[CVE-2026-68116][MODERATE 7.0] vxlan: mdb: Fix source list corruption on a failed replace

[CVE-2026-68116][MODERATE 7.0] vxlan: mdb: Fix source list corruption on a failed replace

[AL-KERNEL]

CVE: CVE-2026-68116
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: vxlan: mdb: Fix source list corruption on a failed replace
Commit: 5bc8fc1d2ff802eec839e03adef5df597421898d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5bc8fc1d2ff802eec839e03adef5df597421898d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68116
Analysis date: Mon, 10 Aug 2026 15:47:41 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A failed VXLAN MDB source-list replace can corrupt the logical source state of an existing multicast remote entry, causing EXCLUDE filters to forward traffic that should be blocked or INCLUDE filters to drop traffic that should be forwarded, with typical triggering requiring CAP_NET_ADMIN or delegated network-control privileges.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68116 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68116

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: NO

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-68116	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-703;CWE-754;CWE-284;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '4.5';DESCR 'A failed VXLAN MDB source list replace can corrupt the logical source state of an existing (*,G) remote entry. During replace, pre existing sources are marked for deletion and new sources are added, but an error in the middle of the add path could remove old sources as well as newly created ones. With an EXCLUDE filter this can allow traffic that should be blocked to be forwarded. With an INCLUDE filter this can drop traffic that should be forwarded. For the CVSS the PR:L is used in the paranoid score because delegated CAP_NET_ADMIN, container root, or a network management service may allow a less than full host administrator to update VXLAN MDB state. The issue is not directly network reachable through VXLAN packet traffic and is triggered through control plane MDB configuration. Impact is a network forwarding policy bypass with limited confidentiality, integrity, and availability impact rather than kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	DANGER INIT ERRORPATH NETWORK  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* **Security / forwarding policy bypass: +2**
  A failed VXLAN MDB source-list replace can remove pre-existing source entries and change multicast forwarding semantics. EXCLUDE mode can forward traffic that should be blocked, and INCLUDE mode can drop traffic that should be forwarded.

* **Confidentiality impact plausible: +1**
  In EXCLUDE mode, traffic may be delivered where policy intended it not to be delivered.

* **Integrity impact plausible: +1**
  The configured multicast forwarding policy can be corrupted logically.

* **Availability impact realistic: +1**
  In INCLUDE mode, traffic that should be forwarded may be dropped.

* **Requires CAP_NET_ADMIN in typical deployments: -2**
  Updating VXLAN MDB state is a control-plane operation and normally requires administrative network privileges.

Conservative total: **3**

Paranoid additions / adjustments:

* **Common data-center networking exposure: +1**
  VXLAN is commonly used in virtualized, containerized, and data-center overlay networks, even though this specific path is not a default Internet-exposed packet parser.

* **Reduced privilege / delegated network admin interpretation: +1 adjustment versus full -2 penalty**
  In practical deployments, VXLAN control may be reachable by container root, orchestrators, service accounts, or delegated CAP_NET_ADMIN rather than full host-root. This supports the paranoid score.

Paranoid total: **5**

No memory-corruption signals are awarded. The patch shows logical source-list rollback corruption, not UAF, OOB write, double-free, refcount takeover, arbitrary write, or privilege escalation.

Call-site confidence: **high**. The replace path, add path, rollback path, and changed helper behavior are visible in the provided diff.

## 3. Reachability analysis

The bug is triggered through VXLAN MDB control-plane configuration, specifically a failed replace of the source list for an existing `(*,G)` remote entry. It is not directly triggered by ordinary VXLAN data packets from the network.

In typical host deployments, the attacker needs CAP_NET_ADMIN or equivalent privileges to manipulate VXLAN MDB state, so the conservative interpretation applies an admin privilege penalty. However, in containerized or orchestrated environments, network administration may be delegated to less-than-full host-root actors, which makes PR:L-style practical triage reasonable for the paranoid score.

The affected functionality is configuration-dependent because VXLAN MDB support and multicast filtering must be in use. It is still relevant for data-center and overlay networking environments where forwarding policy correctness is security-sensitive.

## 4. Severity interpretation

This behaves like an **actionable Moderate** issue rather than a Low correctness-only bug, because it can change forwarding behavior and potentially bypass multicast source filtering policy.

It does **not** look like an Important-class memory-safety issue. The realistic impact is policy corruption, unintended forwarding, or unintended dropping of traffic. Theoretical privilege escalation is not supported by the patch.

Because the issue affects VXLAN forwarding policy and can invert INCLUDE / EXCLUDE behavior, it should not be auto-closed purely because the CVSS-like score is moderate.

## 5. One-sentence report phrase

A failed VXLAN MDB source-list replace can corrupt the logical source state of an existing multicast remote entry, causing EXCLUDE filters to forward traffic that should be blocked or INCLUDE filters to drop traffic that should be forwarded, with typical triggering requiring CAP_NET_ADMIN or delegated network-control privileges.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

YES REQUIRES MANUAL CHECK because this is a network forwarding policy bypass in VXLAN MDB control-plane handling, even though no kernel memory corruption or direct remote packet-triggered path is shown.

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

Patch: vxlan: mdb: Fix source list corruption on a failed replace
Commit: 5bc8fc1d2ff802eec839e03adef5df597421898d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5bc8fc1d2ff802eec839e03adef5df597421898d

Commit description:

When replacing the source list of an MDB remote entry, all existing
sources are first marked for deletion and vxlan_mdb_remote_srcs_add()
is then called to add the new source list. Sources present in the new
list have their deletion mark cleared, and any sources left marked
afterwards are removed.

If vxlan_mdb_remote_srcs_add() fails partway through, its error path
deletes all entries on the remote's source list. That rollback is only
correct for its other caller, vxlan_mdb_remote_add(), where the remote
was just allocated and the list contains solely entries added during
the call. On the replace path the list also holds pre-existing sources,
so a failed replace tears them down together with their (S, G)
forwarding entries instead of leaving the entry unchanged.

This is reachable from an existing (*, G) remote. An EXCLUDE filter
that loses sources starts forwarding traffic that should be blocked,
while an INCLUDE filter that loses sources drops traffic that should be
forwarded.

Mark entries created during the current pass with a new
VXLAN_SGRP_F_NEW flag. On failure, delete only those entries and clear
the deletion mark on the pre-existing ones, so a failed replace leaves
the source list untouched. Retain the flag until the whole operation
succeeds and then clear it. Also stop vxlan_mdb_remote_src_add() from
deleting a pre-existing entry it only looked up when adding that
entry's forwarding entry fails.

Fixes: a3a48de ("vxlan: mdb: Add MDB control path support")
Cc: stable@vger.kernel.org
Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260720160428.249356-1-jamestiotio@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/vxlan/vxlan_mdb.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=5bc8fc1d2ff802eec839e03adef5df597421898d

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

The original Linux kernel CVE announcement for CVE-2026-68116 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68116

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:H/PR:H/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.5

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: 3
  Paranoid ActionableScore: 5

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 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).