[CVE-2026-74735][MODERATE REGULAR] l2tp: fix tunnel and session refcount leak on seq_file release [ Upstream

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

CVE: CVE-2026-74735
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: l2tp: fix tunnel and session refcount leak on seq_file release [ Upstream
Commit: ebe2774e956482dd3c70b8991f6f7654356339e6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebe2774e956482dd3c70b8991f6f7654356339e6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74735
Analysis date: Wed, 26 Aug 2026 13:27:00 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like. Not an Important candidate
Manual review required: NO

Summary:
L2TP seq_file release could leak tunnel and session references when a local user closes procfs or debugfs iteration before EOF, potentially causing resource retention and local denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-74735 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74735

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 REGULAR
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-2026-74735	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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:N/A:H';*CWE-772;*CWE-401;CWE-404;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'L2TP seq_file iteration can leak tunnel and session references when a user closes /proc/net/pppol2tp or /sys/kernel/debug/l2tp/tunnels before reading to EOF. The leaked references are stored in seq_file private data and are not dropped on early release, so repeated open and early close cycles can keep L2TP objects alive and may prevent normal cleanup of sessions, tunnels, or network namespaces. For the CVSS the PR:L is used because a local user or process can trigger the procfs read path when L2TP entries already exist, although creating those entries usually requires CAP_NET_ADMIN and the debugfs path is normally more restricted. The issue is not network reachable and is triggered through local file operations rather than packet traffic. Impact is denial of service through resource leakage and cleanup blockage, with no supported evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like. Not an Important candidate (with actual score 3)	YES	DISK INIT NOMEMCHK LEAK DEBUGFS SYZBOT  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like. Not an Important candidate**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  `/proc/net/pppol2tp` is created with mode `0444`, so a local user may be able to open and close it if L2TP objects already exist.

* Real lifetime corruption: +1
  This is a reference lifetime bug. Stored `pd->tunnel` and `pd->session` references are not dropped on seq_file release.

* Availability impact realistic: +1, paranoid only
  Repeated early close cycles can pin tunnel/session references and may prevent cleanup of L2TP objects or network namespaces. This is resource retention rather than immediate crash.

Negative or limiting signals:

* Rare or configuration-dependent subsystem: -1
  L2TP must be enabled and populated with tunnel/session state. The debugfs path is usually restricted, and creating L2TP objects normally requires `CAP_NET_ADMIN`.

Not triggered:

* No remote/network-triggerable path.
* No UAF, double-free, OOB write, type confusion, arbitrary write, or page-cache corruption.
* No plausible confidentiality or integrity impact from the patch.
* No privilege escalation primitive.

## 3. Reachability analysis

The most realistic trigger is a local process opening `/proc/net/pppol2tp` or `/sys/kernel/debug/l2tp/tunnels` and closing it before EOF. The procfs path may be readable by unprivileged users when L2TP state already exists, but creating the relevant tunnels and sessions typically requires `CAP_NET_ADMIN`. The debugfs path is normally restricted to privileged users. Network traffic alone does not trigger the bug directly. Namespaces may make the issue easier to exercise in environments where L2TP objects are created inside a delegated network namespace, but the impact remains a local resource leak.

Call-site confidence: high. The patch shows both release paths and the exact missing `l2tp_session_put()` and `l2tp_tunnel_put()` calls.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource lifetime issue, not an Important-class vulnerability. The bug can cause reference leaks and cleanup blockage, but the patch does not support memory corruption, information disclosure, privilege escalation, or remote exploitation. The paranoid score reaches 3 only because sustained local abuse could have availability impact by pinning objects.

## 5. One-sentence report phrase

L2TP seq_file release could leak tunnel and session references when a local user closes procfs or debugfs iteration before EOF, potentially causing resource retention and local denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a refcount/resource leak with local reachability and no demonstrated memory corruption or security-boundary bypass. It can be handled as an ordinary Moderate unless the target environment relies heavily on untrusted users accessing preconfigured L2TP namespaces.

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

Patch: l2tp: fix tunnel and session refcount leak on seq_file release [ Upstream
Commit: ebe2774e956482dd3c70b8991f6f7654356339e6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebe2774e956482dd3c70b8991f6f7654356339e6

Changed files:
  net/l2tp/l2tp_debugfs.c
  net/l2tp/l2tp_ppp.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=ebe2774e956482dd3c70b8991f6f7654356339e6

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

The original Linux kernel CVE announcement for CVE-2026-74735 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74735

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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 REGULAR

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).