[CVE-2026-98548][IMPORTANT] sctp: discard the rest of the packet on a stale-cookie error

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

CVE: CVE-2026-98548
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: sctp: discard the rest of the packet on a stale-cookie error
Commit: c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98548
Analysis date: Sun, 18 Oct 2026 22:18:06 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A malicious SCTP peer can preserve a pointer to an RCU-freed transport through bundled stale-cookie and DATA processing, causing a remote kernel use-after-free with subsequent field access, possible post-free writes, live-list corruption, and denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-98548 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98548

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-2026-98548	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98548 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 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:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '9.8';DESCR 'A use-after-free in SCTP packet processing occurs when a bundled ERROR Stale Cookie chunk removes a non-primary transport and a following DATA chunk stores the same transport pointer back into association state. After the receive reference is released and the transport is freed through RCU, a later FWD-TSN or delayed SACK can dereference the dangling pointer. Some paths may also write to the freed transport and link it into the live transport list, creating a stronger memory corruption primitive than a crash-only invalid read. A malicious SCTP peer can trigger the condition through protocol traffic sent from a non-primary address while the association is in COOKIE-ECHOED state. For the CVSS the PR:N value is used because no capabilities or authenticated local user context are required when the attacker can communicate with the exposed SCTP endpoint. The practical attack surface is limited to systems that accept SCTP traffic and permit the required association and address state. Impact is at least remote denial of service through a kernel crash. The confirmed use-after-free write path also supports possible confidentiality and integrity impact and requires manual security review. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 9.8 and the production ActionableScore returned 7 with recommendation Strong Important candidate. 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 5.3 and ActionableScore 5 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 Strong Important candidate (with actual score 7)	MAYBE	READ DANGER UAF NETWORK 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=7
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: 7
* Conservative score: 7
* Raw paranoid score: 7
* Paranoid score: 7
* No floor or cap changed either score.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Positive signals:

* +2 Real UAF. The association retains `last_data_from` after the referenced `sctp_transport` is RCU-freed, and later processing dereferences it.
* +2 Direct network reachability. A malicious SCTP peer can trigger the sequence using bundled stale-cookie ERROR and DATA chunks from a non-primary address.
* +1 Reliable kernel crash. The supplied reproducer produced a KASAN slab-use-after-free in the SCTP receive path.
* +1 Attacker-influenced active-object lifetime. The peer controls the SCTP association, non-primary transport, packet ordering, and protocol transition that removes the transport and restores its pointer into association state.
* +1 Structured stale-object leverage. Later paths read transport fields, may write through the freed object, and may link it into the live transport list. This is meaningfully stronger than an immediate stale-pointer fault.

Not awarded:

* No +2 strong exploitation leverage. Controlled reclaim, attacker-shaped replacement contents, directed write destination, callback control, or arbitrary kernel read/write is not established.
* No separate integrity point. The post-free write and live-list insertion are already represented by structured stale-object leverage.
* No additional availability point. The same crash cannot be counted twice.
* No confidentiality point. No disclosure path is demonstrated.
* No race penalty. The vulnerable behavior follows an attacker-controlled ordered packet sequence rather than a narrow, externally uncontrollable timing race.
* No repeated-release/refcount point. There is no demonstrated second `put`, release, or refcount mutation through the exact dangling reference.
* NEW52 B5-R does not apply. This is not a retry-accumulated heap OOB write.
* NEW53 B3-PENDING does not apply. No shared file-backed or COW backing write is involved.
* NEW54 B4-PENDING does not apply. No authorization decision or omitted mandatory guard is involved.

3. Reachability analysis

A remote malicious SCTP peer can trigger the bug if it can participate in an SCTP association, reach `COOKIE-ECHOED`, use a non-primary address, and send the required bundled chunks. The commit also explicitly identifies a local unprivileged user acting as the peer, with no capabilities required.

No host UID 0, host-admin capability, namespace capability, special hardware, or separate privileged control-plane operation is required for the documented trigger. An SCTP endpoint and suitable association state must exist, but this is protocol exposure rather than evidence of an attacker-required privileged setup.

The trigger is deterministic packet-state manipulation, not a conventional race. The practical exposure is limited to hosts accepting relevant SCTP traffic, but this does not negate direct network reachability.

4. Primitive and severity interpretation

The primitive is an active structured-object UAF:

1. `SCTP_CMD_DEL_NON_PRIMARY` removes the arrival transport.
2. A bundled DATA chunk copies the removed `chunk->transport` back into `asoc->peer.last_data_from`.
3. The receive reference is dropped and the transport is RCU-freed.
4. FWD-TSN or the delayed SACK timer later dereferences the dangling pointer.
5. Additional paths may write into the freed transport and insert it into the live transport list.

