[CVE-2026-74562][MODERATE 7.0] nexthop: take nh->lock for f6i_list walks in replace check and notify [ Upstream

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

CVE: CVE-2026-74562
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nexthop: take nh->lock for f6i_list walks in replace check and notify [ Upstream
Commit: bb2b072c619c1f741a6234257050f72005dc63ef
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb2b072c619c1f741a6234257050f72005dc63ef
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74562
Analysis date: Sat, 15 Aug 2026 15:52:09 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in nexthop replacement can let rtnetlink code walk nh->f6i_list while a concurrent IPv6 route deletion frees a fib6_info object, causing a race-based kernel use-after-free and likely local DoS, with practical reachability depending on CAP_NET_ADMIN delegation or network namespace policy.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74562 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74562

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-2026-74562	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:N/A:H';BEST CVSS score: '7';DESCR 'A race in nexthop replace handling can let fib6_check_nh_list or __nexthop_replace_notify walk nh f6i_list while a concurrent IPv6 route delete unlinks and frees a fib6_info object. This creates a use after free read in the kernel networking path and can crash the system during rtnetlink route or nexthop operations. For the CVSS the PR:L is selected for the paranoid score because a local process with delegated CAP_NET_ADMIN or container root in a network namespace may be enough in practical deployments. The issue is not directly network reachable and is triggered through local control plane operations rather than packet traffic. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to reuse of freed kernel memory. YES REQUIRES MANUAL CHECK because this is a race based UAF in kernel networking code.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-74562 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 081efd18326e353c6fbfdeff903a83edde953f72 YES  NO NO unknown 	MAYBE	REMOTE READ LOCK DANGER RACE UAF NETWORK IPV6  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

### Paranoid score signals

* Local unprivileged trigger: +1
  Possible when unprivileged user namespaces or delegated network namespaces allow a local user or container root to obtain CAP_NET_ADMIN inside a netns and issue rtnetlink route or nexthop operations.

* Memory corruption, weak primitive: +1
  The patch and KASAN reports show slab use-after-free reads of a freed fib6_info object. This is real memory-safety impact, but the demonstrated primitive is stale read during list traversal, not write-after-free, controlled reclaim, callback control, or type confusion.

* Real lifetime corruption: +1
  A fib6_info object can be unlinked and freed by concurrent IPv6 route deletion while another path still walks nh->f6i_list.

* Reliable kernel crash / strong DoS: +1
  KASAN reports slab-use-after-free in rt6_fill_node() and fib6_check_nh_list(). A non-KASAN kernel may crash or enter corrupted networking state.

* Broad/default/common subsystem exposure: +1
  This is in the standard Linux networking control plane: rtnetlink, IPv6 routing, and nexthop handling. It is not a packet receive path, but it is not an obscure driver-only path.

### Conservative score adjustments

* Requires CAP_NET_ADMIN in typical deployments: -2
  In ordinary host configurations, creating or replacing nexthops and deleting IPv6 routes requires CAP_NET_ADMIN. If unprivileged user namespaces are disabled and no container or service has delegated network administration, reachability is limited.

### Not awarded

* Remote reachable / network-triggerable: +0
  This is triggered through local rtnetlink control-plane operations, not by receiving network packets.

* Strong LPE plausibility: +0
  The patch shows UAF reads, but not attacker-controlled reclaim, overwrite, callback dispatch, refcount takeover, or object replacement.

* Confidentiality / Integrity impact: +0 for ActionableScore
  The paranoid CVSS may keep C/I elevated for manual-review sensitivity, but this scoring formula should not award C/I for a race-only UAF read without a concrete leak or write primitive.

* Hard race penalty: 0
  Although this is a race, the reported traces use concurrent route and nexthop operations and appear practically reproducible enough that I would not subtract the timing penalty for the paranoid score.

## 3. Reachability analysis

The bug is reachable by a local actor that can perform rtnetlink nexthop replace operations concurrently with IPv6 route add or delete operations. On a normal host this usually means CAP_NET_ADMIN, so the conservative interpretation is privileged local attack surface.

Namespaces matter. If unprivileged user namespaces are enabled, or if a container root or service account has CAP_NET_ADMIN in a network namespace, the practical privilege level can drop to local low privilege in the product risk model. This is why the paranoid score treats it as locally reachable without full host-root.

The path is not directly network reachable. Packet traffic alone should not trigger it. The affected subsystem is common kernel networking control-plane code, but successful triggering requires specific concurrent route and nexthop manipulation.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup. It is a real race-based UAF in kernel networking code with KASAN evidence and a plausible local DoS outcome. However, the demonstrated primitive is read-after-free during list traversal, not a controlled write or object replacement primitive.

Realistic impact is most strongly local denial of service. Theoretical memory corruption concerns justify manual review, especially because fib6_info is a kernel networking object and the bug occurs in a concurrent lifetime window. Based on the provided patch alone, this is best treated as Actionable Moderate at minimum rather than a clear Strong Important case.

## 5. One-sentence report phrase

A race in nexthop replacement can let rtnetlink code walk nh->f6i_list while a concurrent IPv6 route deletion frees a fib6_info object, causing a race-based kernel use-after-free and likely local DoS, with practical reachability depending on CAP_NET_ADMIN delegation or network namespace policy.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed race-based UAF in core networking control-plane code. The currently demonstrated impact is DoS, but memory lifetime bugs in shared networking objects should not be auto-closed without human review, especially where CAP_NET_ADMIN may be delegated to containers or unprivileged user namespaces.

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

Patch: nexthop: take nh->lock for f6i_list walks in replace check and notify [ Upstream
Commit: bb2b072c619c1f741a6234257050f72005dc63ef
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb2b072c619c1f741a6234257050f72005dc63ef

Changed files:
  net/ipv6/route.c
  net/ipv4/nexthop.c
  net/core/rtnetlink.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=bb2b072c619c1f741a6234257050f72005dc63ef

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

The original Linux kernel CVE announcement for CVE-2026-74562 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74562

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: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: 2
  Paranoid ActionableScore: 5

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 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).