[CVE-2026-99281][IMPORTANT] tun: Require CAP_NET_ADMIN to attach to an ownerless device

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

CVE: CVE-2026-99281
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: tun: Require CAP_NET_ADMIN to attach to an ownerless device
Commit: 2f92c0286cfd58bff9d6d54c01a771073d5eb775
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f92c0286cfd58bff9d6d54c01a771073d5eb775
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99281
Analysis date: Sat, 07 Nov 2026 22:24:56 -0400
ActionableScore: 8
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A local process with access to `/dev/net/tun` could bypass `CAP_NET_ADMIN` and attach to an ownerless persistent TUN/TAP interface, gaining packet queue access.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99281 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99281

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-2026-99281	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99281 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:H/I:H/A:H';CWE-862;CWE-863;CWE-284;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '7.8';DESCR 'This is an authorization logic flaw rather than memory corruption. A local process able to open /dev/net/tun could attach to an existing persistent TUN or TAP device when neither an owner nor a group was configured. The resulting queue access can allow observation and injection of packets carried by that interface and may disrupt traffic belonging to a VPN, virtual machine, container network, or another process. For the CVSS the PR:L value is used because exploitation requires local execution and permission to open the tun device node, but it does not require CAP_NET_ADMIN. The issue is not directly network reachable and also requires a suitable ownerless persistent device to exist. Base impact is limited to the affected interface, while the paranoid HHH interpretation applies when that interface forms a significant network isolation or trusted traffic boundary. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7.8 and the production ActionableScore returned 8 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class 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_TO_HIGH_BASED_ON_GUESSCVSS 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 4.6 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 8)	MAYBE	RACE LEAK NETWORK HARDWARE TEST PACKET INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO B4_CONFIRMED_AUTHORIZATION_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
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: 8.
* Conservative score: 8; no floor or cap applied.
* Raw paranoid score: 8.
* Paranoid score: 8; no unsupported H/H/H effects added.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 Concrete authorization bypass: with both owner and group invalid, `tun_not_capable()` returned false without evaluating `CAP_NET_ADMIN`, changing the intended DENY into ALLOW.
* +2 Practical protected-resource access: successful `tun_set_iff()` attachment grants queue access to an existing persistent TUN/TAP interface, enabling packet observation or injection.
* +1 Local unprivileged trigger: supplied evidence establishes a process that can open `/dev/net/tun` but lacks `CAP_NET_ADMIN`.
* +1 Practical exploit chain: open device node → name ownerless persistent interface → pass vulnerable permission test → attach queue.
* +1 Confidentiality plausible: queue attachment can expose packets carried by the interface.
* +1 Integrity plausible: queue attachment permits packet injection into that interface.
* No crash, memory corruption, arbitrary kernel access, LPE, credential theft, or host-wide availability impact is shown.
* No extra score is assigned for hypothetical VPN/VM/container severity or the legacy H/H/H CVSS interpretation.

3. Reachability analysis

The attacker need not be UID 0 or hold effective host-administrator powers, but must have local access to `/dev/net/tun` and know/select an existing persistent ownerless device. Creation and configuration of that device require a separate privileged actor; later vulnerable attachment does not.

The restored check is `ns_capable(dev_net(tun->dev)->user_ns, CAP_NET_ADMIN)`. The exploit bypasses this capability rather than relying on an attacker-held namespaced capability, so this is not classified as a namespace-scoped privilege trigger. No race, special hardware, or remote-packet reachability is required.

4. Primitive and severity reading

The primitive is a clean authorization decision bypass, not memory corruption: ownerless state suppresses the old capability test, and `tun_set_iff()` proceeds to `security_tun_dev_open()` and queue attachment. The fix permits attachment only through matching configured ownership or `CAP_NET_ADMIN`.

B4-PENDING is confirmed through the concrete path `tun_set_iff()` → defective owner/group test → unauthorized attachment to an existing persistent interface. Packet access is a meaningfully protected resource effect. This supports CLASS_A and mandatory Important review, while stronger claims such as LPE, arbitrary host access, or guaranteed high-impact VM/VPN compromise remain unproven. Auto-downgrade is unsafe because it would discard a demonstrated below-host-admin network-interface authorization bypass.

