CVE-2026-63921
Is CVE-2026-63921 real, exploitable, or a false positive? Here's the community verdict.
signals
public sources
High CVSS base score, but low real-world exploitation probability (EPSS). Likely less urgent than the score implies.
baseline read
auto · not a community verdict
Real, but low real-world risk
A genuine vulnerability on paper, but EPSS shows little real-world exploitation — the base score may overstate urgency. This is not the same as a false positive.
Based on CVSS · FIRST EPSS
Confirm or dispute →CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
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.
References
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Field notes & remediation
Verdicts are the quick signal. Notes are the evidence and fixes behind them.
- 0
This scores high on paper (CVSS 8.8) but the real-world signal is low. It is not in CISA KEV and no public exploit was found, and near-term exploitation looks unlikely (EPSS about 0%). Verify your own exposure before treating it as urgent. It needs local access, so it reads more as a privilege-escalation or post-access risk than a remote one. Not an emergency for most teams, but patch it in your normal cycle and check whether the affected component is actually exposed in your setup.