[CVE-2026-46260][MODERATE REGULAR] ipv6: Fix out-of-bound access in fib6_add_rt2node(). [ Upstream

[CVE-2026-46260][MODERATE REGULAR] ipv6: Fix out-of-bound access in fib6_add_rt2node(). [ Upstream

[AL-KERNEL]

CVE: CVE-2026-46260
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv6: Fix out-of-bound access in fib6_add_rt2node(). [ Upstream
Commit: bcc60ad129ae1837cf809c81bff56ec8bfdb6b11
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bcc60ad129ae1837cf809c81bff56ec8bfdb6b11
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46260
Analysis date: Wed, 03 Jun 2026 16:22:30 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
An IPv6 rtnetlink route creation path can trigger a one-byte slab out-of-bounds read in fib6_add_rt2node when RTA_NH_ID routes lack trailing fib6_nh storage, causing at least local DoS on hardened/debug kernels and requiring manual review due to memory-safety impact.

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

The original Linux kernel CVE announcement for CVE-2026-46260 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46260

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-2026-46260	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-125;*CWE-20;CWE-119;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.6';DESCR 'An out of bounds read in IPv6 fib6_add_rt2node can occur when an IPv6 route is created with RTA_NH_ID and the fib6_info object has no trailing fib6_nh storage. The vulnerable path reads iter fib6_nh fib_nh_gw_family before confirming that the route uses a separate nexthop object, which can read one byte beyond the allocated slab object and trigger a KASAN crash. For the CVSS the PR:L is used because reliable triggering requires local rtnetlink route creation with CAP_NET_ADMIN in a network namespace, and this can be available to a local user on systems with user namespaces or delegated network administration. The issue is not directly network reachable and is not triggered by ordinary IPv6 packet traffic. Impact is at least local denial of service via kernel crash on hardened or debug builds, with limited confidentiality or integrity impact kept only for the paranoid score because the primitive is an out of bounds read and not an observed write or UAF.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4) 	MAYBE	REMOTE READ KASAN OOB NETWORK KERNEL_PANIC_PLUS_UAF HARDWARE IPV6 SYZBOT  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger or namespace-lowered trigger: +1
  Triggering requires IPv6 route creation through rtnetlink. In typical host networking this requires CAP_NET_ADMIN, but unprivileged user namespaces can make CAP_NET_ADMIN available inside a network namespace.

* Reliable kernel crash / strong DoS: +1
  syzbot demonstrates a KASAN slab-out-of-bounds read. On KASAN or hardened debug kernels this is a reliable crash signal. On production kernels the one-byte read may not always crash.

* Local high-control netlink API: +1
  The trigger is rtnetlink route creation with attacker-controlled netlink attributes such as RTA_NH_ID.

* Broad/default/common subsystem: +1
  IPv6 routing and rtnetlink are common kernel networking subsystems.

* Requires CAP_NET_ADMIN in typical deployments: -1
  Host-level route modification normally requires CAP_NET_ADMIN. I use -1 rather than -2 because network namespaces and delegated/container networking can lower the practical privilege barrier.

Paranoid additional signal:

* Integrity impact plausible: +1
  The out-of-bounds byte is used in routing flag logic, so a limited routing state integrity effect is plausible, but there is no demonstrated arbitrary write, UAF, or controllable corruption primitive.

Not counted:

* Remote reachable / network-triggerable: +0
  This is not triggered by ordinary IPv6 packet reception. It requires rtnetlink route creation.

* Generic memory corruption strong primitive: +0
  The demonstrated primitive is a one-byte slab out-of-bounds read, not an OOB write, UAF, double-free, or arbitrary write.

* Privilege escalation plausible: +0
  No realistic LPE primitive is shown. The bug is memory-safety relevant but currently looks like limited OOB read plus DoS.

## 3. Reachability analysis

The bug is reachable by a local actor that can create IPv6 routes using rtnetlink with RTA_NH_ID. On a normal host this usually requires CAP_NET_ADMIN, so a fully unprivileged local user may not trigger it in the initial network namespace. However, systems with unprivileged user namespaces or delegated container networking may allow a local user to obtain CAP_NET_ADMIN inside a network namespace and reach the vulnerable path. The affected path is not packet-processing remote input and should not be scored as AV:N style network reachable. The exploitation conditions are realistic for syzkaller or namespace-enabled local environments, but the observed primitive is only a small OOB read.

## 4. Severity interpretation

This behaves like a borderline actionable Moderate rather than an Important-class vulnerability. The theoretical memory-safety concern is real because fib6_add_rt2node reads past the end of a fib6_info allocation when the object lacks trailing fib6_nh storage. Realistically, the evidence supports local DoS on KASAN or hardened/debug builds and possible limited routing-state integrity effects. There is no demonstrated UAF reclaim, OOB write, arbitrary write, info leak to userspace, or privilege escalation chain.

## 5. One-sentence report phrase

An IPv6 rtnetlink route creation path can trigger a one-byte slab out-of-bounds read in fib6_add_rt2node when RTA_NH_ID routes lack trailing fib6_nh storage, causing at least local DoS on hardened/debug kernels and requiring manual review due to memory-safety impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue should not be auto-closed because it is a kernel memory-safety bug in IPv6 routing and may be reachable from namespace-enabled local environments. It does not currently justify Important severity without additional evidence of disclosure, UAF, OOB write, or privilege escalation.

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

Patch: ipv6: Fix out-of-bound access in fib6_add_rt2node(). [ Upstream
Commit: bcc60ad129ae1837cf809c81bff56ec8bfdb6b11
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bcc60ad129ae1837cf809c81bff56ec8bfdb6b11

Changed files:
  net/ipv6/ip6_fib.c
  lib/dump_stack.c
  mm/kasan/report.c
  net/ipv6/route.c
  net/core/rtnetlink.c
  net/netlink/af_netlink.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  mm/kasan/common.c
  include/linux/kasan.h
  mm/slub.c
  include/linux/slab.h

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=bcc60ad129ae1837cf809c81bff56ec8bfdb6b11

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

The original Linux kernel CVE announcement for CVE-2026-46260 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46260

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:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 4

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).