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CVE-2026-63921

In the Linux kernel, the following vulnerability has been resolved:

ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().

After patch 1/2 in this series, vti6_update() unlinks and relinks the tunnel through t->net. vti6_siocdevprivate() still uses dev_net(dev) for the collision lookup. For a tunnel moved through IFLA_NET_NS_FD, dev_net(dev) is the new netns, not t->net.

SIOCCHGTUNNEL on a migrated tunnel then runs:

net = dev_net(dev) /* migrated netns / t = vti6_locate(net, &p1, false) / misses target in t->net / ... t = netdev_priv(dev) vti6_update(t, &p1, false) / mutates t->net's hash */

A caller in the migrated netns picks params that match a tunnel in the creation netns. The lookup in dev_net(dev) finds nothing. vti6_update() prepends the migrated tunnel at the head of the creation netns hash bucket for those params. Later lookups in the creation netns resolve to the migrated device. xfrm receive delivers the matched packets through a device the caller controls.

Reachable from an unprivileged user namespace (unshare --user --map-root-user --net). Cross tenant scope on container hosts.

Switch the SIOCCHGTUNNEL path on a non fallback device to use t->net for the lookup. The lookup now matches the netns vti6_update() operates on.

Also add ns_capable(self->net->user_ns, CAP_NET_ADMIN) before the lookup. The check at the top of the case is against dev_net(dev)->user_ns, which after migration is the attacker's netns. A caller there can pick params absent from self->net, the lookup returns NULL, t becomes self, and vti6_update() inserts the device into the creation netns hash. The new check requires CAP_NET_ADMIN in the creation netns user_ns too.

SIOCADDTUNNEL and SIOCCHGTUNNEL on the fallback device keep dev_net(dev), which equals init_net there.

  • Published: Jul 19, 2026
  • Updated: Jul 21, 2026
  • CVE: CVE-2026-63921
  • Severity: High
  • Exploit:
  • CISA KEV:

CVSS v3:

  • Severity: High
  • Score: 8.8
  • AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

No CWE or OWASP classifications available.

Frequently Asked Questions

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

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Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

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