[CVE-2026-68169][MODERATE 7.0] mptcp: pm: userspace: fix use-after-free in get_local_id [ Upstream

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

CVE: CVE-2026-68169
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mptcp: pm: userspace: fix use-after-free in get_local_id [ Upstream
Commit: d2c3760b45f2f481a4dd4c5adef4a29dfabd948f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2c3760b45f2f481a4dd4c5adef4a29dfabd948f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68169
Analysis date: Mon, 10 Aug 2026 09:07:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in MPTCP userspace path-manager handling can read a freed address-entry id after concurrent local-address deletion, causing a confirmed UAF read and possible kernel crash, with practical triggering requiring local PM control and tight timing.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68169 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68169

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-2026-68169	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 'A race in MPTCP userspace path manager handling can make mptcp_userspace_pm_get_local_id read an address id after the address entry was deleted and freed by a concurrent MPTCP_PM_CMD_SUBFLOW_DESTROY request. The confirmed primitive is a use-after-free read of one byte, reached when local PM control operations overlap with an MP_JOIN SYN path. For the CVSS the PR:L is used because reliable triggering requires a local process with access to MPTCP PM control operations, typically CAP_NET_ADMIN or an equivalent delegated network administration context. The issue is not purely network reachable because the deletion side is a local control plane request, although the racing MP_JOIN SYN can be supplied by a peer. Impact is at least local denial of service via kernel crash or KASAN detection. In the paranoid score, limited confidentiality or integrity impact is kept for manual-review sensitivity because this is a real UAF read in networking code, but no strong write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE READ OOB DANGER RACE UAF NETWORK LINUS TEST KPANIC 	YES	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

Conservative signals:

* Memory corruption, weak primitive: +1
  Confirmed slab use-after-free, but only a 1-byte stale read of `entry->addr.id` is shown.
* Real lifetime corruption: +1
  The object is freed by `mptcp_userspace_pm_delete_local_addr()` while another path still uses the pointer after dropping `msk->pm.lock`.
* Reliable kernel crash / strong DoS: +1
  KASAN confirms the race is reachable and reports slab-use-after-free.
* Broad/default/common networking exposure: +1
  The bug is in MPTCP networking code, with one side of the race involving MP_JOIN processing.
* Common networking / packet-processing path involvement: +1
  The lookup is reached from MPTCP PM local-id resolution during subflow handling.
* Hard or unreliable race / special timing required: -1
  The commit says the race window is narrow and was reproduced only with a local stress test.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1 conservative refinement
  The destructive PM operation is a local control-plane request, likely requiring network administration capability, but delegated netns/container/service contexts make full -2 too harsh.

Paranoid extra interpretation:

* Local low-privileged or delegated trigger concern: +1
  If an untrusted actor has delegated CAP_NET_ADMIN in a network namespace or service context, triggering may be closer to PR:L practical risk than full host-root.

Race-UAF downgrade applied:

* No Weak LPE concern awarded.
* No strong LPE signal awarded.
* No arbitrary write, callback control, type confusion, refcount takeover, or attacker-controlled reclaim is shown.
* Conservative score capped at 4 and paranoid score capped at 5.

Call-site confidence: high. The patch, KASAN trace, allocation path, free path, and fixed dereference are all present.

## 3. Reachability analysis

The bug requires a race between MPTCP userspace path-manager state lookup and deletion of a local address entry through `MPTCP_PM_CMD_SUBFLOW_DESTROY`. A remote peer can contribute the MP_JOIN SYN side of the race, but the deletion side is a local control-plane operation, so this is not purely remote/network-triggerable.

In typical deployments, the MPTCP PM netlink control operation is expected to require CAP_NET_ADMIN or equivalent network administration authority. In containerized or delegated network namespace setups, that may be available to a less trusted local actor, so PR:L-style practical treatment is reasonable for the paranoid view.

MPTCP is a real networking subsystem, but the userspace PM path and the narrow timing requirement reduce broad exploitability. Realistic exploitation appears to require local orchestration and stress timing rather than a simple remote packet stream.

## 4. Severity interpretation

This is stronger than an ordinary Moderate cleanup because it is a confirmed UAF in networking code, not just a theoretical race. However, the demonstrated primitive is a stale 1-byte read, and the patch does not show attacker-controlled reclaim, write-after-free, type confusion, callback dispatch, or refcount abuse.

Realistic impact is local DoS via kernel crash or KASAN-detected UAF. Theoretical escalation should be reviewed, but current evidence supports Actionable Moderate rather than Strong Important.

## 5. One-sentence report phrase

A race in MPTCP userspace path-manager handling can read a freed address-entry id after concurrent local-address deletion, causing a confirmed UAF read and possible kernel crash, with practical triggering requiring local PM control and tight timing.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed race-dependent UAF in networking code, KASAN-reachable, with local control-plane plus packet-path interaction. The observed primitive is weak, but the lifetime bug class warrants manual review before auto-closure.

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

Patch: mptcp: pm: userspace: fix use-after-free in get_local_id [ Upstream
Commit: d2c3760b45f2f481a4dd4c5adef4a29dfabd948f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2c3760b45f2f481a4dd4c5adef4a29dfabd948f

Changed files:
  net/mptcp/pm_userspace.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=d2c3760b45f2f481a4dd4c5adef4a29dfabd948f

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

The original Linux kernel CVE announcement for CVE-2026-68169 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68169

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).