[CVE-2026-52940][MODERATE 7.0] tun: zero the whole vnet header in tun_put_user() [ Upstream

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

CVE: CVE-2026-52940
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tun: zero the whole vnet header in tun_put_user() [ Upstream
Commit: 5fd1fa5a4254bfdd70571c77f5e3bcb4e43738d5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5fd1fa5a4254bfdd70571c77f5e3bcb4e43738d5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52940
Analysis date: Wed, 24 Jun 2026 05:36:28 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local TUN user can leak 14 bytes of uninitialized kernel stack data per read because `tun_put_user()` copied a partially initialized virtio vnet header to userspace.

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

The original Linux kernel CVE announcement for CVE-2026-52940 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52940

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
======================================================================

CVE-2026-52940	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';*CWE-200;CWE-908;*CWE-457;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'A kernel stack information leak in the tun driver occurs because tun_put_user() used an on-stack virtio_net_hdr_v1_hash_tunnel structure without clearing the whole object. For a non-tunnel skb only the first virtio_net_hdr sized part is initialized, while the remaining num_buffers and hash or tunnel fields can contain stale stack data. A local user that can use a tun file descriptor and set TUNSETVNETHDRSZ to 24 can receive the full header in userspace and leak 14 bytes of kernel stack data per read. For the CVSS the PR:L is used because triggering requires a local user context and access to the tun interface or a passed tun file descriptor, but not full administrator privileges in all deployments. The issue is not network reachable by itself and does not provide a write primitive. Impact is information disclosure only, but repeated kernel stack leaks can assist exploitation of other bugs or bypass address randomization.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-52940  SKIP The Fixes patch not applied yet, so unlikely that actual: 288f30435132d2f9e7a29ec9b9745a4f9dc7fd37 YES  NO NO unknown 	MAYBE	SIMPLEFIX LEAK NETWORK KPANIC PACKET 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local user with access to a TUN fd can set `TUNSETVNETHDRSZ` to 24 and read packets to receive the uninitialized bytes.
* Local high-control kernel data-plane API: +1
  TUN is a user-controlled virtual networking interface where userspace can influence packet flow and vnet header behavior.
* Confidentiality impact plausible: +1
  The bug leaks 14 bytes of kernel stack data per non-tunnel packet read.
* Broad or commonly deployed subsystem: +1
  TUN/TAP is widely used by virtualization, containers, VPNs, and networking tools.
* Paranoid exposure bump: +1
  Repeated kernel stack disclosure can be useful for defeating hardening or supporting another exploit chain, even though this patch does not show memory corruption.

No generic memory corruption, UAF, OOB write, Dirty-Pipe-like primitive, or direct privilege escalation primitive is shown by the patch.

## 3. Reachability analysis

The bug is locally reachable through the TUN read path when vnet header size is set to 24 and a non-tunnel skb is returned to userspace. Creating or configuring a TUN device may require `CAP_NET_ADMIN` in some deployments, but TUN fds are often delegated to less privileged processes by VPNs, containers, emulators, or networking services. Therefore a full `CAP_NET_ADMIN` penalty is not appropriate for the paranoid score. The issue is not remotely network-triggerable by itself because the attacker needs local access to the TUN interface or a passed TUN fd.

## 4. Severity interpretation

This behaves like an actionable information disclosure bug, not a memory corruption bug. The realistic impact is kernel stack disclosure to a local user. The theoretical concern is that repeated stack leaks may expose pointers or other sensitive kernel values and assist exploitation of another vulnerability. There is no evidence of a write primitive, object lifetime corruption, or direct LPE path in this patch.

## 5. One-sentence report phrase

A local TUN user can leak 14 bytes of uninitialized kernel stack data per read because `tun_put_user()` copied a partially initialized virtio vnet header to userspace.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because it is a repeatable kernel stack information leak in a common networking interface, even though it does not by itself demonstrate memory corruption or direct privilege escalation.

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

Patch: tun: zero the whole vnet header in tun_put_user() [ Upstream
Commit: 5fd1fa5a4254bfdd70571c77f5e3bcb4e43738d5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5fd1fa5a4254bfdd70571c77f5e3bcb4e43738d5

Changed files:
  drivers/net/tun.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=5fd1fa5a4254bfdd70571c77f5e3bcb4e43738d5

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

The original Linux kernel CVE announcement for CVE-2026-52940 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52940

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