This is broader active use, not cleanup-only dereferencing or fixed stale storage. The demonstrated consequence is a remotely triggerable kernel UAF and crash. The unresolved consequence is exploitation of the post-free writes and list manipulation after allocator reuse.

The issue remains CLASS_B because controlled reclaim, directed corruption, and a completed privilege-escalation primitive are not demonstrated. It is nevertheless a strong Important candidate because the attacker controls the network sequence, the stale object survives in association-lifetime state, and privileged kernel consumers later perform nontrivial reads and possible writes through it.

`ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` preserve this unresolved remote active-object corruption for manual exploitation-leverage analysis. This does not assert proven remote code execution, privilege escalation, or confidentiality compromise.

5. One-sentence report phrase

A malicious SCTP peer can preserve a pointer to an RCU-freed transport through bundled stale-cookie and DATA processing, causing a remote kernel use-after-free with subsequent field access, possible post-free writes, live-list corruption, and denial of service.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug is remotely reachable without capabilities and establishes an active structured-object UAF with a possible write to freed storage and insertion into a live kernel list.

7. Call-site confidence

Call-site confidence: high

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

Patch: sctp: discard the rest of the packet on a stale-cookie error
Commit: c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a

Commit description:

When an association is in COOKIE-ECHOED state and the peer sends a
bundled [ERROR(Stale Cookie)][DATA] packet from one of its non-primary
addresses, processing the ERROR chunk takes the non-fatal stale-cookie
retry path sctp_sf_do_5_2_6_stale(), which queues
SCTP_CMD_DEL_NON_PRIMARY while keeping the association alive.
sctp_cmd_del_non_primary() removes every non-primary transport -
including the very transport this packet arrived on, which is still
referenced by the receive lookup and shared by all chunks of the
packet via chunk->transport.

sctp_assoc_rm_peer() does redirect asoc->peer.last_data_from away from
the removed transport, but right afterwards the bundled DATA chunk
makes sctp_assoc_bh_rcv() re-register
asoc->peer.last_data_from = chunk->transport unconditionally, undoing
the redirection with the just-removed transport.

Once the packet is done, the receive reference is dropped and the
transport is RCU-freed, while the surviving association keeps the
dangling last_data_from.  A later FWD-TSN (or the delayed SACK timer)
makes sctp_gen_sack() dereference it (->param_flags and friends), and
sctp_make_sack()/sctp_outq_select_transport() may write to the freed
object and link it into the live transport list.  This is a
use-after-free triggerable by any malicious SCTP peer (or a local
unprivileged user acting as one) with no capabilities required:

  BUG: KASAN: slab-use-after-free in sctp_do_sm+0x498a/0x5660
  Read of size 4 at addr ffff88800e1e356c by task poc/115
  Call Trace: sctp_do_sm <- sctp_assoc_bh_rcv <- sctp_inq_push <-
              sctp_rcv <- ip_protocol_deliver_rcu <- ip_rcv
  Allocated: sctp_transport_new <- sctp_assoc_add_peer <-
             sctp_process_init (INIT-ACK processing)
  Freed: kfree <- sctp_transport_destroy_rcu <- rcu_core
         (call_rcu queued by sctp_transport_put at end of sctp_rcv)
  The buggy address is located 364 bytes inside of freed 1024-byte
  region [ffff88800e1e3400, ffff88800e1e3800), cache kmalloc-1k

Note that commit 03a9d10 ("sctp: drop a chunk if its transport
was removed") only covers the window between the receive lookup and
the chunk processing (e.g. an ASCONF DEL-IP racing the socket backlog);
here the transport is removed *while* the packet is being processed,
by an earlier chunk of the same packet, so the drop in sctp_inq_push()
does not reach this path.  Verified with the bundled [ERROR(Stale
Cookie)][DATA] + FWD-TSN reproducer: the KASAN report above still
fires with that commit applied, and is gone with this patch on top.

Fix it by discarding the rest of the packet on this path, as suggested
by Xin.  After the stale-cookie ERROR has sent the association back to
COOKIE-WAIT and removed the non-primary transports, the remaining
chunks of the packet can only run against the restarted handshake
while referencing the removed arrival transport through
chunk->transport: besides the last_data_from registration above,
sctp_cmd_setup_t2() and the sctp_make_*() reply builders would also
copy that pointer into association-lifetime state that
sctp_assoc_rm_peer() has already sanitized.  Let the peer retransmit
them, in line with what sctp_inq_push() does for chunks whose
transport was removed before processing.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Suggested-by: Xin Long <lucien.xin@gmail.com>
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Cc: stable@vger.kernel.org
Signed-off-by: Aohan Mei <henrymei@tencent.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260921093707.1432184-1-ljp1205831794@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sctp/sm_statefuns.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=c0af3fd150fdc43821df3bb77e5ec74faf4dfc7a

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

The original Linux kernel CVE announcement for CVE-2026-98548 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98548

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:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 7

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).