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CVE-2026-63921
ip6: vti: Use ip6_tnl.net in vti6_siocdevprivate().
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/ipv6. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63921 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/ipv6/ip6_vti.c
- What this is
- NETWORKING [IPSEC]MAINTAINERS section matched on net/ipv6/ip6_vti.c · netdev@vger.kernel.org · Maintained
- Kernel module
- ip6_vti
Check it on the machine
grep -w CONFIG_IPV6_VTI /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w ip6_vtiPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switch
- CONFIG_IPV6_VTI
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 8.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.18%, higher than 6.7% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv6
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 3.15 | 5.10.259 | 94ff740a7f9e |
| 3.15 | 5.15.210 | df42ac708acc |
| 3.15 | 6.1.176 | 44d2ff7d2178 |
| 3.15 | 6.6.143 | 1acfb7d9c6fc |
| 3.15 | 6.12.93 | d2236348414b |
| 3.15 | 6.18.35 | 853f6ea482df |
| 3.15 | 7.0.12 | 596f6354c96a |
| 3.15 | 7.1 | 8b484efd5cb4 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- Exploitation requires a local ioctl(SIOCCHGTUNNEL) on a migrated vti6 netdev via socket/dev_ioctl; the bug is in vti6_siocdevprivate(), not in remote packet reception.
- AC:LAttack complexity, low
- The attacker fully controls namespace creation, tunnel migration (IFLA_NET_NS_FD), and ioctl parameters to collide with a victim tunnel; the commit documents a deterministic, reproducible sequence with no uncontrollable race.
- PR:LPrivileges required, low
- Only CAP_NET_ADMIN in the migrated (attacker) network namespace is required, obtainable via unprivileged user namespaces (unshare --user --map-root-user --net); the missing check against the creation namespace's user_ns is the core flaw.
- UI:NUser interaction, none
- No victim interaction is needed; the attacker performs all setup steps (create/migrate tunnel, issue SIOCCHGTUNNEL) autonomously.
- S:CScope, changed
- The attack hijacks IPsec/XFRM traffic destined for tunnels in a different tenant's network namespace on container hosts, crossing container/tenant security boundaries rather than staying within the attacker's own scope.
- C:HConfidentiality impact, high
- After hash poisoning, xfrm receive delivers matched/decapsulated packets to the attacker-controlled netdev, enabling interception of victim VPN/IPsec traffic including potentially sensitive plaintext after decryption.
- I:HIntegrity impact, high
- The attacker gains control over which netdev receives victim tunnel traffic and can read, drop, or forward/modify decapsulated packets, constituting unauthorized manipulation of another tenant's network data path.
- A:HAvailability impact, high
- Hijacking the creation-namespace hash bucket causes the legitimate victim tunnel to stop receiving matched traffic, producing denial of service to the victim's IPsec connectivity.