5. One-sentence report phrase

A local process with access to `/dev/net/tun` could bypass `CAP_NET_ADMIN` and attach to an ownerless persistent TUN/TAP interface, gaining packet queue access.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm affected deployments expose `/dev/net/tun` and contain ownerless persistent interfaces, then assess what protected traffic crosses those interfaces.

7. Call-site confidence

Call-site confidence: high

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UPSTREAM PATCH SUMMARY
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Patch: tun: Require CAP_NET_ADMIN to attach to an ownerless device
Commit: 2f92c0286cfd58bff9d6d54c01a771073d5eb775
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f92c0286cfd58bff9d6d54c01a771073d5eb775

Commit description:

Twenty years ago I made creating a tuntap device require CAP_NET_ADMIN
in commit ca6bb5d ("[NET]: Require CAP_NET_ADMIN to create tuntap
devices.")

The idea was to allow the permissions on /dev/net/tun to be relaxed to
0666 and thus allow owners of tun devices assigned with TUNSETOWNER to
actually open it and get at their devices.

What we missed at the time was that persistent devices with *no* owner
were already essentially world-consumable, and anyone who could open
the device node could also attach to those.

Even then, using persistent devices with no explicitly set owner — on
the (false) presumption that CAP_NET_ADMIN would be required to open
them — wasn't an egregious userspace bug. There are cases where it does
make sense to separate the creation/configuration of the netdevice, from
the process which shovels packets into it.

Since then, TUNSETGROUP was added a year later in commit 8c64462
("[NET]: Allow group ownership of TUN/TAP devices."). Its commit message
noted that if both uid and gid are set, both have to match (in itself a
bizarre choice) — but still not that with *neither* set, no permission
at all was required.

Then in 2012 commit c8d68e6 ("tuntap: multiqueue support") allowed
new queues to be attached to a device from the moment of its creation,
with no way for userspace to close the race between creation and a
subsequent TUNSET{OWNER,GROUP}.

In 2024, commit 3ca459e ("tun: fix group permission check") tried
to fix things to be saner: only require the user *or* the group to
match, in line with standard POSIX permissions, and require
CAP_NET_ADMIN if neither the user nor group authorised the access
(although it put the CAP_NET_ADMIN check first, and would wrongly have
reported the capability as having been used even when it was not).

That fix was reverted in commit a70c7b3 ("tun: revert fix group
permission check") due to allegedly causing a regression in an selinux
test suite which *expected* the attachment to succeed when authorised
by none of user, group, nor CAP_NET_ADMIN:
https://lore.kernel.org/netdev/CAFqZXNtkCBT4f+PwyVRmQGoT3p1eVa01fCG_aNtpt6dakXncUg@mail.gmail.com/

We should not break UAPI. But we don't have to consider an selinux test
case which deliberately exercises a security flaw to be UAPI worth
preserving either. To which end, I have fixed it in selinux-testsuite
commit b2f894f0016b ("tests/tun_tap: set an owner on the device for the
relabel tests").

So: fix the lack of CAP_NET_ADMIN check on devices which *haven't* been
assigned an owner or group, while leaving the weird tun ownership model
as it was (user *and* group have to match, if both are set). If someone
still wants to fix the latter (which wasn't actually the reason the
previous attempt was reverted), they can do it under separate cover.

Permit attaching to an existing device only if an owner and/or group was
set and the caller matches all that were set, or if the caller has
CAP_NET_ADMIN. The only behavioural change is that a device with no
owner or group now requires CAP_NET_ADMIN to attach — precisely the
loophole we want to close.

And put the CAP_NET_ADMIN check in the right place this time.

Fixes: ca6bb5d ("[NET]: Require CAP_NET_ADMIN to create tuntap devices.")
Reported-by: Andrew Lutomirski <luto@kernel.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/3c77f88acff5b3c2617b22d64faad15f6212619c.camel@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=2f92c0286cfd58bff9d6d54c01a771073d5eb775

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

The original Linux kernel CVE announcement for CVE-2026-99281 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99281

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: unknown
  Paranoid ActionableScore: 8

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).