[CVE-2025-40240][MODERATE REGULAR] sctp: avoid NULL dereference when chunk data buffer is missing [ Upstream

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

CVE: CVE-2025-40240
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sctp: avoid NULL dereference when chunk data buffer is missing [ Upstream
Commit: 61cda2777b07d27459f5cac5a047c3edf9c8a1a9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=61cda2777b07d27459f5cac5a047c3edf9c8a1a9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-40240
Analysis date: Mon, 12 Oct 2026 21:28:16 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2025-40240 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-40240

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 REGULAR
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-2025-40240	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-40240 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, 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:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A NULL pointer dereference can occur in the SCTP receive path when sctp_inq_pop replaces chunk skb with a missing frag_list and then dereferences the resulting NULL pointer while handling the invalid state. A remote SCTP sender may be able to reach this path through traffic that produces an skb with all data marked as non linear but without the expected frag_list. The exact ability to create this internal skb layout from ordinary network packets may depend on GSO, GRO, the network driver, and the target configuration. For the CVSS the PR:N value is used because no account or local execution is required if crafted SCTP traffic can produce the affected skb state. The base score uses AC:H because reliable control of that state is not demonstrated. The paranoid score uses AC:L to cover a deployment where the state can be reproduced reliably through network traffic. Impact is denial of service through a kernel Oops or panic. No confidentiality or integrity impact is assigned because the available code shows a NULL dereference and does not expose a UAF, OOB access, information leak, or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 2)	MAYBE	NULLPTR INIT SIMPLEFIX NETWORK SKB LINUS  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=3
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Reliable kernel crash / strong DoS. Assigning a missing `frag_list` to `chunk->skb` makes it NULL, after which `chunk->skb->dev` is dereferenced in the error path.
* +0 Remote reachability. The function is in the SCTP receive path, but the supplied evidence does not prove that ordinary remote packets can reliably create the required internal skb layout.

Paranoid signals:

* +2 Direct network reachability. It is technically defensible that SCTP traffic can influence GSO or GRO packet layout and reach `sctp_inq_pop()` without local execution.
* +1 Reliable kernel crash / strong DoS. The NULL dereference can cause an Oops or panic.

Not awarded:

* No UAF, stale lifetime use, OOB access, write primitive, controlled reclaim, disclosure, or authorization bypass is established.
* No additional Availability point because it would duplicate the same kernel crash.
* No C or I signal because the operation is an invalid NULL dereference rather than corruption of attacker-selected memory.
* No common-networking-path point because SCTP is not sufficiently broad to treat this specialized path as common packet processing.
* No race penalty because this is a deterministic invalid-state sequence, not a timing race.
* B5-R does not apply because there is no retry-accumulated heap OOB write.
* B3-PENDING does not apply because there is no shared-backing write.
* B4-PENDING does not apply because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The immediate vulnerable operation is reached while processing an SCTP chunk with `head_skb` set, `data_len == len`, and no `frag_list`. A remote SCTP sender can supply traffic to the receive path, but the supplied context does not establish whether packet contents alone can reliably produce this inconsistent internal skb representation. GSO, GRO, driver behavior, and SCTP configuration may affect reachability.

No local capability, namespace operation, sysctl, boot option, race, or special hardware requirement is shown. An SCTP-capable kernel and a reachable SCTP path are implicit prerequisites. Because the complete remote trigger is unresolved, host-admin and namespace privilege requirements remain UNKNOWN rather than being inferred as either required or unnecessary.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference. `chunk->skb` is replaced with a NULL `frag_list`, and the subsequent statistics operation dereferences `chunk->skb->dev`.

This is neither active structured-object UAF nor fixed stale storage. No lifetime operation or repeated release exists. The bug is CLASS_C because its demonstrated behavior is invalid-state consumption followed by an immediate fault.

The demonstrated consequence is kernel Oops or panic and denial of service. A remotely triggered crash is technically plausible but not conclusively established by the supplied call path. Memory corruption, privilege escalation, confidentiality impact, integrity impact, and exploitation leverage beyond the crash are unsupported.

ImportantCandidate is NO because there is no independent corruption, ownership, lifetime, or security-boundary primitive. AutoDowngradeAllowed is YES because the established primitive is crash-only and does not leave unresolved post-lifetime or write leverage.

5. One-sentence report phrase

A missing SCTP skb frag_list can cause sctp_inq_pop to replace chunk skb with NULL and dereference it, potentially allowing crafted network traffic to crash the kernel if the required GSO or GRO skb layout is remotely reproducible.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether an unauthenticated remote SCTP sender can reliably produce the required skb state on affected GSO or GRO configurations. This determines whether the practical score is 1 or 3.

7. Call-site confidence

Call-site confidence: medium

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

Patch: sctp: avoid NULL dereference when chunk data buffer is missing [ Upstream
Commit: 61cda2777b07d27459f5cac5a047c3edf9c8a1a9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=61cda2777b07d27459f5cac5a047c3edf9c8a1a9

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=61cda2777b07d27459f5cac5a047c3edf9c8a1a9

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

The original Linux kernel CVE announcement for CVE-2025-40240 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-40240

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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: 1
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).