[CVE-2026-74587][IMPORTANT] sctp: fix use-after-free of cached ASCONF chunk

[CVE-2026-74587][IMPORTANT] sctp: fix use-after-free of cached ASCONF chunk

[AL-KERNEL]

CVE: CVE-2026-74587
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sctp: fix use-after-free of cached ASCONF chunk
Commit: 10459b03e2d9ee12435e96f587de4d4cacdbf435
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10459b03e2d9ee12435e96f587de4d4cacdbf435
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74587
Analysis date: Sat, 22 Aug 2026 15:44:31 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote SCTP peer may trigger a use-after-free and second release of a cached ASCONF chunk during peer restart and delayed authenticated ASCONF-ACK handling, causing at least kernel DoS and requiring manual review for memory-corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-74587 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74587

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-2026-74587	IMPORTANT	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:H/I:H/A:H';*CWE-416;CWE-415;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 can leave addip_last_asconf pointing to a freed ASCONF chunk when peer restart handling purges the ASCONF queue while the association remains alive. A delayed authenticated ASCONF ACK can later reach the ASCONF ACK handling path and use the stale chunk, causing a use after free and a second release in the kernel SCTP stack. For the CVSS the PR:N is selected because a remote SCTP peer can trigger the affected protocol path without local privileges on the victim when SCTP and ASCONF are reachable. The attack is network reachable but AC:H is used because reliable triggering depends on peer restart timing, delayed authenticated ASCONF ACK processing, and a race with T4 expiry. Impact is at least remote denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption, so this should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DANGER RACE UAF NETWORK HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A remote SCTP peer can influence peer restart handling and delayed authenticated ASCONF-ACK delivery when SCTP ASCONF is in use.

* Generic memory corruption, strong primitive: +2
  The commit explicitly describes a stale freed `addip_last_asconf` chunk, use-after-free, and a second release.

* Real lifetime corruption: +1
  This is a real object lifetime bug: cached pointer survives `sctp_chunk_free()` while the association remains alive.

* Reliable kernel crash / strong DoS: +1
  UAF, double release, and the related NULL dereference in T4 expiry can plausibly crash the kernel.

* Hard or unreliable race / special timing required: -1
  Triggering depends on peer restart timing, delayed authenticated ASCONF-ACK processing, and timer expiry interaction.

Paranoid additional signals:

* Weak LPE concern / memory-corruption escalation concern: +1
  UAF plus second release in a network protocol stack is enough to require manual review, although no controlled reclaim or overwrite primitive is shown.

* Confidentiality or integrity impact plausible from the same memory corruption class: +1
  Counted conservatively as one combined triage point to avoid double-counting the same UAF/double-free primitive.

## 3. Reachability analysis

The likely attacker is a remote SCTP peer able to establish or interact with an SCTP association using ASCONF and authenticated ASCONF-ACK behavior. No local user account is required on the victim, so PR:N is reasonable for CVSS-style reachability if the SCTP service and relevant ASCONF path are exposed.

This is not a generic Internet-wide TCP/IP core bug. Exposure depends on SCTP being enabled and reachable, and on the affected ADD-IP or ASCONF behavior being usable in the deployment. Containers and namespaces do not materially lower the requirement because the main concern is remote protocol reachability, not local namespace privilege.

Call-site confidence: high. The commit message gives the concrete path through `sctp_asconf_queue_teardown()`, `sctp_sf_do_dupcook_a()`, `sctp_sf_do_asconf_ack()`, `sctp_process_asconf_ack()`, and `sctp_sf_t4_timer_expire()`.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate or auto-closed DoS-only issue. The realistic minimum impact is remote kernel crash or denial of service under non-trivial SCTP timing conditions. The theoretical worst case is more serious because the bug is a real network-reachable UAF and double-free candidate, even though the patch does not demonstrate attacker-controlled reclaim, arbitrary write, or a reliable privilege-escalation primitive.

Overall this is an Actionable Moderate at minimum and a Strong Important candidate for manual kernel security review.

## 5. One-sentence report phrase

A remote SCTP peer may trigger a use-after-free and second release of a cached ASCONF chunk during peer restart and delayed authenticated ASCONF-ACK handling, causing at least kernel DoS and requiring manual review for memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable SCTP UAF and double-free candidate with a concrete stale-pointer lifetime bug, not merely a WARN or validation cleanup.

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

Patch: sctp: fix use-after-free of cached ASCONF chunk
Commit: 10459b03e2d9ee12435e96f587de4d4cacdbf435
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=10459b03e2d9ee12435e96f587de4d4cacdbf435

Commit description:

addip_last_asconf caches the outstanding outbound ASCONF chunk. The normal
ASCONF-ACK completion path releases the chunk and clears the pointer.

However, sctp_asconf_queue_teardown() releases the cached chunk without
clearing addip_last_asconf. During peer restart handling,
sctp_sf_do_dupcook_a() queues SCTP_CMD_PURGE_ASCONF_QUEUE, which invokes
sctp_asconf_queue_teardown() while the association remains alive and leaves
the pointer dangling.

A delayed authenticated ASCONF-ACK can then reach sctp_sf_do_asconf_ack(),
which accesses the stale chunk and passes it to sctp_process_asconf_ack(),
causing a use-after-free and a second release.

Clearing the pointer exposes a race with T4 expiry. Peer restart handling
queues the timer stop before the purge, but SCTP_CMD_TIMER_STOP uses
timer_delete(), which does not wait for a callback already running on
another CPU. Such a callback can reach sctp_sf_t4_timer_expire() after
the purge and dereference NULL.

Clear addip_last_asconf after releasing the cached chunk, and make
sctp_sf_t4_timer_expire() consume a stale T4 expiry if no outstanding
ASCONF remains.

Fixes: a000c01 ("sctp: stop pending timers and purge queues when peer restart asoc")
Cc: stable@vger.kernel.org
Suggested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260809043806.2768302-1-yangyx22@mails.tsinghua.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sctp/associola.c
  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=10459b03e2d9ee12435e96f587de4d4cacdbf435

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

The original Linux kernel CVE announcement for CVE-2026-74587 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74587

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: 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: 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).