[CVE-2026-98379][MODERATE REGULAR] netfilter: ip6t_rpfilter: reject routes without inet6_dev

[CVE-2026-98379][MODERATE REGULAR] netfilter: ip6t_rpfilter: reject routes without inet6_dev

[AL-KERNEL]

CVE: CVE-2026-98379
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: ip6t_rpfilter: reject routes without inet6_dev
Commit: f4de78756b0fddbd125b2b1b77c74f2eccc8e977
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4de78756b0fddbd125b2b1b77c74f2eccc8e977
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98379
Analysis date: Fri, 09 Oct 2026 22:03:34 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98379 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98379

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: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-98379	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98379 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	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-476;*CWE-20;CWE-754;BEST CVSS score: '5.5';DESCR 'An IPv6 rpfilter lookup can dereference a NULL rt6i_idev when ip6_route_lookup() returns an error free route after the related inet6_dev has been removed. An unprivileged local user can construct this state with rtnetlink inside private user and network namespaces by lowering the external nexthop device MTU below IPV6_MIN_MTU and then sending traffic through the rpfilter path. The resulting NULL pointer dereference occurs in softirq context and can panic the host kernel, causing denial of service. For the CVSS the PR:L is selected because local user execution is required, although the necessary network administration operations can be obtained inside an unprivileged user and network namespace on systems that permit their creation. The issue is not remotely network reachable because an external peer cannot create the required namespace, route, nexthop, and MTU state through packet traffic alone. No confidentiality or integrity impact is assigned because the demonstrated primitive is a NULL pointer read with no UAF, OOB access, information disclosure, or controlled write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	REMOTE KASAN OOB NULLPTR NETFILTER NETWORK HARDWARE LINUS IPV6 PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. The commit explicitly states that an unprivileged user can construct the required route state using rtnetlink in private user and network namespaces.
* +1 Reliable kernel crash / strong DoS. The demonstrated NULL dereference occurs in softirq context and produces a fatal kernel panic.
* +1 Common networking packet-processing path. The invalid route is consumed through IPv6 netfilter rpfilter processing on the receive path.

Negative signals:

* No negative points. Host-admin privilege, difficult race timing, rare hardware, and an unusually inaccessible subsystem are not established.

Material non-awarded signals:

* No UAF, heap OOB access, controlled overwrite, stale lifetime mutation, or arbitrary write is established.
* No confidentiality or integrity point is awarded for a NULL pointer read.
* No remote-reachability point is awarded because remote traffic alone cannot construct the required namespace, route, nexthop, and MTU state.
* No additional Availability point is awarded because it would double-count the same kernel panic.
* No LPE or corruption-leverage point is awarded. The path is an invalid dereference with no useful post-crash primitive.
* B5-R does not apply because there is no heap OOB write or retry-accumulated offset.
* B3-PENDING does not apply because there is no shared-backing write.
* B4-PENDING does not apply because no authorization DENY-to-ALLOW decision is bypassed.

Repeated-release assessment:

* No first release that can end an object lifetime is present.
* No dangling reference survives such a release.
* No second release or refcount mutation exists.
* The dedicated repeated-release point was not awarded.
* Paranoid repeated-refcount leverage was not awarded.
* FLOOR 5 was not applicable.

3. Reachability analysis

The evaluated attacker is an ordinary local user. The supplied commit explicitly establishes that the user can create private user and network namespaces, obtain namespace-scoped network administration authority, lower the relevant nexthop device MTU below IPV6_MIN_MTU, retain the route through the described FIB behavior, configure or exercise IPv6 rpfilter, and send the triggering packet.

The complete demonstrated operation is therefore below host-admin privilege and namespace-scoped. It does not require host UID 0, special hardware, a race, or a separate vulnerability. Unprivileged user-namespace availability and the affected IPv6 netfilter functionality are environmental prerequisites, but they do not establish host-admin-only triggering.

The issue is local rather than remotely network-triggerable. An external peer cannot create the required private namespace and routing state merely by sending IPv6 packets.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference. `ip6_route_lookup()` can return an error-free route with `rt6i_idev == NULL`, after which rpfilter processing dereferences that missing object. The fix rejects this route before any later `rt6i_idev` access.

