[CVE-2026-89478][MODERATE 7.0] sctp: drop a chunk if its transport was removed

[CVE-2026-89478][MODERATE 7.0] sctp: drop a chunk if its transport was removed

[AL-KERNEL]

CVE: CVE-2026-89478
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: sctp: drop a chunk if its transport was removed
Commit: c6c86a5e62a4fec36692ddd64b9144b660f71f96
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c6c86a5e62a4fec36692ddd64b9144b660f71f96
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89478
Analysis date: Sat, 12 Sep 2026 06:44:07 -0400
ActionableScore: 6
ActionableScore lower bound: 5
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-2026-89478 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89478

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-2026-89478	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89478 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-416;CWE-362;*CWE-672;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'An SCTP use after free can occur when a packet keeps a chunk.transport pointer while an authenticated ASCONF DEL-IP removes the same transport from the association. A DATA chunk from that packet can restore the removed transport into asoc peer last_data_from, and a later delayed SACK can carry the stale pointer into transport selection where freed transport state is read. This is a network reachable race condition with AC:H because the vulnerable window depends on packet backlog or direct receive timing around socket locking and transport removal. For the CVSS the PR:N is used for the paranoid score because a remote SCTP peer may trigger the path without local code execution on the target once it can reach and form the required association. If SCTP AUTH keys or association policy restrict the peer set, PR:L is a more realistic base interpretation. Impact is at least remote denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the primitive is UAF in kernel networking code.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)  SKIP CVE-2026-89478 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	UAF NETWORK LINUS KPANIC PACKET  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap changed the score.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation. The commit establishes that an SCTP transport can be removed from the association, later freed by RCU, and then read through a stale pointer during delayed SACK transport selection.
* +2 Remote reachable / directly network-triggerable. The trigger is driven by SCTP protocol traffic, specifically authenticated ASCONF DEL-IP plus affected packet processing. This is not a local syscall-only or host-admin-only path.
* +1 Reliable kernel crash / strong DoS. Reading freed transport state in kernel networking code is a realistic kernel fault / DoS condition.

Paranoid additional signal:

* +1 Attacker-influenced active-object lifetime manipulation. The remote SCTP peer can influence the relevant transport removal and packet/chunk sequence, and the stale object is later consumed in active SCTP output processing rather than only in a cleanup destructor.

Negative signals:

* 0 No hard-race penalty applied. The bug is a race, but the supplied evidence describes reachable backlog and direct receive windows rather than a proven one-shot or externally uncontrollable timing condition.
* 0 No host-admin penalty applied. Host-admin-equivalent privilege is not required by the supplied trigger model.
* 0 No rare-subsystem penalty applied. SCTP is less common than TCP, but the subsystem being optional or less common is not enough by itself for a negative point.

Important non-awarded signals:

* Strong exploitation leverage +2 not awarded. Controlled reclaim, attacker-shaped replacement object, arbitrary read/write, callback control, refcount takeover, or demonstrated LPE is not established.
* Structured stale-object leverage was not separately stacked beyond the paranoid lifetime-manipulation point. The same stale transport consumption is the basis for the unresolved active-object concern.
* Confidentiality and integrity points not awarded. The patch supports UAF and DoS, but not a concrete disclosure path or unauthorized modification primitive.
* Extra availability point not awarded. The same stale transport use supports the kernel crash/DoS signal and is not counted again.
* Concrete repeated release/refcount signal not awarded. The later vulnerable operation reads freed transport state, but the patch does not establish a second put/free/refcount mutation on the same dangling reference.

Repeated release/refcount check:

* First release can end lifetime: yes, sctp_transport_free drops the association reference and the transport can later be freed after remaining references go away.
* Exact same reference survives: yes, the stale transport pointer can be carried through chunk->transport and asoc->peer.last_data_from.
* Second operation is a real lifetime/refcount mutation: no, the later described operation is reading freed transport state in sctp_outq_select_transport.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no explicit floor needed because the conservative raw score is already 5.

