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CVE-2026-74625
netfilter: bridge: release template ct on non-IP path
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem net/bridge. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74625 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 | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. 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
A bridge nftables ct zone set rule can attach a conntrack template to an skb before nf_ct_bridge_pre() sees it. For non-IPv4 and non-IPv6 EtherTypes, nf_ct_bridge_pre() currently overwrites skb->_nfct with IP_CT_UNTRACKED without releasing the existing template reference. That makes the per-cpu template, and any temporary templates allocated for concurrent use, unreachable and leaks memory until the host runs out of slab. Reset the skb conntrack state before marking the frame untracked so the existing template reference is dropped on the non-IP path.
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/bridge/netfilter/nf_conntrack_bridge.c
- What this is
- NETFILTERMAINTAINERS section matched on net/*/netfilter/ · netfilter-devel@vger.kernel.org · Maintained
- Kernel module
- nf_conntrack_bridge
Check it on the machine
grep -wE 'CONFIG_BRIDGE|CONFIG_NETFILTER|CONFIG_NF_CONNTRACK_BRIDGE' /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 nf_conntrack_bridgePrints 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 switches
- CONFIG_BRIDGE, CONFIG_NETFILTER, CONFIG_NF_CONNTRACK_BRIDGE
- Reads as
- all on means the file is compiled; any one off means it is not.
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
- 2026-08-22 (2 months ago)source
- CVSS
- 7.5 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.50%, higher than 41.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/bridge
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.3 | 5.10.265 | fc90df375406 |
| 5.3 | 5.15.216 | daa6e070f8e1 |
| 5.3 | 6.1.183 | bd7b16494dac |
| 5.3 | 6.6.152 | 6ea88401e10e |
| 5.3 | 6.12.104 | 46d559f00b1a |
| 5.3 | 6.18.45 | c58d34fe8b7e |
| 5.3 | 7.1.9 | 7cff440d7026 |
| 5.3 | 7.2 | d45cc8020d7c |
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 |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | 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
- Non-IP L2 frames (ARP/LLDP) injected into a Linux bridge reach NF_BR_PRE_ROUTING via br_handle_frame→nf_hook_bridge_pre; on overlay/container hosts (VXLAN/GRE bridges in Docker/k8s/OpenStack) remote peers can deliver those frames across routed networks to the victim bridge port.
- AC:LAttack complexity, low
- Once a bridge nftables ct zone set rule runs before NF_IP_PRI_CONNTRACK (-200), every non-IPv4/IPv6 EtherType frame deterministically leaks template references; the attacker controls rule priority, zone value, and can flood ARP/LLDP to accelerate the leak without races.
- PR:NPrivileges required, none
- On bridge firewalls/routers with deployed nftables ct zone marking, exploitation needs only unauthenticated L2 packet delivery—no host UID, capability, or namespace privilege; CAP_NET_ADMIN (obtainable via user namespaces) is an alternate Low path for self-setup but not required on pre-configured appliances.
- UI:NUser interaction, none
- Bridge netfilter processes received frames automatically in softirq; no victim mount, click, or administrative action is required beyond normal bridge forwarding of attacker-supplied non-IP traffic.
- S:UScope, unchanged
- Impact is unbounded kernel slab consumption from leaked nf_conn template objects within the host kernel; it does not cross VM, container, IOMMU, or hypervisor security boundaries.
- C:NConfidentiality impact, none
- The bug is a missing nf_conntrack_put()—a reference-count leak—not a use-after-free, out-of-bounds read, or disclosure primitive; orphaned templates are unreachable and their contents are never returned to the attacker.
- I:NIntegrity impact, none
- Only template reference counts are leaked via skb->_nfct overwrite; no heap corruption, type confusion, or attacker-controlled writes occur, so integrity cannot be compromised beyond availability loss from memory pressure.
- A:HAvailability impact, high
- Each leaked per-CPU and temporary conntrack template is never freed; sustained non-IP frame flooding can exhaust kernel slab until OOM, causing severe host unavailability or panic on memory-allocation failure paths.