[CVE-2026-89790][LOW] ipv6: avoid divide by zero in rt6_multipath_rebalance [ Upstream

[CVE-2026-89790][LOW] ipv6: avoid divide by zero in rt6_multipath_rebalance [ Upstream

[AL-KERNEL]

CVE: CVE-2026-89790
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ipv6: avoid divide by zero in rt6_multipath_rebalance [ Upstream
Commit: f30cf8fd9872299c0c27f9916252ba2b9f422dce
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f30cf8fd9872299c0c27f9916252ba2b9f422dce
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89790
Analysis date: Wed, 16 Sep 2026 05:15:08 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89790 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89790

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89790	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89790 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:N/I:N/A:H';CWE-369;CWE-362;CWE-755;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 IPv6 multipath route rebalancing can make the first validation pass calculate a zero total nexthop weight while the second pass observes an eligible nexthop. rt6_upper_bound_set can then divide by zero and cause an Oops or kernel panic, resulting in a local denial of service. For the CVSS the PR:L value is used for the paranoid score because CAP_NET_ADMIN may be delegated to container root or a restricted network management service without granting full host administration. AC:H applies because reliable triggering requires a timing window between an RTM_NEWROUTE operation and a concurrent ignore_routes_with_linkdown update. The issue is not directly network reachable because received IPv6 traffic does not perform the required control plane operations. No UAF, OOB access, information disclosure, or write primitive is visible, so the supported impact is availability loss only.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-89790 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE UAF NETWORK IPV6  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash / strong DoS. A zero denominator reaches `DIV_ROUND_CLOSEST_ULL`, producing a divide error and Oops, with possible panic depending on system policy.
* +1 Common networking routing path. The defect is in IPv6 multipath route installation and rebalancing, although it is a control-plane path rather than remotely triggered packet processing.
* No race penalty. Concurrency is required, but the evidence does not show a narrow, one-shot, or externally uncontrollable timing window.
* No privilege penalty. `RTM_NEWROUTE` and the sysctl update require administrative networking access, but the supplied evidence does not resolve whether the complete trigger requires host-equivalent privilege or works entirely in an attacker-accessible namespace.
* No UAF, OOB access, write primitive, disclosure, persistent corruption, authorization bypass, or privilege-escalation primitive is established.
* No extra Availability point. The Oops or panic is the same consequence already counted as strong DoS.
* Remote reachability is not awarded. Network packets do not directly perform the required route and sysctl control-plane operations.
* Paranoid escalation is withheld because the patch establishes only division by zero and crash behavior.

3. Reachability analysis

The complete trigger requires concurrent IPv6 `RTM_NEWROUTE` processing and modification of `ignore_routes_with_linkdown`. Route administration normally involves `CAP_NET_ADMIN`, but the supplied material does not establish which user namespace governs every required check or whether an ordinary user can create a namespace and perform the complete sequence there. Therefore both host-admin and namespace-scoped reachability remain unknown.

No special hardware is required. IPv6 and multipath routing must be active, but no separate privileged provisioning prerequisite is established beyond access to the relevant control-plane operations. An actor controlling both operations could plausibly repeat attempts, so the race is not scored as intrinsically hard.

4. Primitive and severity interpretation

The exact primitive is an integer division by zero caused by an inconsistent two-pass view of eligible nexthops. It is a race-induced invalid-state bug, not a lifetime violation or memory-corruption primitive.

`PrimitiveClass=CLASS_C` because the demonstrated result is an Oops or kernel panic. There is no stale object, post-free access, repeated release, attacker-controlled overwrite, disclosure path, or useful behavior beyond the crash. Consequently, this is not an Important candidate and automatic downgrade is allowed. The likely operational severity is local control-plane denial of service.

5. One-sentence report phrase

A race between IPv6 multipath route insertion and an ignore_routes_with_linkdown update can produce a zero total nexthop weight and trigger a divide error and kernel crash, but no memory corruption or direct remote trigger is established.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a bounded crash-only divide-by-zero condition without a corruption, lifetime, authorization, disclosure, or privilege-escalation primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: ipv6: avoid divide by zero in rt6_multipath_rebalance [ Upstream
Commit: f30cf8fd9872299c0c27f9916252ba2b9f422dce
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f30cf8fd9872299c0c27f9916252ba2b9f422dce

Changed files:
  net/ipv6/route.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=f30cf8fd9872299c0c27f9916252ba2b9f422dce

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

The original Linux kernel CVE announcement for CVE-2026-89790 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89790

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: unknown
  Paranoid ActionableScore: 2

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).