[CVE-2026-74588][IMPORTANT] sctp: keep chunk->transport in step with the list it is queued on

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

CVE: CVE-2026-74588
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sctp: keep chunk->transport in step with the list it is queued on
Commit: 6575fb17230814b48b471727c8410c0aadff9274
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6575fb17230814b48b471727c8410c0aadff9274
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74588
Analysis date: Sat, 22 Aug 2026 16:36:30 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An SCTP peer can trigger a stale `chunk->transport` use-after-free after ASCONF Delete-IP and SACK reneging, causing at least a remote kernel crash and requiring manual review for possible memory-corruption impact beyond DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74588 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74588

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-2026-74588	IMPORTANT	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-825;CWE-362;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'SCTP outqueue retransmit handling can leave a gap-acked chunk on one transport transmitted list while its stored chunk transport pointer still names another transport. If the peer removes the old transport with ASCONF Delete-IP and later sends SACKs that renege the TSN, later accounting in sctp_check_transmitted can dereference the freed transport and trigger a kernel use-after-free. For the CVSS the PR:N is used because the triggering traffic comes from the SCTP association peer and does not require local privileges on the victim, assuming the SCTP service is reachable and accepts the association. The issue is network reachable over SCTP, although practical exploitation usually requires a reachable SCTP endpoint with ASCONF or multihoming behavior and careful packet sequencing. Impact is at least remote denial of service via kernel crash, and because this is a kernel use-after-free with a stale transport pointer, the paranoid assessment keeps possible confidentiality and integrity impact for manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-74588 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DANGER UAF NETWORK LINUS KPANIC 	-	-	checked

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

ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The commit states that both ASCONF Delete-IP removal and later SACKs come from the SCTP association peer.
* Memory corruption, strong corruption primitive: +2. The bug is a real kernel use-after-free involving `chunk->transport` pointing to an RCU-freed `sctp_transport`.
* Real lifetime corruption: +1. The stale pointer survives transport removal and is later used from a live transmitted list.
* Reliable kernel crash / strong DoS: +1. KASAN reports slab-use-after-free in `sctp_check_transmitted()`, and this is plausibly a kernel crash.
* Hard or special sequencing required: -1. Triggering requires a specific SCTP state sequence involving gap-acked data, ASCONF Delete-IP, SACK reneging, and subsequent SACK processing.

Conservative total: 5.

Paranoid additional signals:

* Weak LPE concern: +1. The freed transport is reached through a stale pointer and used in accounting logic, including a read-modify-write style update to `flight_size`, but no controlled reclaim or object replacement is shown.
* Confidentiality impact plausible: +1. Kernel UAF classes can sometimes expose stale object contents, but no concrete disclosure path is demonstrated.
* Integrity impact plausible: +1. The stale pointer can affect freed-object memory through transport accounting, but no arbitrary write primitive is demonstrated.

Paranoid total: 8.

## 3. Reachability analysis

The bug is triggerable by an SCTP association peer, not by local code on the victim. No local privileges are required if the target exposes an SCTP service and accepts the relevant association behavior. The path is network reachable over SCTP, but not as broadly exposed as TCP or UDP core paths. Realistic exploitation likely requires SCTP multihoming or ASCONF support, a live association, a gap-acked chunk, peer transport removal via ASCONF Delete-IP, and SACK reneging. This makes exploitation more complex than a simple packet parser crash, but still security-relevant because the entire sequence is controlled by the association peer.

Namespaces and containers do not materially reduce the remote attack requirement on the victim side. If SCTP is disabled, blocked by firewall policy, or not used by exposed services, practical exposure is much lower.

Call-site confidence: high. The commit message identifies the exact stale pointer, freeing path, later dereference path, and KASAN UAF location.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation bug. It is a network-reachable kernel lifetime bug with a concrete use-after-free and a demonstrated crash path. Realistically, the most supported impact is remote denial of service. Theoretical escalation beyond DoS is less certain because the patch does not show attacker-controlled reclaim, type confusion, callback control, or arbitrary write. However, because it is a remote SCTP kernel UAF with stale pointer use after RCU free, it should be treated as Actionable Moderate at minimum and as a Strong Important candidate for manual review.

## 5. One-sentence report phrase

An SCTP peer can trigger a stale `chunk->transport` use-after-free after ASCONF Delete-IP and SACK reneging, causing at least a remote kernel crash and requiring manual review for possible memory-corruption impact beyond DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: network-reachable kernel UAF with a concrete stale pointer and KASAN report. Even though practical LPE or remote code execution is not demonstrated, the bug class and remote SCTP control make auto-close unsafe.

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

Patch: sctp: keep chunk->transport in step with the list it is queued on
Commit: 6575fb17230814b48b471727c8410c0aadff9274
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6575fb17230814b48b471727c8410c0aadff9274

Commit description:

__sctp_outq_flush_rtx() moves a gap-acked chunk onto another transport's
transmitted list without updating chunk->transport:

	if (chunk->tsn_gap_acked) {
		list_move_tail(&chunk->transmitted_list,
			       &transport->transmitted);
		continue;
	}

The chunk then sits on a live transport's list while chunk->transport still
names a different one.  If that transport is removed - sctp_assoc_rm_peer()
from an ASCONF Delete-IP - sctp_transport_free() RCU-frees it and the chunk
is left with a dangling pointer.  sctp_assoc_rm_peer() scrubs
peer->transmitted and asoc->outqueue.out_chunk_list, but the chunk is on
neither.

The pointer is not followed while tsn_gap_acked is set.  A SACK that
reneges on the TSN clears the flag, and the next SACK reaches

	tchunk->transport->flight_size -= sctp_data_size(tchunk);

inside the freed transport.  KASAN reports a slab-use-after-free read in
sctp_check_transmitted(), freed from sctp_assoc_rm_peer().  Both the
removal and the SACKs come from the association peer.

Set chunk->transport at the move.  The ordinary resend path needs nothing:
it reaches its list_move_tail() only after sctp_packet_append_chunk()
returned SCTP_XMIT_OK, and __sctp_packet_append_chunk() has rebound the
chunk by then.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260729160028.54546-1-baul.lee@xbow.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sctp/outqueue.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=6575fb17230814b48b471727c8410c0aadff9274

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

The original Linux kernel CVE announcement for CVE-2026-74588 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74588

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: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: 8

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: YES
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).