Linux Kernel: security vulnerability (CVE-2026-99281)
Published Last modified Sources: CVE.org
What is affected, what to do
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.
Update Linux Kernel to version 6.18.61, Linux Kernel to version 7.3.2, Linux Kernel to version 7.4-rc1.
- Affected
- Linux Kernel
lt 2.6.18..6.18.61 - Linux Kernel
lt 6.19..7.3.2 - Linux Kernel
lt 7.4..7.4-rc1
- Linux Kernel
- Fixed in
- Linux Kernel
6.18.61 - Linux Kernel
7.3.2 - Linux Kernel
7.4-rc1
- Linux Kernel
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CVSS v3
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Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 30.00 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
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+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.
Source: cvelistv5, NVD (NIST)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Linux | Linux |
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Vendors and products
References & sources
- git.kernel.orgstable/c/2f92c0286cfd58bff9d6d54c01a771073d5eb775
- git.kernel.orgstable/c/e53d1cf05da8ea58a5743b7ce5894fb51b0b9700
- git.kernel.orgstable/c/ffdf678dd16610e3f006dac904bd37c09684673c
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
ReceivedLinux
Description added or changed
Show 5 original changes
Individual changes of this event as published by NVD Type Action Before After Description added —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+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.Affected versions (publisher) added —New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/99xxx/CVE-2026-99281.json">CVE-2026-99281</a>Reference added —https://git.kernel.org/stable/c/2f92c0286cfd58bff9d6d54c01a771073d5eb775Reference added —https://git.kernel.org/stable/c/e53d1cf05da8ea58a5743b7ce5894fb51b0b9700Reference added —https://git.kernel.org/stable/c/ffdf678dd16610e3f006dac904bd37c09684673cSource identifier: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
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