[CVE-2026-89476][MODERATE 7.0] sctp: fix stream->outcnt underflow on duplicate RECONF responses

[CVE-2026-89476][MODERATE 7.0] sctp: fix stream->outcnt underflow on duplicate RECONF responses

[AL-KERNEL]

CVE: CVE-2026-89476
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: sctp: fix stream->outcnt underflow on duplicate RECONF responses
Commit: 2d867663c563e8009257b9edf9c8ec75d9bbf19a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d867663c563e8009257b9edf9c8ec75d9bbf19a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89476
Analysis date: Sat, 12 Sep 2026 06:42:05 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89476 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89476

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-2026-89476	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89476 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:L/I:L/A:H';CWE-191;CWE-841;CWE-682;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An integer underflow in the SCTP stream reconfiguration handling can occur because strreset_outstanding was tracked as a counter instead of per request type state. A cached RECONF chunk may contain multiple request parameters, and a duplicate response can process the same ADD_OUT request again while another request is still outstanding, causing stream outcnt rollback to be applied twice. This can corrupt SCTP association stream accounting and may lead to denial of service, with a plausible worst case of limited confidentiality or integrity impact if the underflow drives later out of bounds stream handling. For the CVSS the PR:N value is used because a remote SCTP peer can trigger the protocol state after reaching an SCTP endpoint and establishing an association. The issue is network reachable for systems that expose SCTP services using stream reconfiguration, but practical exposure is limited to deployments where SCTP is enabled and reachable from untrusted peers.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	REMOTE OOB INIT NETWORK KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap changed the conservative score.
* No floor or cap changed the paranoid score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable. A remote SCTP peer can send duplicate RECONF responses to an SCTP endpoint using stream reconfiguration.
* +1 Weak / indirect corruption candidate. The bug can underflow stream->outcnt, corrupting active SCTP association stream accounting.
* +1 Reliable DoS / strong availability concern. A corrupted stream count can plausibly break later SCTP stream handling and cause denial of service.

Paranoid additional signal:

* +1 Active structured state underflow with unresolved downstream impact. stream->outcnt is an active protocol state field used by later stream handling, so follow-on OOB or invalid stream processing requires manual review.

Non-awarded signals:

* RealUAF not awarded. No object lifetime violation or stale post-free use is established.
* Concrete repeated release/refcount not awarded. No first release, surviving exact reference, or second lifetime mutation exists.
* Strong LPE / strong exploitation leverage not awarded. No controlled reclaim, arbitrary write, callback control, credential impact, or demonstrated privilege escalation is shown.
* Confidentiality and integrity bonuses are not awarded conservatively. The patch establishes an integer underflow and stream accounting corruption, not a proven disclosure or unauthorized write primitive.
* Extra availability not awarded. The same likely DoS condition is already counted once.
* Race/timing penalty not applied. This is a protocol state sequencing issue with duplicate responses, not a narrow uncontrollable timing race.
* Niche subsystem penalty not applied. SCTP exposure depends on deployment, but subsystem niche status alone is not concrete negative evidence.

3. Reachability analysis

The evaluated attacker is a remote SCTP peer that can reach an SCTP endpoint where stream reconfiguration is enabled or used. The trigger is protocol-level: a cached RECONF chunk can contain multiple request parameters, and a duplicate response can cause the same ADD_OUT request to be processed again while another request remains outstanding.

Host administrator privileges are not required by the attacker once the SCTP service is reachable. Prior legitimate setup of an SCTP endpoint is required, but this is normal service exposure rather than attacker-required host-admin setup. Namespace-scoped privilege is not relevant to the remote trigger. No special hardware, boot option, or local device access is indicated. The issue is not a hard race.

4. Primitive and severity interpretation

The exact primitive is an integer underflow of stream->outcnt caused by treating strreset_outstanding as a count instead of per-request-type state. The affected object is active structured SCTP association state, not fixed stale storage. The demonstrated issue is protocol state corruption that can lead to denial of service.

This is not an ordinary UAF and not a concrete repeated release/refcount bug. It is also not CLASS_A because the patch does not establish controlled memory corruption, arbitrary read/write, controlled reclaim, or privilege escalation. It is CLASS_B because a remotely influenced active kernel protocol state field can underflow and the downstream exploitation impact of that corrupted count is unresolved.

ImportantCandidate=YES because a network-reachable integer underflow in active stream accounting should not be automatically downgraded without checking later users of stream->outcnt. AutoDowngradeAllowed=NO because the issue has meaningful unresolved exploitation leverage beyond a purely local crash-only bug, even though Strong Important is not proven.

5. One-sentence report phrase

A remote SCTP peer can trigger duplicate RECONF response handling that rolls back stream outcnt twice, causing an integer underflow in active SCTP stream accounting and a likely denial of service with unresolved follow-on impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable SCTP protocol state corruption is established, and later use of the underflowed stream count may affect stream indexing or bounds-sensitive logic.

7. Call-site confidence

Call-site confidence: medium

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

Patch: sctp: fix stream->outcnt underflow on duplicate RECONF responses
Commit: 2d867663c563e8009257b9edf9c8ec75d9bbf19a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d867663c563e8009257b9edf9c8ec75d9bbf19a

Commit description:

A cached RECONF chunk may contain more than one request parameter.  A
duplicate response can therefore find and process the same ADD_OUT request
again while another parameter is still outstanding, rolling back outcnt
twice and possibly underflowing it.

Track outstanding request types as bits and clear each bit after its first
response.  Later responses for the same request are then ignored.

Fixes: 11ae76e ("sctp: implement receiver-side procedures for the Reconf Response Parameter")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Link: https://lore.kernel.org/netdev/20260730110225.37371-1-juny24602@gmail.com/
Suggested-by: Xin Long <lucien.xin@gmail.com>
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Link: https://patch.msgid.link/20260824081832.98717-3-juny24602@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/sctp/structs.h
  net/sctp/stream.c
  net/sctp/sm.h
  net/sctp/stream_sched.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=2d867663c563e8009257b9edf9c8ec75d9bbf19a

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

The original Linux kernel CVE announcement for CVE-2026-89476 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89476

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:L/A:H
  The Best / paranoid CVSS score: 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: 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).