[CVE-2026-98846][IMPORTANT] sctp: check RCV_SHUTDOWN after the sendmsg connect wait

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

CVE: CVE-2026-98846
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: sctp: check RCV_SHUTDOWN after the sendmsg connect wait
Commit: e8f972b1759d26bbefe4c524caae1ed7c8a593bd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e8f972b1759d26bbefe4c524caae1ed7c8a593bd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98846
Analysis date: Fri, 23 Oct 2026 21:44:21 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A local unprivileged SCTP user can race sendmsg, backlog ABORT processing, and SHUT_RD so that sctp_sendmsg_to_asoc continues with a freed association, causing at least kernel DoS and leaving stronger UAF consequences unresolved.

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

The original Linux kernel CVE announcement for CVE-2026-98846 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98846

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
======================================================================

CVE-2026-98846	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98846 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME INDEPENDENT_LEGACY=MODERATE_KPANIC INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=MODERATE_KPANIC (final reconciliation agrees; DECISION: both IMPORTANT)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A lifetime race in the SCTP sendmsg connection wait can leave sctp_sendmsg_to_asoc using an association after its final reference has been released. sctp_wait_for_connect drops the socket lock while sleeping, allowing an OOTB ABORT from the backlog to unlink the association while a concurrent shutdown with SHUT_RD causes the waiter to return with no error. The final sctp_association_put can free the object and execution can then continue through a dangling association pointer. Triggering requires local sendmsg and shutdown operations on an SCTP socket plus correctly timed ABORT processing. A remote peer may provide the ABORT, but remote traffic alone is not sufficient for the demonstrated race. For the CVSS the PR is L because the complete trigger requires an unprivileged local user context operating an SCTP socket. The bug provides a kernel use after free primitive rather than only a NULL dereference or resource leak. Impact is at least local DoS and may include limited confidentiality or integrity impact due to memory corruption. Reliable privilege escalation has not been demonstrated, but the explicit dangling pointer and possible reuse of the freed association justify a paranoid CIA:H/H/H assessment and manual review. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 6 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. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE_KPANIC to IMPORTANT because a specific evidence based protection or manual review veto was applied and 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 has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. 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 6)	MAYBE	INIT SIMPLEFIX NETWORK LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Conservative CVSS:3.1: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H (5.8)
* Paranoid CVSS:3.1: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H (6.9)
* CWE: CWE-416, CWE-362, CWE-667
* No floor or cap changed the scores.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF. The final sctp_association_put can free the association while sctp_sendmsg_to_asoc retains and subsequently uses the same pointer.
* +1 Local unprivileged trigger. Creating and operating an SCTP socket with sendmsg and shutdown does not normally require host administrator privileges.
* +1 Active structured stale object. The dangling object is an active sctp_association that remains in the send path rather than a fixed stale buffer used only during cleanup.
* +1 Availability impact. Continued processing through the freed association can fault or corrupt kernel state and crash the host.
* +1 paranoid integrity concern. Continued send processing can consume or mutate association state after its lifetime ended. The exact post-free write sink is not visible in the supplied diff, so this point applies only to the paranoid score.
* No remote reachability bonus. A peer may supply the ABORT, but remote traffic alone cannot perform the required local sendmsg and shutdown sequence.
* No hard-race penalty. The trigger is timing dependent, but the attacker can repeatedly drive the local socket operations. The supplied evidence does not establish a one-shot or externally uncontrollable race.
* Controlled reclaim, arbitrary read, arbitrary write, callback control, and privilege escalation are not demonstrated.
* No repeated release or refcount point is awarded. The first sctp_association_put can end the lifetime, but the supplied path establishes later dangling-pointer use rather than a concrete second put or refcount mutation.
* Additional availability points are not awarded for multiple manifestations of the same potential kernel crash.
* B5-R, B3-PENDING, and B4-PENDING do not apply.

3. Reachability analysis

The local call path is sctp_sendmsg to sctp_sendmsg_to_asoc to sctp_wait_for_connect. The wait releases the socket lock while sleeping. During that interval an OOTB ABORT can be processed from the socket backlog and unlink the association. A concurrent shutdown with SHUT_RD sets RCV_SHUTDOWN and can make the waiter stop with err equal to zero before checking asoc base dead. The final association reference can then be dropped and the send path can resume with a dangling pointer.