3. Reachability analysis

The evaluated attacker is a remote SCTP peer that can form or participate in an SCTP association and send authenticated ASCONF DEL-IP traffic. The issue is network-triggerable through SCTP packet processing and does not require local code execution on the target host.

The ASCONF DEL-IP requirement means the peer must satisfy SCTP authentication or association policy, so deployments with strict SCTP AUTH keys or trusted peer lists may reduce practical exposure. That restriction does not make the issue host-admin-only.

HostAdminRequired is NO because the supplied trigger is a protocol peer, not a host administrator. NamespaceScopedPrivilege is NO because the trigger is not based on namespace-scoped Linux capabilities. No prior local privileged setup by the attacker is established, although the vulnerable endpoint must have SCTP in use and reachable.

The race is timing-dependent, but the same remote peer can influence the packet sequence and transport removal. The supplied evidence does not justify treating it as a rare one-shot race for ActionableScore purposes.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free of an SCTP transport object. The stale object is an active structured networking object, not fixed stale storage. The stale use is broader than cleanup-only behavior because the freed transport pointer can be placed into asoc->peer.last_data_from and later consumed by delayed SACK output transport selection.

This is CLASS_B. It is a concrete kernel lifetime bug with active-object stale use, but the patch does not establish CLASS_A evidence such as controlled reclaim, directed write, arbitrary read/write, callback influence, credential manipulation, or demonstrated privilege escalation.

Demonstrated consequence is remote kernel DoS through UAF-triggered invalid access. Speculative consequence is stronger exploitation through post-free object reuse, but that is not proven. Because the UAF is network reachable and involves active SCTP transport state rather than a narrow cleanup-only stale dereference, Important-vs-Moderate exploitation leverage remains unresolved.

ImportantCandidate is YES because this is a network-reachable active-object UAF requiring manual exploitation-leverage review. AutoDowngradeAllowed is NO because automatic closure would discard a concrete unresolved kernel lifetime primitive in packet-processing code.

Likely operational severity is at least Actionable Moderate. It is a Strong Important candidate only if later evidence shows controlled reuse, useful stale field consumption after replacement, information disclosure, memory corruption beyond stale read, or privilege escalation.

5. One-sentence report phrase

An SCTP peer can trigger a race where an authenticated ASCONF DEL-IP removes a transport while queued DATA processing later reuses the stale transport pointer for delayed SACK selection, causing a network-reachable kernel UAF and possible DoS.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real network-reachable active-object UAF in SCTP transport handling, with DoS demonstrated and unresolved exploitation leverage beyond crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: sctp: drop a chunk if its transport was removed
Commit: c6c86a5e62a4fec36692ddd64b9144b660f71f96
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c6c86a5e62a4fec36692ddd64b9144b660f71f96

Commit description:

sctp_rcv() resolves the transport once per packet and leaves it in
chunk->transport. The lookup reference, or the one sctp_add_backlog() takes
if the socket is owned by userspace, keeps it around until the chunk has
been processed.

An authenticated ASCONF DEL-IP can remove it in the meantime.
sctp_assoc_rm_peer() takes the transport out of the association and calls
sctp_transport_free(), which tags it dead and drops the reference the
association held. There is a window on both paths: the packet can sit on
the socket backlog, and on the direct path the lookup completes before
bh_lock_sock().

The DATA chunk in that packet puts the removed transport back into
asoc->peer.last_data_from. Once the packet is done that reference goes
away and the transport is freed by RCU, so the next delayed SACK carries
the pointer into the SACK chunk and sctp_outq_select_transport() reads the
freed transport's state.

Drop the chunk in sctp_inq_push(), next to the existing rcvr->dead check.
Both paths reach it with the association's socket lock held. The peer
retransmits it.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/aoUJHQmxL0LFIMCw@v4bel
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=c6c86a5e62a4fec36692ddd64b9144b660f71f96

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

The original Linux kernel CVE announcement for CVE-2026-89478 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89478

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:L/UI:N/S:U/C:L/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: 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: 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).