[CVE-2026-98122][MODERATE 7.0] vxlan: mdb: Fix use-after-free in vxlan_mdb_remote_src_del() [ Upstream

[CVE-2026-98122][MODERATE 7.0] vxlan: mdb: Fix use-after-free in vxlan_mdb_remote_src_del() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98122
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vxlan: mdb: Fix use-after-free in vxlan_mdb_remote_src_del() [ Upstream
Commit: e12c903fd13c6261b98ca18d8f70ac26e739e2f8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e12c903fd13c6261b98ca18d8f70ac26e739e2f8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98122
Analysis date: Fri, 25 Sep 2026 12:46:05 -0400
ActionableScore: 6
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98122 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98122

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 7.0
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-2026-98122	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98122 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:H/I:H/A:H';*CWE-416;CWE-825;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A use after free in the VXLAN MDB control path occurs when an all zero source address causes a derived S G entry key to alias the original star G entry key. Removing the aliased forwarding entry frees the source entry, after which hlist_del operates on the freed object and reads pprev and next pointers before writing through them. An unprivileged local user can perform MDB operations from a new user and network namespace where unprivileged namespace creation is permitted. For the CVSS the PR:L reflects that local user context and namespace creation are required even though host administrator privileges are not required. The issue is not directly network reachable because it is triggered through the netlink control plane rather than VXLAN packet traffic. Impact is at least a local denial of service via kernel memory corruption and in the worst case may allow confidentiality integrity and privilege impact due to the write through use after free primitive. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	READ OOB DANGER UAF NETWORK TEST IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=6
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6
* Raw paranoid score: 6
* Paranoid score: 6
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF and active lifetime violation. The forwarding-entry deletion frees `ent`, and `vxlan_mdb_remote_src_entry_del(ent)` subsequently uses the same freed object.
* +1 Constrained post-free corruption. `hlist_del()` reads `pprev` and `next` from the freed object and writes through the resulting stale list pointers. This is more than a read-only stale dereference, but attacker-controlled target selection or controlled reclaim is not established.
* +1 Local unprivileged trigger. The supplied commit explicitly states that MDB operations are netns-scoped and can be performed by an unprivileged user in a new user and network namespace.
* +1 Attacker-influenced active-object lifetime manipulation. The attacker controls MDB source attributes, creates the colliding entries, and triggers replacement and deletion through the MDB netlink control plane.
* +1 Reliable kernel crash or strong DoS. KASAN reports an in-task slab-use-after-free, and stale `hlist_del()` list-pointer operations can produce kernel faults or list corruption.

Important non-awarded signals:

* Controlled reclaim or attacker-controlled replacement of the freed slab object is not demonstrated.
* Arbitrary or attacker-directed kernel write is not demonstrated. The stale `hlist_del()` write target is derived from stale kernel list-link fields.
* Confidentiality and integrity bonuses are not awarded. The patch establishes kernel memory corruption, but no disclosure, protected-object modification, credential modification, or policy bypass is demonstrated.
* Remote reachability is not established. The trigger is rtnetlink MDB control-plane input, not VXLAN packet traffic.
* No concrete repeated release or refcount mutation was established. The stale operation is `hlist_del()`, not a second `put`, `release`, or refcount decrement on the same object.
* No additional paranoid point is awarded. Generic theoretical slab reuse is insufficient without evidence of attacker-controlled reclamation or post-free structured-object substitution.
* No race penalty applies. The vulnerable sequence is an ordered aliasing and deletion path, not a narrow timing race.

3. Reachability analysis

The trigger is a local rtnetlink MDB operation against a VXLAN device. The supplied context explicitly states that MDB operations are scoped to the network namespace and that an unprivileged user can execute them after creating a user and network namespace.

The complete vulnerable operation appears namespace-scoped: the attacker can create/configure the namespace-local VXLAN and MDB state, submit the malformed all-zero source configuration, and invoke the replacement path that performs deletion. This conclusion depends on systems permitting unprivileged user-namespace creation. If user namespaces are disabled by policy, the practical attacker must instead possess delegated or host network-administration capability.

No special hardware, firmware interaction, external network access, or privileged host-side VXLAN setup is identified by the supplied evidence. The triggering sequence is deterministic state construction through netlink rather than a timing-sensitive race.

4. Primitive and severity interpretation

The exact primitive is an active structured-object use-after-free in the VXLAN MDB control plane. An all-zero source address causes a derived `(S,G)` key to alias its parent `(*,G)` key. Deleting the derived forwarding entry therefore deletes and frees the source entry itself. A later `hlist_del()` operates on the freed source entry and reads stale linkage fields before writing through them.

This is not fixed async stale storage and not merely a cleanup-only dereference. The object is an active forwarding/MDB entry, the attacker controls the malformed key relationship, and the vulnerable path performs a post-free list mutation. However, the evidence does not establish controlled slab reclaim, attacker-shaped replacement contents, callback control, arbitrary write, information disclosure, or practical privilege escalation.

The vulnerability is CLASS_B. It is stronger than a crash-only invalid dereference because the stale operation performs writes through list pointers obtained from freed memory. It is not CLASS_A because the supplied patch does not demonstrate attacker control over the freed memory contents or resulting write destination.

ImportantCandidate is YES because a below-host-admin attacker can reach a concrete active-object UAF with a post-free write operation, while the practical exploitation leverage of the stale list mutation remains unresolved. AutoDowngradeAllowed is NO because automatically treating this as an ordinary local crash would discard a concrete kernel corruption primitive requiring manual exploitability review.

Likely operational severity is actionable local kernel DoS at minimum. Possible confidentiality, integrity, or privilege effects remain unproven and require allocator, object-layout, and list-corruption reachability analysis.

5. One-sentence report phrase

A local unprivileged user can trigger a VXLAN MDB use-after-free through all-zero source key aliasing, causing `hlist_del()` to write through stale list pointers and potentially crash or corrupt the kernel.

6. Manual review recommendation

MANUAL CHECK REQUIRED

A namespace-reachable active-object UAF performs a concrete post-free list-pointer write, but controlled reclaim and practical post-free write influence are not resolved by the patch alone.

7. Call-site confidence

Call-site confidence: high

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

Patch: vxlan: mdb: Fix use-after-free in vxlan_mdb_remote_src_del() [ Upstream
Commit: e12c903fd13c6261b98ca18d8f70ac26e739e2f8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e12c903fd13c6261b98ca18d8f70ac26e739e2f8

Changed files:
  drivers/net/vxlan/vxlan_mdb.c
  tools/testing/selftests/net/test_vxlan_mdb.sh

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=e12c903fd13c6261b98ca18d8f70ac26e739e2f8

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

The original Linux kernel CVE announcement for CVE-2026-98122 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98122

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 6

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).