An ordinary local user can normally create an SCTP socket and invoke sendmsg and shutdown on its own descriptor. Host UID 0 and host administrator capabilities are not required for these operations. No namespace capability is required, so this is below host administration rather than a namespace-scoped privileged operation.

A remote SCTP peer can participate by causing ABORT processing, but the demonstrated trigger is not remote-only. The attacker must also control or coordinate local socket operations. SCTP and the affected stream interleave path must be available. No special hardware, boot-time privilege, or prior host administrator configuration is established beyond subsystem availability.

The race requires ordering between connection waiting, backlog ABORT processing, and SHUT_RD. AC:H remains appropriate for CVSS because successful triggering depends on concurrent timing, even though repeated attempts may be possible.

4. Primitive and severity interpretation

The exact primitive is a use after free of struct sctp_association. This is an active structured object rather than fixed stale storage. It is not restricted to a destructor or cleanup dereference because sctp_sendmsg_to_asoc resumes normal send-path processing after the final reference may have freed the object.

The established sequence contains one lifetime-ending operation followed by stale use. It does not establish a second release, second refcount mutation, controlled replacement, or directed overwrite. Therefore this is CLASS_B rather than CLASS_A.

The patch-supported consequence is unsafe kernel access through a dangling association pointer, with local kernel DoS as the minimum credible impact. Limited confidentiality or integrity impact is defensible because active association fields may be consumed after free. Arbitrary kernel read or write and reliable privilege escalation are not established. CIA:H/H/H is reserved for the paranoid CVSS vector because a real kernel UAF exists, but it remains a worst-case assessment requiring source and allocator review.

ImportantCandidate is YES because this is a strong active-object UAF reachable below host administrator privilege and its post-free send-path leverage remains unresolved. AutoDowngradeAllowed is NO because automatic crash-only classification would discard a concrete lifetime violation with active post-lifetime processing. This does not prove Important severity or local privilege escalation.

The fix checks RCV_SHUTDOWN immediately after sctp_wait_for_connect and returns ESRCH before reusing the association. ESRCH also instructs the outer sctp_sendmsg path not to free an association that may already have been destroyed.

5. One-sentence report phrase

A local unprivileged SCTP user can race sendmsg, backlog ABORT processing, and SHUT_RD so that sctp_sendmsg_to_asoc continues with a freed association, causing at least kernel DoS and leaving stronger UAF consequences unresolved.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Review all association accesses after sctp_wait_for_connect, determine whether any post-free writes or callbacks are reachable, inspect the association allocation cache and reclaim opportunities, and verify whether one unprivileged actor can repeatedly generate the complete ABORT and shutdown sequence.

7. Call-site confidence

Call-site confidence: high

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

Patch: sctp: check RCV_SHUTDOWN after the sendmsg connect wait
Commit: e8f972b1759d26bbefe4c524caae1ed7c8a593bd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e8f972b1759d26bbefe4c524caae1ed7c8a593bd

Commit description:

sctp_wait_for_connect() drops the socket lock while it sleeps. An
out-of-the-blue ABORT can then be processed from the socket backlog and
unlink the association. If a concurrent shutdown(fd, SHUT_RD) sets
RCV_SHUTDOWN, the waiter breaks with err == 0 before checking
asoc->base.dead. Its final sctp_association_put() can then free the
association, leaving sctp_sendmsg_to_asoc() to continue with a dangling
pointer.

Check RCV_SHUTDOWN along with the wait error in sctp_sendmsg_to_asoc()
before using the association again. The check only accesses the socket,
so it needs no additional association reference. Return the existing
-ESRCH so that sctp_sendmsg() skips freeing a new association that may
already have been destroyed.

Keep sctp_wait_for_connect() unchanged to preserve its behavior for the
connect() caller.

Fixes: 668c9be ("sctp: implement assign_number for sctp_stream_interleave")
Cc: stable@vger.kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Signed-off-by: Jun Yang <junvyyang@tencent.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260926095606.68601-1-juny24602@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sctp/socket.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=e8f972b1759d26bbefe4c524caae1ed7c8a593bd

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

The original Linux kernel CVE announcement for CVE-2026-98846 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98846

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: 5
  Paranoid ActionableScore: 6

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).