* [CVE-2026-99074][MODERATE REGULAR] ipv4: stop PMTU walk when nexthop group shrinks
@ 2026-10-27 4:06 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 4:06 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99074
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv4: stop PMTU walk when nexthop group shrinks
Commit: 7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99074
Analysis date: Tue, 27 Oct 2026 00:06:12 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A namespace-capable local attacker can race IPv4 multipath nexthop replacement against PMTU walking to trigger a host-kernel NULL-pointer dereference.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99074 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99074
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-2026-99074 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99074 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final 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:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;**CWE-476;CWE-667;BEST CVSS score: '4.7';DESCR 'A race in the IPv4 PMTU multipath walk occurs because fib_info_num_path and fib_info_nhc can observe different RCU protected nexthop groups during concurrent replacement. If a two member group shrinks after the loop accepts index 1, fib_info_nhc can return NULL and update_or_create_fnhe then dereferences it, causing a kernel oops or panic. For the CVSS the PR:L value reflects that a local process must perform nexthop replacement with CAP_NET_ADMIN in its network namespace, which an unprivileged user may obtain when unprivileged user and network namespaces are enabled. AC:H reflects the required race window. The demonstrated primitive is a NULL pointer dereference with availability impact only, not UAF, OOB access, information disclosure, or privilege escalation. The issue is not independently remotely exploitable because local control over nexthop replacement remains necessary even if network activity helps generate PMTU updates. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 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. 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 V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE REMOTE KASAN NULLPTR NETWORK PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
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 base score: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Local unprivileged trigger: where unprivileged user namespaces are enabled, a local user can obtain namespace-scoped CAP_NET_ADMIN and replace the nexthop group.
* +1 Common packet-processing path: the inconsistent group observations occur during IPv4 PMTU route processing.
* +1 Reliable kernel crash/strong DoS: a reproduced NULL `nhc` reaches `update_or_create_fnhe()` and causes a KASAN NULL-pointer fault.
* -1 Hard race: replacement must shrink the group between two independent RCU-protected loads; the reproducer used a probe to widen this window.
* No extra availability point: it would duplicate the same oops/panic.
* No UAF, OOB write, controlled corruption, disclosure, authorization bypass, or integrity primitive is shown.
* Remote reachability is not awarded: attacker-controlled local nexthop replacement remains necessary.
3. Reachability analysis
The demonstrated actor is a local user on systems permitting unprivileged user and network namespaces. That user can obtain CAP_NET_ADMIN in the new network namespace, create/replace the namespace-local nexthop group, and race PMTU processing. Host UID 0 and host CAP_NET_ADMIN are not required for this evidenced configuration. There is no special hardware or prior host provisioning, but disabling unprivileged user namespaces removes this below-host-admin path.
4. Primitive and severity reading
The exact primitive is an inconsistent RCU snapshot: the loop bound observes the old two-member group while indexed lookup observes the replacement one-member group and returns NULL. The later consumer immediately dereferences that NULL pointer. This is CLASS_C crash behavior, not stale-object lifetime use or a useful corruption primitive. ImportantCandidate is NO and auto-downgrade is allowed because no non-crash leverage or unresolved trust-boundary corruption is present.
5. One-sentence report phrase
A namespace-capable local attacker can race IPv4 multipath nexthop replacement against PMTU walking to trigger a host-kernel NULL-pointer dereference.
6. Manual review advice
MANUAL CHECK RECOMMENDED
Confirm product user-namespace policy and whether namespace-triggered kernel oopses panic the host.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ipv4: stop PMTU walk when nexthop group shrinks
Commit: 7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
Commit description:
Commit 7d3f3b4 ("net: ipv4: Cache pmtu for all packet paths if
multipath enabled") made __ip_rt_update_pmtu() update every path.
For nexthop objects, fib_info_num_path() and fib_info_nhc()
independently load nh->nh_grp. RCU protects each group's lifetime but
does not make the loads observe the same group. If replacement shrinks
the group after the loop accepts an index, fib_info_nhc() returns NULL
and update_or_create_fnhe() dereferences it.
A deterministic probe only widened the existing window between the real
operations. On v7.2, an RTM_NEWNEXTHOP replacement published a
one-member group after the PMTU reader accepted index 1 from a two-member
group, producing:
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: update_or_create_fnhe+0x45/0x15b0
Stop when the indexed path is absent. Groups are dense, so no later
index exists in that snapshot. The same real-writer window completed
114,423 PMTU iterations without an oops with this check. A reproducer
is available on request.
Replacement requires CAP_NET_ADMIN in the network namespace. Where
unprivileged user namespaces are permitted, a local user can obtain it
in a new user and network namespace.
Fixes: 7d3f3b4 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled")
Cc: [email protected]
Signed-off-by: Daehyeon Ko <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Link: https://patch.msgid.link/4fb87ed994dfc067178fc845cdd8bc9b5446b039.1790915964.git.4ncienth@gmail.com
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/ipv4/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=7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99074 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99074
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: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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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