[CVE-2026-64109][MODERATE 7.0] af_unix: Fix UAF read of tail->len in unix_stream_data_wait() [ Upstream

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

CVE: CVE-2026-64109
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: af_unix: Fix UAF read of tail->len in unix_stream_data_wait() [ Upstream
Commit: 26342087fac93b3932e6af61dc91ec029cb8a623
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=26342087fac93b3932e6af61dc91ec029cb8a623
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64109
Analysis date: Sun, 19 Jul 2026 15:26:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local unprivileged user can race AF_UNIX stream peeking and normal receive operations so that `unix_stream_data_wait()` reads `tail->len` from a freed SKB, causing a UAF read with realistic DoS impact and limited but unproven escalation concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64109 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64109

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64109	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:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'Use-after-free read in the AF_UNIX stream receive path can occur because unix_stream_data_wait reads the length of the tail SKB after observing the receive queue without a lock that protects against dequeue and free. A local process can trigger the race by using AF_UNIX stream sockets with peeking and normal receive operations on the same socket so that one thread waits on stale queue state while another thread removes and frees the SKB. For the CVSS the PR:L because attacker controlled local code is needed to create the socket state and race the recv operations, but no administrative privilege is required. The issue is not network reachable because AF_UNIX sockets are local IPC. Impact is at least local denial of service via kernel warning or crash, and in the paranoid score limited confidentiality or integrity impact is kept possible because this is a real UAF read of a freed kernel object, not just a pure logic hang.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE LOCK DANGER RACE UAF SKB TEST  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  AF_UNIX stream sockets, socketpair, SO_PEEK_OFF, MSG_PEEK, recv, and shutdown are reachable by ordinary local users.

* Memory corruption, weak primitive: +1
  This is a real UAF read of `tail->len`, but the patch only shows a read from a freed `sk_buff`, not write-after-free, type confusion, callback control, refcount takeover, or attacker-controlled reclaim.

* Real lifetime corruption: +1
  The bug is caused by an SKB being dequeued and freed while another thread still uses a stale pointer from the receive queue.

* Broad/default/common subsystem: +1
  AF_UNIX is a common local IPC subsystem and is normally enabled on Linux systems.

* Availability impact realistic: +1 in paranoid score
  A UAF read from a freed kernel object can plausibly lead to kernel warning, KASAN report, Oops, or crash depending on allocator state and hardening. This is not counted as strong reliable DoS in the conservative score because the demonstrated access is only a small read of `len`.

Subtracted signals:

* Hard or unreliable race / special timing required: -1
  Triggering requires a specific race between peeking recv and normal recv on the same socket, with a narrow lifetime window.

Not awarded:

* Remote reachable: +0
  AF_UNIX is local IPC, not network reachable.

* Strong LPE plausibility: +0
  No attacker-controlled reclaim, object replacement, write primitive, callback dispatch, or type confusion is shown.

* Confidentiality / Integrity impact in conservative score: +0
  The concrete primitive is only a UAF read of `tail->len`. Paranoid CVSS may keep C:L/I:L for manual-review sensitivity, but the ActionableScore should not assume a practical disclosure or overwrite primitive.

## 3. Reachability analysis

A local unprivileged process can create the required AF_UNIX stream socketpair and race `MSG_PEEK` receive with a normal receive on the same socket. No root privileges or special capabilities are required. Namespaces do not materially reduce reachability because AF_UNIX sockets are ordinary local IPC objects available in typical containers and user sessions.

The path is default-enabled on normal Linux systems. Exploitation is timing-dependent and requires a specific concurrent interaction, but the commit provides a concrete reproducer-style race pattern, so this should not be treated as a purely theoretical bug.

Call-site confidence: high. The patch and commit include the vulnerable call path, the stale pointer lifetime, and the exact dereference removed by the fix.

## 4. Severity interpretation

This behaves like an actionable Moderate kernel memory-lifetime issue, not an ordinary low-risk DoS-only cleanup. The realistic demonstrated primitive is a local race-triggered UAF read and likely impact is crash or warning. Theoretical privilege escalation is not strongly supported because there is no demonstrated reclaim, write, type confusion, callback control, or refcount abuse.

The conservative score is capped at 4 under the race-UAF downgrade rule. The paranoid score is 5 because the bug is a real local unprivileged UAF in a common kernel IPC path and should not be auto-closed without manual review.

## 5. One-sentence report phrase

A local unprivileged user can race AF_UNIX stream peeking and normal receive operations so that `unix_stream_data_wait()` reads `tail->len` from a freed SKB, causing a UAF read with realistic DoS impact and limited but unproven escalation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes a local unprivileged UAF read in a broad default subsystem. The demonstrated primitive is weak and race-dependent, but memory lifetime bugs in common local IPC paths should receive manual review rather than automatic closure.

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

Patch: af_unix: Fix UAF read of tail->len in unix_stream_data_wait() [ Upstream
Commit: 26342087fac93b3932e6af61dc91ec029cb8a623
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=26342087fac93b3932e6af61dc91ec029cb8a623

Changed files:
  net/unix/af_unix.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=26342087fac93b3932e6af61dc91ec029cb8a623

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

The original Linux kernel CVE announcement for CVE-2026-64109 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64109

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:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4
  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: 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).