[CVE-2026-53259][MODERATE 7.0] ipv6: anycast: insert aca into global hash under idev->lock [ Upstream

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

CVE: CVE-2026-53259
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: anycast: insert aca into global hash under idev->lock [ Upstream
Commit: 15be7e9fdbff831fb3e89b83cc337a4f85ad3310
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15be7e9fdbff831fb3e89b83cc337a4f85ad3310
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53259
Analysis date: Thu, 25 Jun 2026 12:32:52 -0400
ActionableScore: 8
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in IPv6 anycast handling can leave a freed ifacaddr6 object linked in the global RCU hash, allowing later IPv6 anycast lookups to dereference freed memory and causing at least a local kernel crash with potential UAF-based escalation concerns in namespace-delegated environments.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53259 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53259

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53259	MODERATE	CHECK	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:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in IPv6 anycast handling can leave a freed ifacaddr6 object linked in the global inet6_acaddr_lst RCU hash. The vulnerable window appears when __ipv6_dev_ac_inc publishes the object to the device anycast list before the global hash insertion, while concurrent device teardown removes it from the device list and performs a hash removal that becomes a no op because the object is not in the hash yet. A later reader such as ipv6_chk_acast_addr can then walk the global hash and dereference a freed slab object. For the CVSS the PR:L is used because triggering generally requires a local process with IPv6 anycast or network administration capability, for example CAP_NET_ADMIN in a network namespace, rather than unauthenticated remote packet access. The issue is not directly network reachable as AV:N because remote traffic alone does not create the join versus teardown race. Impact is at least local denial of service via kernel crash. For the paranoid score, this should be treated as a real race based use after free with possible confidentiality and integrity impact in addition to availability impact, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)  SKIP CVE-2026-53259  SKIP The Fixes patch not applied yet, so unlikely that actual: eb1ac9ff6c4a5720b1a1476233be374c5dc44bff YES  NO NO unknown 	MAYBE	REMOTE LOCK DANGER UAF NETWORK HARDWARE IPV6 SYZBOT KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The report describes a real slab use-after-free of `struct ifacaddr6`.
* Real lifetime corruption: +1. The freed object remains linked in the global RCU hash.
* Reliable kernel crash / strong DoS: +1. syzbot observed a UAF splat in `ipv6_chk_acast_addr()`.
* Broad networking subsystem exposure: +1. This is IPv6 networking state, though anycast itself is not a common default path.
* Hard or unreliable race / special timing required: -1. Triggering requires a race between anycast join and device teardown.
* Requires CAP_NET_ADMIN in typical deployments: -2. Joining IPv6 anycast and tearing down devices normally requires network administration capability.

Paranoid additional signals:

* Local unprivileged trigger: +1. In environments with unprivileged user namespaces or delegated network namespaces, CAP_NET_ADMIN may be obtainable inside the namespace.
* Weak LPE concern: +1. This is a real UAF with a dangling RCU hash node, but no demonstrated arbitrary write or reclaim primitive is shown.
* Confidentiality impact plausible: +1. A stale RCU hash node can theoretically lead to reads of reused slab contents.
* Integrity impact plausible: +1. UAF object reuse may theoretically affect control flow or object interpretation, though this is not demonstrated.
* Common networking packet-processing path: +1. `ipv6_chk_acast_addr()` is a lookup path reachable by IPv6 address checks after the corrupted hash state exists.

## 3. Reachability analysis

The bug is locally triggerable through IPv6 anycast membership changes racing with network device teardown. In typical host deployments this requires CAP_NET_ADMIN, so the conservative score applies a privilege penalty. In containerized or namespace-enabled systems, CAP_NET_ADMIN may be delegated to a less trusted actor, so the paranoid interpretation treats this closer to PR:L practical exposure.

This is not directly remote network-triggerable. Remote packets may reach `ipv6_chk_acast_addr()` after the dangling hash entry exists, but remote traffic alone does not create the join versus teardown race. Exploitation is timing-sensitive and currently evidenced as a syzbot-detected slab UAF / crash rather than a demonstrated LPE.

## 4. Severity interpretation

Conservatively, this is a local privileged race UAF with reliable DoS behavior, so it is at least Borderline / manual-review Moderate. Paranoid triage should treat it as a Strong Important candidate because it is a real lifetime bug in an RCU hash with freed slab reuse, and namespace delegation can reduce the effective privilege barrier.

Theoretical memory corruption potential exists due to UAF. Realistic exploitation evidence currently supports crash/DoS, with LPE remaining plausible but not demonstrated.

## 5. One-sentence report phrase

A race in IPv6 anycast handling can leave a freed ifacaddr6 object linked in the global RCU hash, allowing later IPv6 anycast lookups to dereference freed memory and causing at least a local kernel crash with potential UAF-based escalation concerns in namespace-delegated environments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a real race-based slab use-after-free in networking RCU state, with possible namespace-lowered triggering and plausible but unproven escalation potential.

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

Patch: ipv6: anycast: insert aca into global hash under idev->lock [ Upstream
Commit: 15be7e9fdbff831fb3e89b83cc337a4f85ad3310
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15be7e9fdbff831fb3e89b83cc337a4f85ad3310

Changed files:
  net/ipv6/anycast.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=15be7e9fdbff831fb3e89b83cc337a4f85ad3310

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

The original Linux kernel CVE announcement for CVE-2026-53259 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53259

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:N/I:N/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: 3
  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: 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).