[CVE-2026-99072][LOW] ipv4: stop route notification sizing when nexthop group shrinks

[CVE-2026-99072][LOW] ipv4: stop route notification sizing when nexthop group shrinks

[AL-KERNEL]

CVE: CVE-2026-99072
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ipv4: stop route notification sizing when nexthop group shrinks
Commit: 72d9d17092bb37b0717f12042a0817d01a1f9e78
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72d9d17092bb37b0717f12042a0817d01a1f9e78
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99072
Analysis date: Mon, 26 Oct 2026 15:38:54 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A concurrent nexthop-group shrink can make IPv4 notification sizing dereference NULL, potentially crashing the kernel without a shown memory-corruption primitive.

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

The original Linux kernel CVE announcement for CVE-2026-99072 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99072

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: LOW
Manual review required: NO

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-2026-99072	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99072 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=0 LEGACY=LOW RESULT=DIFF (final Legacy and Aegis differ; FINAL: LOW, Aegis: 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:N/I:N/A:H';CWE-362;CWE-367;**CWE-476;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A race in IPv4 FIB notification sizing can occur when RTM_NEWNEXTHOP replaces a nexthop group with fewer paths while fib_nlmsg_size is processing an asynchronous hardware flag notification. fib_info_num_path and fib_info_nhc can observe different RCU snapshots, causing fib_info_nhc to return NULL for an index that existed in the older group and allowing fib_nexthop_nlmsg_size to dereference it. The demonstrated primitive is a NULL pointer dereference that may cause a kernel oops or panic, with no supported UAF, OOB access, information disclosure, or write primitive. For the CVSS the PR:L value in the paranoid vector represents delegated CAP_NET_ADMIN, container network administration, or a route management service, while exploitation also requires winning a race with the asynchronous notification path. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 1.9 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	REMOTE SIMPLEFIX NETWORK HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 0 (+1 crash, -1 narrow asynchronous race).
* Conservative score: 0; no floor or cap applied.
* Raw paranoid score: 1 (+1 crash; race penalty withheld because attacker repeatability is unresolved).
* Paranoid score: 1; no floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash/strong DoS: `fib_info_nhc()` can return NULL after the nexthop group shrinks, and the result is dereferenced by `fib_nexthop_nlmsg_size()`.
* -1 conservative hard-race requirement: replacement must occur between two independent RCU snapshot loads during an asynchronous hardware-flag notification.
* No UAF, OOB access, write, disclosure, authorization bypass, or attacker-controlled corruption is shown.
* No extra availability point: the possible Oops/panic is the same crash effect.
* NEW52/B5-R does not apply: there is no heap OOB write, retry-accumulated index, or later consumption of corrupted structured state.
* No networking exposure bonus: ordinary remote packets do not invoke this route-management race.

3. Reachability analysis

The modifying operation is `RTM_NEWNEXTHOP`, ordinarily capability-gated for the target networking namespace. The evidence does not establish whether one below-host-admin actor can both manage the relevant nexthop group and induce the asynchronous hardware notification. Consequently, host-admin and complete namespace-scoped reachability remain unknown. Hardware-offload configuration may be involved, but no rare-hardware penalty is justified. The trigger requires a narrow concurrent shrink between the path-count and path-pointer loads.

4. Primitive and severity reading

The exact primitive is a NULL-pointer dereference caused by inconsistent RCU snapshots of a dense nexthop group. It is Class C crash behavior, not active-state corruption or a lifetime violation. The patch-supported effect is a kernel Oops or possible panic; non-crash exploitation is unsupported. ImportantCandidate is NO and automatic downgrade is allowed because no material corruption or unresolved trust-boundary primitive remains.

5. One-sentence report phrase

A concurrent nexthop-group shrink can make IPv4 notification sizing dereference NULL, potentially crashing the kernel without a shown memory-corruption primitive.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and call chain establish only a race-dependent NULL dereference.

7. Call-site confidence

Call-site confidence: high

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

Patch: ipv4: stop route notification sizing when nexthop group shrinks
Commit: 72d9d17092bb37b0717f12042a0817d01a1f9e78
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72d9d17092bb37b0717f12042a0817d01a1f9e78

Commit description:

Commit 680aea0 ("net: ipv4: Emit notification when fib
hardware flags are changed") added an RCU-only fib_nlmsg_size() call
for asynchronous hardware flag notifications.

fib_nlmsg_size() checks fib_info_num_path() before each iteration,
then fib_info_nhc() independently reloads nh->nh_grp. If
RTM_NEWNEXTHOP replaces a group with fewer paths between those loads,
fib_info_nhc() returns NULL and fib_nexthop_nlmsg_size() dereferences
it.

Stop sizing when the indexed path is absent. Nexthop groups are
dense, so the current snapshot has no later path.

Fixes: 680aea0 ("net: ipv4: Emit notification when fib hardware flags are changed")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/1054fa46165bba1b473782fa41367804aa908e86.1790915964.git.4ncienth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/fib_semantics.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=72d9d17092bb37b0717f12042a0817d01a1f9e78

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

The original Linux kernel CVE announcement for CVE-2026-99072 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99072

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:H/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:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 0
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).