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CVE-2026-81000

net: tun: bound receive headroom

7.8 EPSS 0.4% AV:L

Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem drivers/net. Debian urgency: not yet assigned.

What to do, per Debian release

Debian's own status for CVE-2026-81000 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	fixedDLA-4841-1 · 2026-10-17 · text	6.1.190-1	Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts.
Debian 13trixie	fixedDSA-6528-1 · 2026-09-29 · text	6.12.111-1	Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts.
Debian 14forky	fixed	7.2.6-1	Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts.
Debian unstablesid	fixed	7.2.6-1	Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts.
Debian 12bookworm · linux-6.12	fixedDLA-4817-1 · 2026-10-04 · text	6.12.111-1~deb12u1	Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

tun_get_user() uses tun->align both as skb headroom and when choosing how much packet data to keep linear. OVS can propagate an oversized headroom request from another port to TUN or TAP. When align is larger than the usable space in a one-page skb head, SKB_MAX_HEAD(align) underflows and the result becomes negative when stored in good_linear. That value later wraps when assigned to the size_t linear variable, and tun_alloc_skb() can place skb->data outside the allocated head. Bound the headroom stored by TUN to the one-page skb-head budget and the largest non-sentinel 16-bit skb header offset. Leave one linear byte for raw TUN and a complete Ethernet header for TAP, including NET_IP_ALIGN. Also pull the raw-TUN protocol byte and the TAP Ethernet header before accessing them, so these checks remain safe for nonlinear skbs supplied by other allocation paths.

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.

drivers/net/tun.c

What this is: TUN/TAP DRIVERMAINTAINERS section matched on drivers/net/tun* · Maintained
Kernel module: tun

Check it on the machine

grep -w CONFIG_TUN /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 tun

Prints 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, sid, bookworm · linux-6.12m

m: a loadable module on every flavour - check lsmod, can be blacklisted

Build switch: CONFIG_TUN

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

Debian tracker CVE record NVD kernel CNA record

Signals

Published: 2026-09-11 (37 days ago)source
CVSS: 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it
Attack vector: local (AV:L)source
EPSS: 0.36%, higher than 27.9% of all CVEssource
CISA KEV: not listedthe KEV catalogue
Debian urgency: not yet assignedsource
Subsystem: drivers/net

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
4.6	5.10.270	ad715e713610
4.6	5.15.221	708e87937de9
4.6	6.1.188	18ef24cdb2eb
4.6	6.6.157	010eee265d6b
4.6	6.12.109	379d85c7f25f
4.6	6.18.50	e098d9cc8859
4.6	7.2.4	0ada54ea63e4
4.6	7.3	447c9303942c

Debian advisories

Advisory	Date	Shipped to	Debian
DLA-4841-1	2026-10-17	bookworm 6.1.190-1	tracker
DLA-4817-1	2026-10-04	bookworm 6.12.111-1~deb12u1	tracker
DSA-6528-1	2026-09-29	trixie 6.12.111-1	tracker

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: tun_get_user() runs on write/sendmsg to /dev/net/tun after ndo_set_rx_headroom stores an oversized tun->align. That is a local tun/tap injection path, not parsing of frames received from a remote peer.
AC:LAttack complexity, low: The attacker fully controls both sides: inflate rx headroom via netkit IFLA_NETKIT_HEADROOM or the OVS/veth/VXLAN and bridge/veth/GRETAP loops, then write a TUN frame. No race or uninfluenced victim state is required.
PR:LPrivileges required, low: TUNSETIFF uses ns_capable(net->user_ns, CAP_NET_ADMIN) and OVS vport commands use GENL_UNS_ADMIN_PERM, so an unprivileged user with a user+net namespace can create the topology and trigger the write.
UI:NUser interaction, none: The attacker opens /dev/net/tun, configures local net devices, and writes the frame; no separate victim action is required.
S:UScope, unchanged: The out-of-bounds access occurs in the same kernel that handles the local TUN write and yields privilege escalation within that kernel, not a VM/IOMMU boundary crossing.
C:HConfidentiality impact, high: SKB_MAX_HEAD underflow wraps the linear length so tun_alloc_skb()/skb_reserve() can place skb->data outside the allocated head, turning later header reads and packet processing into out-of-bounds kernel reads.
I:HIntegrity impact, high: After skb->data is moved outside the allocated head, skb_copy_datagram_from_iter copies attacker-controlled frame bytes relative to that pointer, an out-of-bounds write usable for kernel corruption and control-flow hijacking.
A:HAvailability impact, high: The misplaced skb leads to a kernel oops, BUG() in eth_type_trans/__skb_pull on a nonlinear TAP skb, or panic when the out-of-bounds access is hit.