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tun: Require CAP_NET_ADMIN to attach to an ownerless device

Assessment

Severity
Important
CVSS base score
7.1
Impact
Not stated
Exploitation detected
Unknown: not stated by MSRC
Exploitability
Not assessed
Publicly disclosed
Unknown: not stated by MSRC
EPSS · 30-day forecast
No EPSS score collected
Customer action
Required: follow the MSRC remediation guidance
More details
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Public detection template
No Nuclei template is listed in the current collected catalog
Component
Mariner
Weakness
—
Issued by
Linux
Published
9 Nov 2026 · November 2026
Last revised
9 Nov 2026
CVSS 3.1 base metricsRed raises the risk · green lowers it
Attack vector
LocalAV:L
Attack complexity
LowAC:L
Privileges required
LowPR:L
User interaction
NoneUI:N
Scope
UnchangedS:U
Confidentiality
HighC:H
Integrity
HighI:H
Availability
NoneA:N

Updates that fix it

0 KBs

No KB listed

No KB update is listed in the collected MSRC record. Check the MSRC advisory for product releases and other remedies.

Affected products

Open Source Software
1 affected product
  • azl3 kernel 6.6.161.1-1 on Azure Linux 3.0
Page history

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No retained field changes for this selection.

Explore global source statements, affected packages and advisory history for CVE-2026-99281. The catalog has a separate collection scope from Microsoft release records.

Collected exploitation and prediction evidence

Risk source coverage

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

cve-program · CVE-2026-99281

Published

tun: Require CAP_NET_ADMIN to attach to an ownerless device

In the Linux kernel, the following vulnerability has been resolved: tun: Require CAP_NET_ADMIN to attach to an ownerless device Twenty years ago I made creating a tuntap device require CAP_NET_ADMIN in commit ca6bb5d7ab22 ("[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 8c644623fe7e ("[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 c8d68e6be1c3 ("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 3ca459eaba1bf ("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 a70c7b3cbc06 ("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+[email protected]/ 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.

Source published
7 Nov 2026 14:08:47 UTC
Source modified
10 Nov 2026 16:57:40 UTC
Observed here
10 Nov 2026 18:21:22 UTC
Source terms
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CVSS 3.1 · 7.8 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

ChangeIntel

An IT change radar: releases, security, known issues, retirements, documentation changes, and service status from public sources. Every item links to supporting evidence; dates and statuses can change after they are read.

Sources read 10 Nov 18:48 UTC · 261 of 275 readable · 260 fully read · documentation 469/469 current