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CVE-2026-74480
net: bridge: stop fast-leave after deleting a port group
Published 2026-08-15 (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-2026-74480 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 | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. 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
br_multicast_leave_group() iterates mp->ports with pp = &p->next in its fast-leave path. After br_multicast_del_pg() removes p, continuing the loop advances pp through the deleted entry. If multicast-to-unicast was enabled, the bridge can hold multiple port groups for the same port and group with different source MAC addresses. Once multicast-to-unicast is disabled, br_port_group_equal() matches those entries by port only. A fast leave can then delete one entry and continue from its stale next pointer, leaving mp->ports pointing at a deleted port group. Fast leave only needs to remove one matching port group. Break after br_multicast_del_pg() so the loop stops before dereferencing the removed entry.
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/br_multicast.c
- What this is
- ETHERNET BRIDGEMAINTAINERS section matched on net/bridge/ · bridge@lists.linux.dev · Maintained
- Kernel module
- bridge
Check it on the machine
grep -wE 'CONFIG_BRIDGE|CONFIG_BRIDGE_IGMP_SNOOPING' /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 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_BRIDGE_IGMP_SNOOPING
- 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
- 2026-08-15 (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
- 0.58%, higher than 46.5% 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 |
|---|---|---|
| 4.11 | 5.10.265 | d6c32e2e25a9 |
| 4.11 | 5.15.216 | 482bcb85139a |
| 4.11 | 6.1.183 | 1a109cc9890d |
| 4.11 | 6.6.151 | 159ad90cb929 |
| 4.11 | 6.12.103 | 4695430e8132 |
| 4.11 | 6.18.44 | 0309ebbc5700 |
| 4.11 | 7.1.8 | 4c57056ca6aa |
| 4.11 | 7.2 | a39789f211b8 |
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
- The bug is reached when the bridge network stack processes attacker-supplied IGMP/MLD Leave packets received on a bridge port (br_handle_frame_finish -> br_multicast_rcv -> br_ip4/ip6_multicast_leave_group -> br_multicast_leave_group), requiring no local syscall access.
- AC:LAttack complexity, low
- Once a bridge has IGMP snooping, fast-leave, and leftover duplicate port groups (common on WiFi APs that used multicast-to-unicast), an attacker can reliably trigger the bug by sending IGMP/MLD joins then a Leave; no uncontrollable race or rare timing is required.
- PR:NPrivileges required, none
- No privileges or authentication are required on the target; any host on an attached bridge port can inject IGMP/MLD Leave packets that are processed and reach the vulnerable fast-leave deletion loop.
- UI:NUser interaction, none
- Exploitation requires only attacker-sent multicast control packets; no victim user action such as opening files, clicking links, or mounting filesystems is needed.
- S:UScope, unchanged
- Impact is kernel heap corruption and use-after-free within the bridge/multicast subsystem on the same host; it does not cross a VM/hypervisor or IOMMU security boundary by itself.
- C:HConfidentiality impact, high
- Deleting a port group then continuing the list walk dereferences freed net_bridge_port_group memory and can leave mp->ports dangling, giving a use-after-free read primitive exploitable for arbitrary kernel memory disclosure.
- I:HIntegrity impact, high
- Corrupting the mp->ports linked list via stale next pointers after br_multicast_del_pg() enables attacker-influenced kernel heap corruption that can be developed into arbitrary write and privilege escalation.
- A:HAvailability impact, high
- The use-after-free and corrupted multicast database pointers cause kernel oops/panics when subsequent bridge multicast forwarding or MDB operations traverse the broken port-group list, enabling repeatable denial of service.