[CVE-2026-74586][IMPORTANT] sctp: clear new_transport when removing a peer

[CVE-2026-74586][IMPORTANT] sctp: clear new_transport when removing a peer

[AL-KERNEL]

CVE: CVE-2026-74586
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sctp: clear new_transport when removing a peer
Commit: c0f973bb5118dd1b146cda3fcc8af6f6057befec
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0f973bb5118dd1b146cda3fcc8af6f6057befec
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74586
Analysis date: Sat, 22 Aug 2026 18:16:13 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An authenticated remote SCTP peer can use ASCONF add and wildcard DEL-IP sequencing to leave `asoc->new_transport` pointing to a freed transport, causing a confirmed use-after-free in `sctp_outq_select_transport()` and at least remote kernel crash DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74586 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74586

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74586	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:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;CWE-362;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'An SCTP use after free can occur when an authenticated remote peer sends an ASCONF chunk that adds a new transport and then removes it again with a wildcard DEL-IP parameter in the same control flow. The removed transport can remain stored in asoc new_transport and later be attached to a queued HEARTBEAT without holding a valid transport reference. After RCU frees the transport, later ASCONF_ACK processing can reach sctp_outq_select_transport and read the freed transport state, causing a kernel crash and creating a memory corruption primitive. For the CVSS the PR:L is used because the attacker needs an authenticated SCTP peer context able to send ASCONF operations, but does not need local code execution on the victim. The issue is network reachable over SCTP when SCTP AUTH and dynamic address reconfiguration are enabled and reachable by the peer. Impact is at least remote denial of service via kernel crash and in the paranoid case may allow confidentiality or integrity impact due to a real use after free in kernel networking code.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	READ OOB DANGER UAF NETWORK LINUS KPANIC 	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. Trigger comes from an authenticated remote SCTP peer sending ASCONF operations.
* Generic memory corruption: +2. KASAN confirms slab-use-after-free in `sctp_outq_select_transport()`.
* Real lifetime corruption: +1. `asoc->new_transport` remains a stale pointer after the peer transport is removed and later freed by RCU.
* Reliable kernel crash / strong DoS: +1. The reproducer triggered the same KASAN UAF in 2 of 2 unpatched runs.
* Rare AND difficult-to-reach subsystem/configuration: -1. Requires SCTP, SCTP AUTH, ASCONF / dynamic address reconfiguration, and a valid authenticated peer context.
* Common networking protocol path: +1. Although SCTP is less common than TCP/UDP, this is still a network protocol state machine reachable through packet/control processing.

Paranoid additional signals:

* Confidentiality impact plausible: +1. A real remote UAF in kernel networking code may expose stale object contents in some reuse scenarios, although no leak primitive is demonstrated.
* Integrity impact plausible: +1. A stale freed transport object can be reused by the allocator, so corruption consequences beyond DoS are technically plausible, but not proven.
* No privilege escalation bonus is added. The patch shows UAF read and crash, but not attacker-controlled reclaim, overwrite, callback dispatch, or refcount takeover.

## 3. Reachability analysis

The bug is remotely triggerable by an authenticated SCTP peer that can send ASCONF chunks. The attacker does not need local code execution on the victim, but does need an established authenticated SCTP association with ASCONF capabilities. This is not a general Internet-reachable bug for all systems because SCTP and dynamic address reconfiguration are not universally deployed, and the peer must be authenticated. Namespaces do not materially reduce the network-side requirement, but containerized services exposing SCTP could make the vulnerable path reachable from less trusted networks.

Call-site confidence: high. The provided description includes the stale pointer lifecycle, RCU free, queued HEARTBEAT behavior, and the later `sctp_outq_select_transport()` UAF read.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a network-reachable kernel UAF with a confirmed KASAN reproducer. Realistic demonstrated impact is remote DoS through kernel crash. Theoretical impact can be higher because the stale pointer is a freed kernel networking object accessed after RCU, but the supplied evidence does not demonstrate controlled reclaim, write-after-free, type confusion, or a direct LPE/RCE primitive. Therefore the conservative score is Actionable Moderate, while the paranoid score reaches Strong Important candidate for manual review sensitivity.

## 5. One-sentence report phrase

An authenticated remote SCTP peer can use ASCONF add and wildcard DEL-IP sequencing to leave `asoc->new_transport` pointing to a freed transport, causing a confirmed use-after-free in `sctp_outq_select_transport()` and at least remote kernel crash DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed remote-triggerable UAF in kernel networking code with authenticated peer requirements and plausible but unproven impact beyond DoS.

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

Patch: sctp: clear new_transport when removing a peer
Commit: c0f973bb5118dd1b146cda3fcc8af6f6057befec
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0f973bb5118dd1b146cda3fcc8af6f6057befec

Commit description:

sctp_process_asconf_param() stores a newly added peer transport in
asoc->new_transport. After all parameters in the ASCONF chunk have been
processed, sctp_sf_do_asconf() uses this pointer to send a HEARTBEAT to the
new transport.

An authenticated ASCONF from a remote SCTP peer can add a transport and
remove it again with a wildcard DEL-IP parameter in the same chunk. The
wildcard deletion preserves the transport on which the ASCONF arrived, but
removes the newly added transport through
sctp_assoc_del_nonprimary_peers(). The removal does not clear
asoc->new_transport, leaving it pointing to the removed transport.

sctp_sf_do_asconf() then creates a HEARTBEAT whose chunk->transport points
to the removed transport without holding a transport reference. During
local address replacement, src_out_of_asoc_ok keeps this HEARTBEAT on
control_chunk_list. After the transport is freed by RCU, a successful
ASCONF_ACK for the replacement address releases the queued HEARTBEAT and
sctp_outq_select_transport() reads the freed transport's state.

The issue was found during a static audit of SCTP objects. With an
authenticated peer, the reproducer triggered the same KASAN report in 2
of 2 unpatched runs on a KASAN-enabled netdev/main kernel:

  BUG: KASAN: slab-use-after-free in sctp_outq_select_transport
  Read of size 4 at addr ffff88800b9bd95c by task python3/197

  Call Trace:
   sctp_outq_select_transport+0x549/0x8b0 [sctp]
   sctp_outq_flush+0x306/0x2c60 [sctp]
   sctp_transport_immediate_rtx+0xaf/0x260 [sctp]
   sctp_process_asconf_ack+0xa48/0xf70 [sctp]

  Allocated by task 197:
   sctp_transport_new+0x68/0x650 [sctp]
   sctp_assoc_add_peer+0x258/0x12a0 [sctp]
   sctp_process_asconf+0x5e9/0x1090 [sctp]

  Last potentially related work creation:
   __call_rcu_common.constprop.0+0x77/0xb70
   sctp_assoc_del_nonprimary_peers+0x7c/0xd0 [sctp]
   sctp_process_asconf+0xd9c/0x1090 [sctp]

The first invalid access was a four-byte read of transport->state at
net/sctp/outqueue.c:833. The same reproducer completed the full
authenticated ASCONF and local-address replacement sequence with this
change without a KASAN report or oops.

Clear new_transport when its peer is removed, before it can be used to
create the HEARTBEAT.

Fixes: 6af29cc ("sctp: Bundle HEAERTBEAT into ASCONF_ACK")
Cc: stable@vger.kernel.org
Signed-off-by: Qing Ming <a0yami@mailbox.org>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260811152803.5629-1-a0yami@mailbox.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sctp/outqueue.c
  net/sctp/associola.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=c0f973bb5118dd1b146cda3fcc8af6f6057befec

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

The original Linux kernel CVE announcement for CVE-2026-74586 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74586

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 6
  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).