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CVE-2026-89951
batman-adv: fix stale receive device on merged fragments
Published 2026-09-16 (33 days ago) by the Linux kernel CNA. Subsystem net/batman-adv. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89951 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
Fragment reassembly reuses the skb from the highest-numbered buffered fragment as the merged packet. When that fragment was received on a hard interface which is deleted before the chain completes, the merged skb can re-enter the receive path with a stale skb->dev and skb_iif. batadv_batman_skb_recv() passes such merged packets through the normal receive handlers again. DAT and bridge loop avoidance both derive the ARP header length from skb->dev, so they can dereference the freed net_device before the packet reaches the local mesh interface. Refresh the receive device metadata from the current receive device before running the packet handlers. This keeps internally reinjected merged fragments consistent with the normal receive path after hard interface teardown.
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/batman-adv/main.c
- What this is
- BATMAN ADVANCEDMAINTAINERS section matched on net/batman-adv/ · b.a.t.m.a.n@lists.open-mesh.org · Maintained
- Kernel module
- batman_adv
Check it on the machine
grep -w CONFIG_BATMAN_ADV /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 batman_advPrints 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, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.
- bookworm, trixie, forky, sid, bookworm · linux-6.12Mnot built on amd64/cloud-amd64, arm64/cloud-arm64
- M
- a module where it is built at all; some flavours do not build it
- Build switch
- CONFIG_BATMAN_ADV
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
- 2026-09-16 (33 days ago)source
- CVSS
- 8.8 high
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- adjacent network (AV:A)source
- EPSS
- 0.34%, higher than 26.4% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/batman-adv
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 3.13 | 5.10.270 | 63d86250fa7c |
| 3.13 | 5.15.221 | f830c2dfc143 |
| 3.13 | 6.1.188 | f4b4ae763b5d |
| 3.13 | 6.6.157 | 6df64825f8b1 |
| 3.13 | 6.12.110 | a2c272da99c2 |
| 3.13 | 6.18.51 | e91d2cc7441d |
| 3.13 | 7.2.5 | 2dffc8c44b6e |
| 3.13 | 7.3 | ad46c907d7d9 |
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:AAttack vector, adjacent network
- Merged BATADV_UNICAST_FRAG packets re-enter batadv_batman_skb_recv() after ETH_P_BATMAN receive on a mesh hard interface; an attacker must inject those frames on the same L2 mesh segment (WiFi IBSS/802.11s, Ethernet backbone, or community-mesh overlay), not via arbitrary Internet routing.
- AC:LAttack complexity, low
- The attacker fully controls fragment headers, inner ARP payload, and both send steps (buffer the highest-numbered fragment, then complete the chain on a remaining active hard interface). In-window fragments are not purged on NETDEV_UNREGISTER, so the stale skb->dev UAF needs no uninfluenceable heap layout.
- PR:NPrivileges required, none
- batadv_batman_skb_recv() is an unauthenticated packet_type handler; any adjacent mesh peer can send BATADV_UNICAST_FRAG frames. Originator presence is obtained by sending OGMs or spoofing a MAC learned from the mesh, with no local account or CAP_NET_ADMIN on the victim.
- UI:NUser interaction, none
- Exploitation requires only attacker-sent mesh fragments; the kernel buffers, merges, and runs DAT ARP parsing in NET_RX_SOFTIRQ without any victim click, mount, or extra administrative action at exploit time.
- S:UScope, unchanged
- The use-after-free is of a kernel net_device in batman-adv on the victim host and does not cross a VM/host, IOMMU, or sandbox security authority.
- C:HConfidentiality impact, high
- After hard-interface unregister, batadv_arp_get_type() calls arp_hdr_len(skb->dev) on the merged skb and reads a freed net_device. A kernel-object UAF is high confidentiality impact because reclaim/spray of that object can disclose heap contents.
- I:HIntegrity impact, high
- This is a use-after-free of struct net_device on the fragment-merge receive path. Heap spraying the freed object yields attacker-controlled device fields consumed during packet parsing, a memory-corruption primitive suitable for integrity compromise or control-flow hijacking.
- A:HAvailability impact, high
- Dereferencing a freed net_device from NET_RX_SOFTIRQ can oops, panic, or hang when the object is poisoned or unmapped, denying mesh and host availability even without a full exploit.