[CVE-2025-21759][MODERATE 7.0] ipv6: mcast: extend RCU protection in igmp6_send() [ Upstream

[CVE-2025-21759][MODERATE 7.0] ipv6: mcast: extend RCU protection in igmp6_send() [ Upstream

[AL-KERNEL]

CVE: CVE-2025-21759
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: mcast: extend RCU protection in igmp6_send() [ Upstream
Commit: 81b25a07ebf53f9ef4ca8f3d96a8ddb94561dd5a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81b25a07ebf53f9ef4ca8f3d96a8ddb94561dd5a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21759
Analysis date: Mon, 12 Oct 2026 22:11:14 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2025-21759 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21759

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 7.0
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-2025-21759	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-21759 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in igmp6_send can cause a potential use after free because the network namespace and net ipv6 igmp_sk pointers were fetched and used without continuous RTNL or RCU protection. Concurrent network namespace or device teardown may free the referenced state while igmp6_send continues to prepare an MLD packet and associates the skb with the stale socket. The most direct impact is a kernel crash and local denial of service. Operations on a reused struct sock may also corrupt socket accounting or other recycled kernel memory, so confidentiality and integrity impact cannot be excluded without additional analysis. For the CVSS the PR:L value is used because a local user or process is needed to initiate multicast related activity and arrange or race with namespace or device teardown. AC:H is used because successful triggering depends on a narrow concurrency window. The issue is not directly network reachable because an external IPv6 peer cannot normally control the required local object teardown. Impact is at least a local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	REMOTE LOCK UAF NETWORK SKB IPV6 KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* No floor or cap changed either score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation. `net` and `net->ipv6.igmp_sk` could lose lifetime protection while `igmp6_send()` continued using them.
* +1 Concrete RCU referenced-object lifetime mismatch. The fix fetches `net`, `idev`, and `igmp_sk` inside one extended RCU read-side section, directly establishing that the previous protection interval was insufficient.
* +1 Realistic kernel crash / strong DoS. A stale `struct net` or `struct sock` is passed into allocation, socket-accounting, flow-initialization, and packet-transmission processing, making a fatal kernel fault technically credible.
* +1 Broad exposure. IPv6 multicast and MLD processing are broadly present and exercised on ordinary IPv6-enabled systems.
* Total: 2 + 1 + 1 + 1 = 5.

Paranoid-only signal:

* +1 Nontrivial active structured-object consumption. The stale socket is not merely inspected during cleanup. It participates in skb ownership and socket-memory accounting, and the exact effect if its storage has already been reused remains unresolved.
* Total: 5 + 1 = 6.

No negative points:

* No host-admin penalty because host-admin-equivalent privilege is not positively established.
* No race penalty because the evidence does not establish that the timing is one-shot, externally uncontrollable, or impractical to repeat.
* No niche-subsystem penalty. IPv6 multicast is not rare.
* No remote-reachability bonus. Network traffic alone is not shown to control the required local teardown race.

Important non-awarded signals:

* Controlled reclaim or attacker-shaped replacement contents are not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Structured stale-object leverage is not separately awarded because attacker influence over reclaimed contents is unresolved.
* Strong or weak LPE points are not awarded merely from the UAF.
* Confidentiality and integrity points are not awarded merely because stale socket accounting may touch reused memory.
* No extra Availability point is awarded for the same possible kernel crash.
* No concrete repeated release or refcount operation on the same dangling reference is established. The dedicated repeated-release point and FLOOR 5 rule do not apply.
* NEW52 B5-R does not apply because there is no retry-accumulated heap OOB write.
* NEW53 B3-PENDING does not apply because there is no shared-backing write.
* NEW54 B4-PENDING does not apply because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The immediate send path can be reached through IPv6 multicast and MLD activity, while the UAF requires concurrent destruction or reassignment of network-namespace-related state. An ordinary process can perform some multicast operations, but the supplied evidence does not establish that the same below-host-admin actor can perform the complete setup, teardown, and racing send sequence.

Network namespaces are involved, but that alone does not prove that every required operation is available through attacker-created user and network namespaces. Therefore both host-admin requirement and complete namespace-scoped reachability remain unknown. No requirement for special hardware, firmware, unusual boot options, or a rare kernel subsystem is present.

The issue is not directly remotely triggerable from the supplied evidence. A remote IPv6 peer may influence multicast traffic, but it is not shown to control the necessary local namespace or device lifetime transition. Repeatability is unresolved, so neither an attacker-control bonus nor a hard-race penalty is applied.

4. Primitive and severity interpretation

The primitive is a concrete RCU lifetime violation involving active structured kernel objects. The old code fetched `struct net` and `struct sock` pointers before entering the relevant RCU critical section and then used them after their protected lifetime could have ended. The fix moves the sleeping allocation before `rcu_read_lock()`, obtains the pointers under RCU, and keeps socket ownership and subsequent processing inside the protected interval.

This is not fixed stale storage and is not merely a destructor-side invalid read. The stale `struct sock` participates in active skb ownership and memory-accounting behavior. However, no controlled reclaim, chosen replacement type, attacker-controlled post-free contents, callback control, or arbitrary kernel read/write is established.

There is no concrete first-release followed by a second release or refcount mutation on the exact same reference. This is therefore an active-object RCU UAF, not the repeated-release subtype.

The demonstrated practical consequence is potential kernel crash and local DoS. A stronger post-reuse corruption consequence is technically unresolved but not proven. This supports CLASS_B rather than CLASS_A.

ImportantCandidate is YES because the active socket object is used in nontrivial accounting and transmission processing after its lifetime may have ended. AutoDowngradeAllowed is NO because automatically reducing the issue to an ordinary crash-only race would discard unresolved post-lifetime mutation through an active kernel object. The likely operational classification is actionable Moderate, with Important-class review warranted before downgrade.

5. One-sentence report phrase

Insufficient RCU protection in `igmp6_send()` can expose stale network namespace or IPv6 multicast socket state to active skb accounting and packet processing, creating a local race-driven kernel UAF with credible crash impact and unresolved post-reuse corruption leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Review the complete `igmp6_send()` call paths, namespace teardown synchronization, accessibility from attacker-created namespaces, and whether stale `igmp_sk` storage can be predictably reused before socket-accounting operations modify it.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ipv6: mcast: extend RCU protection in igmp6_send() [ Upstream
Commit: 81b25a07ebf53f9ef4ca8f3d96a8ddb94561dd5a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=81b25a07ebf53f9ef4ca8f3d96a8ddb94561dd5a

Changed files:
  net/ipv6/mcast.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=81b25a07ebf53f9ef4ca8f3d96a8ddb94561dd5a

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

The original Linux kernel CVE announcement for CVE-2025-21759 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21759

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).