This is neither active structured-object UAF nor fixed stale storage. There is no demonstrated post-lifetime access, repeated release, refcount mutation, shared-page write, authorization bypass, or controlled memory corruption.

The demonstrated consequence is an unprivileged local host kernel panic. Stronger consequences such as information disclosure, arbitrary kernel write, privilege escalation, or code execution are unsupported. The issue is therefore CLASS_C rather than CLASS_A or CLASS_B.

ImportantCandidate is NO because no independently useful corruption or security-boundary primitive survives after excluding the crash. AutoDowngradeAllowed is YES because the evidence supports a constrained crash-only vulnerability, although product-specific exposure should still be checked due to the below-host-admin host-wide DoS.

5. One-sentence report phrase

An unprivileged local user can construct an IPv6 route without inet6_dev inside private user and network namespaces and trigger a NULL pointer dereference in ip6t_rpfilter, causing a host kernel panic without establishing memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether unprivileged user namespaces and IPv6 rpfilter are enabled in the evaluated product, since those settings determine practical exposure to the demonstrated local host-wide DoS.

7. Call-site confidence

Call-site confidence: high

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

Patch: netfilter: ip6t_rpfilter: reject routes without inet6_dev
Commit: f4de78756b0fddbd125b2b1b77c74f2eccc8e977
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4de78756b0fddbd125b2b1b77c74f2eccc8e977

Commit description:

ip6_route_lookup() can return an error-free route whose rt6i_idev is
NULL. Lowering an external nexthop device's MTU below IPV6_MIN_MTU tears
down its inet6_dev while fib6_ifdown() leaves routes using nexthop objects
in the FIB. An unprivileged user can construct this state with rtnetlink
in a private user and network namespace, then trigger a NULL dereference
through an IPv6 rpfilter lookup:

  Oops: general protection fault, probably for non-canonical address
  0xdffffc0000000000
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: rpfilter_mt (net/ipv6/netfilter/ip6t_rpfilter.c:75)
  Call Trace:
  ip6t_do_table (net/ipv6/netfilter/ip6_tables.c:316)
  nf_hook_slow (net/netfilter/core.c:619)
  ipv6_rcv (net/ipv6/ip6_input.c:351)
  __netif_receive_skb_one_core (net/core/dev.c:6216)
  process_backlog (net/core/dev.c:6680)
  __napi_poll (net/core/dev.c:7739)
  net_rx_action (net/core/dev.c:7959)
  handle_softirqs (kernel/softirq.c:622)
  do_softirq.part.0 (kernel/softirq.c:523)
  __local_bh_enable_ip (kernel/softirq.c:450)
  __dev_queue_xmit (net/core/dev.c:4913)
  packet_sendmsg (net/packet/af_packet.c:3139)
  __sys_sendto (net/socket.c:2252)
  __x64_sys_sendto (net/socket.c:2259)
  do_syscall_64 (arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception in interrupt

Reject routes without an inet6_dev immediately after lookup. Such routes
are not eligible for reverse-path filtering, and the check protects all
later rt6i_idev dereferences.

Fixes: e26f9a4 ("netfilter: add ipv6 reverse path filter match")
Reported-by: co+459f67f4d8af8ce6@bugs.sh
Closes: https://lore.kernel.org/all/VtWUkE8QzJt5CroTj2V2v3ZQ0gwbXZ7nq7I3@bugs.sh/
Suggested-by: Florian Westphal <fw@strlen.de>
Assisted-by: Claude:gpt-5
Cc: stable@vger.kernel.org
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/netfilter/ip6t_rpfilter.c
  net/ipv6/netfilter/ip6_tables.c
  net/netfilter/core.c
  net/ipv6/ip6_input.c
  net/core/dev.c
  kernel/softirq.c
  net/packet/af_packet.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=f4de78756b0fddbd125b2b1b77c74f2eccc8e977

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

The original Linux kernel CVE announcement for CVE-2026-98379 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98379

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: Not available
  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: 3

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).