[CVE-2026-97598][MODERATE REGULAR] ipv4: fib: bound automatic table ID allocation

[CVE-2026-97598][MODERATE REGULAR] ipv4: fib: bound automatic table ID allocation

[AL-KERNEL]

CVE: CVE-2026-97598
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv4: fib: bound automatic table ID allocation
Commit: bc8d01d856db106f284b19961af6112e10c3e3b4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bc8d01d856db106f284b19961af6112e10c3e3b4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97598
Analysis date: Fri, 25 Sep 2026 13:15:46 -0400
ActionableScore: 1
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-2026-97598 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97598

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 REGULAR
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-2026-97598	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97598 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:N/A:H';**CWE-400;CWE-407;CWE-770;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'IPv4 automatic routing table allocation can consume excessive CPU while holding RTNL because fib_empty_table searches sequential table IDs and repeatedly walks growing hash chains for dense table sets. A local actor able to create many IPv4 policy rules with automatic table selection can delay network control plane operations and potentially cause soft lockups or denial of service. For the CVSS the PR:L is used for the paranoid score because CAP_NET_ADMIN may be delegated to a service account, container root, or a user in a network namespace. The issue is not directly network reachable and is triggered through local routing rule configuration rather than packet traffic. Impact is denial of service only. No memory corruption, information disclosure, integrity impact, or privilege escalation primitive is indicated.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	REMOTE NETWORK  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable control plane denial of service. A dense set of IPv4 FIB table IDs can cause fib_empty_table to perform a very long sequential search while RTNL is held. This can stall routing and network configuration operations and may cause a CPU soft lockup.
* No remote reachability bonus. The vulnerable operation is performed through local rtnetlink rule configuration, not packet processing.
* No local unprivileged trigger bonus. CAP_NET_ADMIN is relevant, but the supplied material does not establish that an ordinary attacker can complete the full setup and trigger sequence below host administrative privilege.
* No memory corruption, UAF, OOB access, invalid free, refcount misuse, disclosure, integrity impact, authorization bypass, or privilege escalation primitive is established.
* No resource leak signal. The problem is algorithmic CPU consumption and prolonged RTNL lock holding, not an unreleased resource.
* No extra Availability point. The same RTNL stall and possible soft lockup are already represented by the single DoS point.
* No negative host-admin penalty. Host-admin-equivalent privilege is not positively established, so a -2 penalty is not justified.
* No namespace reachability bonus. Rtnetlink capability checks may be namespace-aware, but the supplied evidence does not prove that an attacker can create a suitable namespace and complete all required operations while causing meaningful host-wide RTNL contention.

3. Reachability analysis

The direct trigger is local IPv4 policy-routing configuration using automatic table selection, for example a rule using table 0. The attacker must create a dense set of table IDs and then invoke automatic allocation so fib_empty_table scans occupied IDs while RTNL is held.

The relevant configuration interface normally requires CAP_NET_ADMIN. The supplied patch does not show the exact rtnetlink capability check or prove whether a user with CAP_NET_ADMIN only in an attacker-created network namespace can complete the full high-cost sequence and cause host-wide impact through the global RTNL lock. Therefore HostAdminRequired and NamespaceScopedPrivilege remain UNKNOWN.

No prior hardware, firmware, boot parameter, or special device is required. The legacy automatic table allocation path is explicitly described as IPv4-only, undocumented, not covered by selftests, and unused by NetworkManager and systemd. This reduces practical exposure, but no score penalty is applied because the path can still be directly invoked through low-level routing configuration.

4. Primitive and severity interpretation

The established primitive is excessive algorithmic work under RTNL. The function sequentially probes table IDs and repeatedly performs hash-table lookups. A dense table-ID allocation pattern creates increasing hash-chain traversal cost while RTNL remains locked.

This is not a lifetime bug, UAF, stale-object bug, ownership violation, memory corruption issue, or security-boundary bypass. There is no attacker-controlled kernel memory corruption primitive after excluding the availability effect. The demonstrated consequence is local network control-plane denial of service through prolonged RTNL lock contention, high CPU consumption, and possible soft lockups.

PrimitiveClass is NONE because the patch establishes no meaningful corruption, lifetime, authorization, ownership, or semantic reinterpretation primitive. ImportantCandidate is NO because the issue is a bounded-resource and locking DoS without evidence of cross-boundary resource exhaustion, persistent state damage, confidentiality impact, integrity impact, or privilege escalation. AutoDowngradeAllowed is YES because no unresolved concrete kernel-corruption or security-boundary primitive would be discarded by ordinary Moderate handling.

5. One-sentence report phrase

IPv4 automatic routing table allocation can hold RTNL for excessive time when dense table IDs force repeated hash-chain scans, causing local network control-plane denial of service and possible CPU soft lockups.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Verify the exact capability check and whether an untrusted user with CAP_NET_ADMIN in an attacker-created network namespace can create the required dense table set and cause meaningful host-wide RTNL contention.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ipv4: fib: bound automatic table ID allocation
Commit: bc8d01d856db106f284b19961af6112e10c3e3b4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bc8d01d856db106f284b19961af6112e10c3e3b4

Commit description:

fib_empty_table() probes every table ID from 1 until it finds a
free one.  IPv4 tables are stored in a 256-bucket hash table, so a
dense set of IDs makes each probe walk a growing hash chain while
RTNL is held.

Automatic table assignment ("ip rule ... table 0") is an IPv4-only
legacy path.  Bound the automatically allocated ID to 4096 so the
RTNL hold stays bounded, without changing lookups of explicitly
specified table IDs.

This changes user-visible behavior.  A table-0 rule previously
received the lowest free ID in 1..RT_TABLE_MAX (0xFFFFFFFF).  After
this patch the search stops at 4096 and the rule add fails with
ENOBUFS if that range is fully occupied.  Explicit table IDs above
4096 remain usable.

The automatic path is unused in practice: it is IPv4-only, not
documented by ip-rule, uncovered by kernel selftests, and both
NetworkManager and systemd refuse table 0.

Fixes: b801f54 ("[NET]: Increate RT_TABLE_MAX to 2^32")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Link: https://patch.msgid.link/6f2f2a7a136aee005512a2e1ac8ede62ac8c7bb6.1788258884.git.zihanx@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/fib_rules.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=bc8d01d856db106f284b19961af6112e10c3e3b4

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

The original Linux kernel CVE announcement for CVE-2026-97598 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97598

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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

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).