[CVE-2026-53275][MODERATE 7.0] ipv6: mcast: Fix use-after-free when processing MLD queries

[CVE-2026-53275][MODERATE 7.0] ipv6: mcast: Fix use-after-free when processing MLD queries

[AL-KERNEL]

CVE: CVE-2026-53275
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: mcast: Fix use-after-free when processing MLD queries
Commit: 1354271c89d0e5fbf8b3d94097ff0216695209c7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1354271c89d0e5fbf8b3d94097ff0216695209c7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53275
Analysis date: Thu, 25 Jun 2026 08:57:01 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An adjacent-network attacker may trigger a stale skb header pointer dereference in IPv6 MLD query processing after `pskb_may_pull()` reallocates the header, causing a use-after-free read and likely kernel DoS.

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

The original Linux kernel CVE announcement for CVE-2026-53275 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53275

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-2026-53275	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-825;*CWE-20;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'IPv6 MLD query processing can dereference a stale multicast group address pointer after pskb_may_pull reallocates the skb header, resulting in a use-after-free read in __mld_query_work. An adjacent network attacker can potentially trigger this with a crafted MLD query because MLD is an ICMPv6 link-local control protocol and is not normally routable across the Internet. For the CVSS the PR:N is used because no credentials or local account on the victim are required when the attacker is on the same L2 segment. AC:H is used because reliable triggering depends on skb layout and header reallocation conditions. Impact is at least denial of service via kernel crash. The paranoid score keeps limited confidentiality and integrity impact because this is a real UAF read in networking code, but there is no clear arbitrary write primitive in the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	REMOTE READ OOB DANGER UAF NETWORK SKB IPV6 KPANIC PACKET 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  MLD queries are received from the network. This is best treated as adjacent-network reachable rather than Internet-routable remote exposure.
* Memory corruption, weak/indirect corruption candidate: +1
  The bug is a confirmed stale pointer / UAF read, but the patch shows a read of an old skb header pointer, not an attacker-controlled write, callback overwrite, or type confusion.
* Real lifetime corruption: +1
  The pointer survives skb header reallocation after `pskb_may_pull()` / `pskb_expand_head()`.
* Reliable kernel crash / strong DoS: +1
  KASAN reports slab-use-after-free in `__mld_query_work()`. Availability impact is realistic.
* Common networking packet-processing path: +1
  IPv6 multicast / MLD processing is a networking receive path, not a purely local control-plane interface.
* Hard or unreliable trigger conditions: -1
  Reliable triggering depends on skb layout and whether the header is reallocated after pull operations.

Conservative total: 5

Paranoid adjustment:

* Broad/default networking exposure or UAF triage sensitivity: +1
  IPv6 and multicast support are common enough that a network-adjacent UAF in packet processing should not be auto-closed without review.

Paranoid total: 6

## 3. Reachability analysis

An attacker needs the ability to send crafted IPv6 MLD traffic to the target on the same L2 segment or otherwise in a position where MLD packets are accepted. This is not normally Internet-routable, so AV:A is more appropriate than full AV:N in CVSS terms. No local account or credentials on the victim are required.

Namespaces and containers do not materially reduce the main network-adjacent concern, because the trigger is packet reception by the host networking stack. Practical exploitation depends on network topology, IPv6 multicast handling, and skb reallocation conditions.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate because it is a confirmed UAF in a network packet-processing path. However, the demonstrated primitive is a stale read from freed skb header memory, not a write-after-free, controlled object replacement, callback dispatch, or known privilege-escalation primitive.

Realistic impact is primarily kernel crash / DoS. Theoretical confidentiality or integrity concerns are limited and justify manual review, but current evidence does not support treating it as a Strong Important candidate.

## 5. One-sentence report phrase

An adjacent-network attacker may trigger a stale skb header pointer dereference in IPv6 MLD query processing after `pskb_may_pull()` reallocates the header, causing a use-after-free read and likely kernel DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed UAF plus network-adjacent packet reachability. The available evidence supports Actionable Moderate handling, while manual review should confirm whether any reclaim, disclosure, or stronger corruption path exists beyond DoS.

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

Patch: ipv6: mcast: Fix use-after-free when processing MLD queries
Commit: 1354271c89d0e5fbf8b3d94097ff0216695209c7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1354271c89d0e5fbf8b3d94097ff0216695209c7

Commit description:

When processing an MLD query, a pointer to the multicast group address
is retrieved when initially parsing the packet. This pointer is later
dereferenced without being reloaded despite the fact that the skb header
might have been reallocated following the pskb_may_pull() calls, leading
to a use-after-free [1].

Fix by copying the multicast group address when the packet is initially
parsed.

[1]
BUG: KASAN: slab-use-after-free in __mld_query_work (net/ipv6/mcast.c:1512)
Read of size 8 at addr ffff8881154b8e90 by task kworker/4:1/118

Workqueue: mld mld_query_work
Call Trace:
<TASK>
dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
print_address_description.constprop.0 (mm/kasan/report.c:378)
print_report (mm/kasan/report.c:482)
kasan_report (mm/kasan/report.c:595)
__mld_query_work (net/ipv6/mcast.c:1512)
mld_query_work (net/ipv6/mcast.c:1563)
process_one_work (kernel/workqueue.c:3314)
worker_thread (kernel/workqueue.c:3397 kernel/workqueue.c:3478)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
</TASK>

[...]

Freed by task 118:
kasan_save_stack (mm/kasan/common.c:57)
kasan_save_track (mm/kasan/common.c:78)
kasan_save_free_info (mm/kasan/generic.c:584)
__kasan_slab_free (mm/kasan/common.c:253 mm/kasan/common.c:285)
kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6251 mm/slub.c:6566)
pskb_expand_head (net/core/skbuff.c:2335)
__pskb_pull_tail (net/core/skbuff.c:2878 (discriminator 4))
__mld_query_work (net/ipv6/mcast.c:1495 (discriminator 1))
mld_query_work (net/ipv6/mcast.c:1563)
process_one_work (kernel/workqueue.c:3314)
worker_thread (kernel/workqueue.c:3397 kernel/workqueue.c:3478)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

Fixes: 97300b5 ("[MCAST] IPv6: Check packet size when process Multicast")
Reported-by: Leo Lin <leo@depthfirst.com>
Reviewed-by: David Ahern <dahern@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260603101811.612594-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/mcast.c
  lib/dump_stack.c
  mm/kasan/report.c
  kernel/workqueue.c
  kernel/kthread.c
  arch/x86/kernel/process.c
  arch/x86/entry/entry_64.S
  mm/kasan/common.c
  mm/kasan/generic.c
  include/linux/kasan.h
  mm/slub.c
  net/core/skbuff.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=1354271c89d0e5fbf8b3d94097ff0216695209c7

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

The original Linux kernel CVE announcement for CVE-2026-53275 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53275

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.4

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).