CVE-2026-63921
Local user namespace bug in Linux ip6 vti allows tunnel hijack across netns
Is CVE-2026-63921 being exploited?
Not confirmed. CVE-2026-63921 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.18% probability of exploitation in the next 30 days.
How severe is CVE-2026-63921?
CVE-2026-63921 is rated High with a CVSS score of 8.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-63921?
Yes. A fix has been recorded for CVE-2026-63921. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-63921 affect?
CVE-2026-63921 affects Linux kernel (IPv6 vti/vti6 tunnel handling), Container hosts using user namespaces. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-63921?
Apply vendor kernel updates and reboot hosts; restrict unprivileged user namespaces until patched.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Unprivileged userns can hijack/migrate vti6 tunnels, causing cross-namespace packet delivery and local privilege escalation on container hosts.
Immediate action required
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < 94ff740a7f9ef5c010784a325dca00cbf228f941
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < df42ac708acc3399bbb6dc5ca16e0540adda7bbf
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < 44d2ff7d2178503b93151140a45dfa2ad49c9906
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < 1acfb7d9c6fc7e209ed7789392697e97e03edd33
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < d2236348414bdd6558385f35aa7fdc9bf5634011
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < 853f6ea482dfcd3404bbef458ab4d68364eed838
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < 596f6354c96a891e58c04a09cbfb7b0d1ec00dab
- affected>= 61220ab349485d911083d0b7990ccd3db6c63297 and < 8b484efd5cb4eeef9021a661e198edc5349dacf6
- affected3.15
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.