softirq can lock up.

A remote SCTP peer can trigger this after establishing an association on
a kernel built with CONFIG_IP_SCTP and configured with
net.sctp.addip_enable=1 and net.sctp.auth_enable=1. The reproducer did
not require application credentials, a shared SCTP AUTH key, or
net.sctp.addip_noauth_enable=1.

On commit f967455 ("seg6: reset IP6CB after IPv6 decapsulation"),
one zero-length ASCONF caused repeated
watchdog soft-lockup reports in a two-vCPU KVM guest. All 3 pre-trigger
health probes succeeded, while 36 of 37 post-trigger probes failed. With
this change, all 37 post-trigger probes succeeded and no equivalent
soft-lockup signature appeared.

Reject chunks shorter than the generic SCTP header at the shared inqueue
parser boundary. Mark the packet for discard before either caller can
continue processing it, while preserving the four-byte generic minimum.
Declared-length 1 through 4 controls and kernel-generated ASCONF traffic
remained healthy. The patched sctp_hello selftest passed for IPv4 and
IPv6.

The complete private reproducer and validation evidence are available
directly to maintainers on request.

Fixes: bbd0d59 ("[SCTP]: Implement the receive and verification of AUTH chunk")
Cc: stable@vger.kernel.org
Signed-off-by: Charles Vosburgh <theminershive@gmail.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260827-sctp-zero-chunk-inqueue-v2-1-2e7669c6a6cb@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sctp/inqueue.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=40507f59f230dcf60ae13cb48d34f8c6e9f40659

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

The original Linux kernel CVE announcement for CVE-2026-99128 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99128

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: unknown
  Paranoid ActionableScore: 3

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).