[CVE-2026-74561][MODERATE 7.0] nexthop: avoid unlocked f6i_list walk in nh_rt_cache_flush [ Upstream commit 4787a6d2629b4e8c0b6bacab1f75c1660eca44d9 ]

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

CVE: CVE-2026-74561
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nexthop: avoid unlocked f6i_list walk in nh_rt_cache_flush [ Upstream commit 4787a6d2629b4e8c0b6bacab1f75c1660eca44d9 ]
Commit: 44f53e4331a30fabc38a411fae7524341b618db3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=44f53e4331a30fabc38a411fae7524341b618db3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74561
Analysis date: Sat, 15 Aug 2026 10:10:46 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An unlocked nexthop IPv6 `f6i_list` traversal can race with route deletion during rtnetlink nexthop replacement, causing a `fib6_info` use-after-free and a likely local kernel crash, with manual review recommended because delegated `CAP_NET_ADMIN` environments may make the trigger accessible to less privileged users.

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

The original Linux kernel CVE announcement for CVE-2026-74561 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74561

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
======================================================================

CVE-2026-74561	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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 'An unlocked walk of the nexthop IPv6 f6i_list in nh_rt_cache_flush can race with IPv6 route add or delete operations that mutate the same list and free fib6_info entries. This creates a slab use-after-free during nexthop replace through rtnetlink, with KASAN showing a stale fib6_info read in nh_rt_cache_flush. For the CVSS the PR:L is used because reliable triggering requires a local process with route administration capability such as CAP_NET_ADMIN, which can be delegated to containers or service accounts and should not be treated as full host root in practical risk terms. The issue is not directly network reachable because the trigger is a local netlink control plane operation, not packet processing. Impact is at least local denial of service via kernel crash and in the paranoid score may allow confidentiality or integrity impact because the primitive is a kernel UAF race rather than a pure NULL dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74561 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 081efd18326e353c6fbfdeff903a83edde953f72 YES  NO NO unknown 	MAYBE	REMOTE READ LOCK DEADLOCK DANGER INIT UAF NETWORK IPV6  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered signals:

* Memory corruption, weak primitive: +1
  KASAN reports slab-use-after-free in `nh_rt_cache_flush()` while reading a freed `fib6_info`. This is a real UAF, but the patch shows a stale list traversal/read rather than attacker-controlled reclaim, overwrite, callback control, or type confusion.

* Real lifetime corruption: +1
  The bug is caused by an unlocked `f6i_list` walk racing with IPv6 route add/delete that can free `fib6_info` entries.

* Reliable kernel crash / strong DoS: +1
  The commit includes a concrete KASAN UAF trace in the nexthop replace path. A crash is realistic.

* Broad/default networking control-plane exposure: +1
  The affected code is in the core nexthop/rtnetlink routing control path. It is not packet-path remote, but it is a standard Linux networking subsystem.

Paranoid-only signal:

* Local unprivileged trigger through delegated netns capability: +1
  In typical host networking this needs `CAP_NET_ADMIN`, but in environments with unprivileged user namespaces or containers, a local user may obtain `CAP_NET_ADMIN` in a network namespace and reach rtnetlink route/nexthop operations. This makes the higher score defensible.

Not awarded:

* Remote reachable / network-triggerable: +0
  Trigger is rtnetlink control plane, not packet processing.

* Strong LPE plausibility: +0
  No evidence of controlled reclaim, stale object replacement, write-after-free, callback dispatch, refcount takeover, or arbitrary write.

* Confidentiality / integrity impact: +0 conservative
  The demonstrated primitive is stale read/deref and crash. C/I are only a paranoid CVSS concern, not strong enough for extra ActionableScore points.

* Hard race penalty: 0
  Although this is a race, the commit shows a concrete KASAN reproducer path involving route add/delete versus nexthop replace. I would not subtract for an unrealistic timing window.

## 3. Reachability analysis

The bug is triggered locally through rtnetlink operations involving nexthop replacement and concurrent IPv6 route add/delete activity. In normal host configurations this requires route administration privileges, usually `CAP_NET_ADMIN`.

Namespaces matter. If unprivileged user namespaces are enabled, or if a container/service account is delegated `CAP_NET_ADMIN` in a network namespace, the practical privilege level may be closer to local low privilege than full host root. The vulnerability is not directly reachable from the network because packets do not invoke the vulnerable control-plane sequence.

The affected subsystem is common Linux networking infrastructure, but the vulnerable condition requires a specific concurrent control-plane sequence. Realistic impact is at least local kernel crash. More serious exploitation is not demonstrated but deserves review because this is a kernel UAF in a networking lifetime/list-management path.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a real UAF with a KASAN trace in kernel networking code. Conservative handling should be borderline/manual-review rather than auto-close.

It is not clearly Important on the available evidence because the patch demonstrates stale traversal and crash, not controlled object reuse, write-after-free, callback hijack, or a proven LPE chain. The paranoid score reaches Actionable Moderate because namespace-delegated `CAP_NET_ADMIN` can make the trigger more accessible and because kernel UAF races are historically worth manual review.

## 5. One-sentence report phrase

An unlocked nexthop IPv6 `f6i_list` traversal can race with route deletion during rtnetlink nexthop replacement, causing a `fib6_info` use-after-free and a likely local kernel crash, with manual review recommended because delegated `CAP_NET_ADMIN` environments may make the trigger accessible to less privileged users.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed race-based kernel UAF in a core networking control-plane path, with realistic DoS and a defensible paranoid concern for higher impact, even though no controlled reclaim or LPE primitive is shown.

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

Patch: nexthop: avoid unlocked f6i_list walk in nh_rt_cache_flush [ Upstream commit 4787a6d2629b4e8c0b6bacab1f75c1660eca44d9 ]
Commit: 44f53e4331a30fabc38a411fae7524341b618db3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=44f53e4331a30fabc38a411fae7524341b618db3

Commit description:

nh_rt_cache_flush() walks nh->f6i_list during an RTNL-serialized nexthop
replace without holding nh->lock, racing the unlocked IPv6 route
add/delete that mutate the list under nh->lock and free fib6_info
entries (nh_rt_cache_flush() is inlined into rtm_new_nexthop()):

  BUG: KASAN: slab-use-after-free in nh_rt_cache_flush (net/ipv4/nexthop.c:2243)
  Read of size 8 at addr ffff888012953e18 by task exploit/146
   nh_rt_cache_flush (net/ipv4/nexthop.c:2243)
   replace_nexthop (net/ipv4/nexthop.c:2610)
   rtm_new_nexthop (net/ipv4/nexthop.c:3323)
   rtnetlink_rcv_msg (net/core/rtnetlink.c:7076)

Unlike the other f6i_list walks, this one bumps each route's sernum via
fib6_update_sernum_upto_root(), which needs tb6_lock; taking nh->lock
around it would invert the established tb6_lock -> nh->lock order and
deadlock. As the only purpose is to invalidate cached dsts, bump the
IPv6 sernum for the whole netns with rt_genid_bump_ipv6() instead,
mirroring the rt_cache_flush() already done for IPv4 just above.

Fixes: 081efd18326e ("ipv6: Protect nh->f6i_list with spinlock and flag.")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260722002951.2614721-2-xmei5@asu.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Diffstat -rw-r--r-- net/ipv4/nexthop.c 12

Changed files:
  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=44f53e4331a30fabc38a411fae7524341b618db3

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

The original Linux kernel CVE announcement for CVE-2026-74561 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74561

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: 4
  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).