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VulnerabilityAwaiting classification

CVE-2026-63921

HIGH · 8.8 CVSS v3.1 Published 2026-07-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
90/100
Remediation priority · Urgent
Zero-click 5 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In the Linux kernel's IPv6 VTI (Virtual Tunnel Interface), vti6_siocdevprivate() uses dev_net(dev) for tunnel collision lookup, but vti6_update() operates on t->net. After a tunnel is migrated via IFLA_NET_NS_FD, these differ, allowing an attacker in an unprivileged user namespace to manipulate SIOCCHGTUNNEL and insert the migrated tunnel into the wrong namespace's hash table. This enables cross-namespace hash collision and potential xfrm packet interception.

MitigationApply the kernel patch (or upgrade to a patched kernel version) that switches SIOCCHGTUNNEL to use t->net for lookups and adds ns_capable() verification for CAP_NET_ADMIN in the creation netns. This is a kernel-level fix requiring system update and reboot.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check kernel version for patch presence
    Run 'uname -r' to get the kernel version, then search kernel release notes or git logs for the CVE-2026-63921 fix commit (vti6_siocdevprivate namespace correction)
    Affected if Kernel version predates the patch that changes dev_net(dev) to t->net in vti6_siocdevprivate() for SIOCCHGTUNNEL operations
  2. Identify existing IP6 VTI tunnel interfaces
    Run 'ip -6 tunnel show' or check /sys/class/net/ for tunnel devices, then identify VTI6 type tunnels with 'ip -6 link show'
    Affected if Any VTI6 (ip6_vti) tunnel interfaces exist on the system
  3. Detect namespace-migrated VTI tunnels
    Run 'ip -6 link show' for each VTI tunnel and check for 'link netns' entries indicating migration via IFLA_NET_NS_FD, or inspect netlink attributes via 'ip -6 -detail link'
    Affected if Any VTI6 tunnel shows a link netns reference different from the initial creation namespace
  4. Verify tunnel hash table namespace alignment
    Use 'ip -6 tunnel show' in each network namespace (ip netns exec <ns> ip -6 tunnel show) and compare tunnel presence against the namespace where each tunnel's network device actually resides
    Affected if A VTI6 tunnel's hash table entry exists in a different namespace than the tunnel's actual device namespace
  5. Check for unprivileged user namespace VTI operations
    Review audit logs (auditd) or dmesg for SIOCCHGTUNNEL ioctl calls from user namespaces, or monitor with 'ip -6 monitor' for tunnel attribute changes
    Affected if SIOCCHGTUNNEL operations on VTI6 tunnels are performed from unprivileged user namespaces after namespace migration

User is affected if running a kernel older than the fix AND has VTI6 tunnels that were migrated across namespaces, allowing cross-namespace packet redirection via hash table collision attacks.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the kernel patch (or upgrade to a patched kernel version) that switches SIOCCHGTUNNEL to use t->net for lookups and adds ns_capable() verification for CAP_NET_ADMIN in the creation netns. This is a kernel-level fix requiring system update and reboot.

Recommended fix High confidence

Latest stable Linux kernel (6.6.y, 6.12.y, or later stable branch)

  1. Upgrade the Linux kernel to a version that includes the vti6 namespace fix (commit series fixing the dev_net vs t->net mismatch in vti6_siocdevprivate)
  2. Ensure the kernel is built with CONFIG_XFRM_VTI enabled
  3. After kernel upgrade, verify vti6 tunnels function correctly in containerized environments
  4. If using custom network namespace migration (IFLA_NET_NS_FD), test SIOCCHGTUNNEL operations post-migration to confirm correct namespace behavior
Caveat Kernel update may require system reboot; verify compatibility with custom kernel configurations and container runtimes before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA3.0 h
15.0 hours of engineering $2,560
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-63921 in production — separate from our analysis above.

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