[CVE-2026-72421][LOW] ipv4: fib: Don't ignore error route in local/main tables. [ Upstream

[CVE-2026-72421][LOW] ipv4: fib: Don't ignore error route in local/main tables. [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72421
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ipv4: fib: Don't ignore error route in local/main tables. [ Upstream
Commit: a29e95fbc51e8a1b932773fd0e259b2080881443
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a29e95fbc51e8a1b932773fd0e259b2080881443
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72421
Analysis date: Sat, 15 Aug 2026 04:39:31 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-72421 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72421

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-2026-72421	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';CWE-754;CWE-670;CWE-693;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '4.4';DESCR 'ipv4 fib_lookup could ignore an error route from the local or main table when CONFIG_IP_MULTIPLE_TABLES is enabled and no custom FIB rules are installed. The lookup could continue into the default table and return a normal route even though an unreachable route should have stopped resolution. This may weaken routing policy expectations and allow traffic to be routed where the administrator intended a local error route to block it. For the CVSS the PR:H is used for the base score because changing system routing policy normally requires administrative network privileges. For the CVSS the PR:L is used for the paranoid score because delegated CAP_NET_ADMIN or reduced capability container environments may allow a less privileged local actor to influence a network namespace. The issue is not directly network reachable and does not indicate memory corruption or privilege escalation. Impact is limited to routing policy integrity and possible availability effects from incorrect route selection.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-72421 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown
* +1 Common networking core path. `fib_lookup()` is a central IPv4 route lookup helper.
* +1 Broad/default/common subsystem. IPv4 FIB routing is broadly deployed.
* +1 Integrity impact plausible. An `unreachable` error route can be ignored, weakening intended routing policy.
* +1 Availability impact plausible. Incorrect route selection can change expected reachability behavior.
* -2 Requires admin/root/CAP_NET_ADMIN in typical deployments. Creating relevant routes and policy state normally requires network administration privileges.
* No memory corruption signal. The patch changes error handling and lookup continuation only.
* No LPE signal. No UAF, OOB write, refcount issue, type confusion, or attacker-controlled kernel memory primitive is indicated.
* Call-site confidence: high. The affected helper and behavior are directly shown.

3. Reachability analysis
The issue is triggered through routing table configuration and lookup behavior, not through packet parsing alone. In typical deployments, modifying routes requires CAP_NET_ADMIN. Unprivileged user namespaces may allow a user to reproduce the behavior inside their own network namespace, but that generally affects only resources they already control and does not imply host-level compromise. The path is common in IPv4 routing, but realistic security impact depends on an administrator or delegated network-management actor relying on `unreachable` routes for policy enforcement.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like logic bug, not an Important-class vulnerability. The theoretical impact is a routing policy bypass or inconsistent reachability decision. Realistic exploitation evidence does not show memory corruption, privilege escalation, information disclosure, kernel crash, or a strong cross-boundary isolation bypass.

5. One-sentence report phrase
`ipv4 fib_lookup could ignore an unreachable route from the local or main table and continue lookup in the default table, causing routing policy expectations to be bypassed when CONFIG_IP_MULTIPLE_TABLES is enabled and no custom FIB rules are installed.`

6. Manual review recommendation
NO MANUAL CHECK NEEDED.
The issue is a privileged routing logic bug with low practical impact and no demonstrated memory corruption, privilege escalation, or remote attack primitive.

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

Patch: ipv4: fib: Don't ignore error route in local/main tables. [ Upstream
Commit: a29e95fbc51e8a1b932773fd0e259b2080881443
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a29e95fbc51e8a1b932773fd0e259b2080881443

Changed files:
  include/net/ip_fib.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=a29e95fbc51e8a1b932773fd0e259b2080881443

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

The original Linux kernel CVE announcement for CVE-2026-72421 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72421

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:H/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:N/I:L/A:L
  The Best / paranoid CVSS score: 4.4

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).