[CVE-2026-99123][IMPORTANT] vsock/virtio: validate packet source for connected sockets [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-99123
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: vsock/virtio: validate packet source for connected sockets [ Upstream
Commit: 6cd829de69efd9ed3a53aa2421004a0396eb2bd7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cd829de69efd9ed3a53aa2421004a0396eb2bd7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99123
Analysis date: Fri, 30 Oct 2026 11:17:50 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
An unprivileged local AF_VSOCK attacker can exploit missing connected-peer source validation to establish and inject chosen data into another UID’s socket stream.

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

The original Linux kernel CVE announcement for CVE-2026-99123 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99123

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: IMPORTANT
Manual review required: YES

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-2026-99123	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99123 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, 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:L/A:H';CWE-346;CWE-345;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.1';DESCR 'A source validation error in the virtio vsock receive path allows the destination only bound socket fallback to select a connected socket lacking verifying the packet source against its stored peer. An unprivileged local attacker can discover the target tuple through AF_VSOCK sock_diag, spoof a RESPONSE to transition a victim socket from TCP_SYN_SENT to TCP_ESTABLISHED, and then inject attacker chosen RW data across a UID boundary. For the CVSS the PR:L value is selected because exploitation requires a local process and AF_VSOCK access, but no capabilities or elevated account are required in the reproduced scenario. The demonstrated primitive is cross process stream data injection with integrity impact and possible connection disruption. There is no demonstrated kernel memory corruption, information disclosure, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 6.1 and the production ActionableScore returned 7 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and TEMP_KPANIC_MERGE_IMPORTANT_VETO and B4_CONFIRMED_AUTHORIZATION_PRESERVE and MANUAL_REVIEW_REQUIRED and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 4 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	MAYBE	REMOTE OOB LEAK LINUS PACKET INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 TEMP_KPANIC_MERGE_IMPORTANT_VETO B4_CONFIRMED_AUTHORIZATION_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=YES
B4PendingConfirmed=YES

1. ActionableScore

* Raw base score: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; no additional speculative impact awarded.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 concrete authorization/source-authentication bypass: destination-only lookup accepts packets for a connected socket without validating the stored peer CID and port.
* +2 practical security-boundary bypass: the invalid source is accepted as the victim connection’s peer, allowing cross-UID stream injection.
* +1 local unprivileged trigger: reproduction used capability-empty processes under different UIDs.
* +1 practical exploit chain: unprivileged `sock_diag` tuple discovery, spoofed RESPONSE state transition, then attacker-chosen RW delivery is demonstrated.
* +1 integrity impact: the victim read 16 attacker-chosen bytes while the intended peer read none.
* No memory-corruption, UAF, disclosure, privilege escalation, protected-file access, or arbitrary kernel write points.
* No availability point: connection disruption is possible, but host-wide or otherwise strong DoS is not established.
* B4-PENDING is confirmed: the concrete path is `virtio_transport_recv_pkt()` destination-only fallback → missing peer-source validation → connected victim socket accepts RESPONSE/RW packets.

3. Reachability analysis

The complete demonstrated attacker is a local, capability-empty, non-UID-0 process with AF_VSOCK access. No host-administrator capability, namespace capability, prior privileged setup, special hardware, or race is shown as necessary. The attacker can discover the target tuple through unprivileged AF_VSOCK diagnostics and materially control the spoofed packet source and payload.

4. Primitive and severity reading

The primitive is an authentication/endpoint-identity failure, not memory corruption. A connected socket remains discoverable through the bound-table fallback, and the old code checks only transport identity—not the packet source CID and port. This permits an unrelated source to transition `TCP_SYN_SENT` to `TCP_ESTABLISHED` and inject chosen stream bytes.

The fix cleanly reinstates the mandatory peer check under the socket lock. The protected resource impact is confirmed because attacker data crosses a UID boundary into a victim’s established stream. This supports CLASS_A and B4-PENDING preservation. It does not prove confidentiality loss, LPE, kernel panic, or arbitrary access beyond the affected stream.

ImportantCandidate is YES and auto-downgrade is disallowed because a complete below-host-admin cross-principal data-injection chain is reproduced.

5. One-sentence report phrase

An unprivileged local AF_VSOCK attacker can exploit missing connected-peer source validation to establish and inject chosen data into another UID’s socket stream.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirmed B4-PENDING authorization bypass with a demonstrated protected cross-UID stream integrity impact.

7. Call-site confidence

Call-site confidence: high

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

Patch: vsock/virtio: validate packet source for connected sockets [ Upstream
Commit: 6cd829de69efd9ed3a53aa2421004a0396eb2bd7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cd829de69efd9ed3a53aa2421004a0396eb2bd7

Changed files:
  include/net/af_vsock.h
  net/vmw_vsock/af_vsock.c
  net/vmw_vsock/virtio_transport_common.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=6cd829de69efd9ed3a53aa2421004a0396eb2bd7

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

The original Linux kernel CVE announcement for CVE-2026-99123 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99123

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:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 6.1

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: 7

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

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).