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CVE-2026-74743
macvlan: inherit needed_headroom and needed_tailroom from lowerdev
Published 2026-08-26 (2 months 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-2026-74743 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-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.105-1 | Update the kernel and reboot. Fixed in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
macvlan devices inherit hard_header_len from lowerdev during macvlan_init(), but leave needed_headroom and needed_tailroom set to 0. When the underlying lowerdev requires extra headroom or tailroom for headers/trailers (e.g. macsec, ipsec, wireguard, tunnels, or veth with rx headroom), upper layers calculating packet headroom and tailroom fail to reserve sufficient space. This can result in reallocation overhead, skb headroom underflows, or KASAN slab-use-after-free crashes when dev_hard_header() / macvlan_hard_header() prepends header data or when lower devices append tailroom. Fix this by: 1. Inheriting needed_headroom and needed_tailroom from lowerdev in macvlan_init(). 2. Propagating needed_headroom and needed_tailroom updates to attached macvlans in macvlan_device_event() when receiving NETDEV_FEAT_CHANGE events.
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/macvlan.c
- What this is
- NETWORKING DRIVERSMAINTAINERS section matched on drivers/net/ · netdev@vger.kernel.org · Maintained
- Kernel module
- macvlan
Check it on the machine
grep -w CONFIG_MACVLAN /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 macvlanPrints 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_MACVLAN
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
- 2026-08-26 (2 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.54%, higher than 44.0% 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 |
|---|---|---|
| 2.6.23 | 5.10.271 | c2ef3f134b06 |
| 2.6.23 | 5.15.222 | 7b0eb3e0f363 |
| 2.6.23 | 6.1.184 | 8cd90e850e43 |
| 2.6.23 | 6.6.153 | 28afc87bd8da |
| 2.6.23 | 6.12.105 | 8f6a05dbac05 |
| 2.6.23 | 6.18.46 | 96fa90b74385 |
| 2.6.23 | 7.1.10 | bc9a00fb78e3 |
| 2.6.23 | 7.2 | cef51860becd |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | 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:NAttack vector, network
- The bug fires on the macvlan transmit path (ip_finish_output2/neigh_resolve_output/macvlan_hard_header) when kernel-generated replies or forwarded traffic egress a macvlan netdev; remote peers trigger this by sending TCP/UDP/ICMP to an address on a pre-configured macvlan interface.
- AC:LAttack complexity, low
- Once macvlan is stacked on a lowerdev with nonzero needed_headroom (WireGuard, MACsec, IPsec, tunnels, veth RX headroom), ordinary packets reliably hit under-reserved skb headroom/tailroom during dev_hard_header() without races or uncontrollable timing.
- PR:NPrivileges required, none
- A remote attacker needs no local account or capabilities; sending routable traffic to a macvlan address suffices. Administrative macvlan setup is environmental (like other netdev CVEs), not an attacker privilege requirement.
- UI:NUser interaction, none
- Exploitation requires only normal network delivery to a reachable macvlan address; no victim must open files, mount filesystems, or perform any interactive action beyond existing connectivity.
- S:UScope, unchanged
- Impact is kernel skb heap corruption, slab-use-after-free, or panic within the host network stack; it does not cross VM, IOMMU, or container security boundaries into a separate authority.
- C:HConfidentiality impact, high
- The fix commit documents KASAN slab-use-after-free from skb headroom underflow during header prepending; this class of kernel heap corruption is exploitable for arbitrary memory disclosure beyond a bounded leak.
- I:HIntegrity impact, high
- Insufficient headroom causes skb_push underflows and slab UAF during pskb_expand_head/reallocation, providing kernel heap write primitives that can be leveraged for control-flow hijacking and privilege escalation.
- A:HAvailability impact, high
- The failure mode includes skb_under_panic (kernel BUG/panic) and KASAN-detected slab UAF crashes, causing immediate host kernel denial of service on affected transmit paths.