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CVE-2026-93203
batman-adv: bla: avoid CRC corruption due to parallel claim add
Published 2026-09-17 (31 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-2026-93203 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
batadv_bla_add_claim() is used to add claims and modify the backbone of claims for CLAIM frames from remote backbones and local packets. When it handles a claim, it needs to either * add the new claim's CRC to the backbone CRC * remove the already existing claim's CRC from the old backbone and add it to the new backbone But when the "new" claim code was running in parallel to the "change backbone" code, it can happen that the CRC was invalid because the backbone_gw of the claim was changed twice in the "new" claim code path: * CPU0 creates the claim for gateway A and publishes it in the claim hash. The crc16 of the address has not yet been added to A's crc at this point. * CPU1 processes a claim frame of gateway B for the same client, finds the just published claim, and performs the ownership change: it switches the pointer to B, removes the crc16 from A's crc - which never contained it - and adds it to B's crc. * CPU0 continues behind the creation branch, unconditionally switches the pointer back to A without compensating B's crc (its remove_crc is false for the creation path), and finally adds the crc16 to A's crc The CRC is then wrong for both: * claim belongs to A: but CRC is not part of backbone A's CRC * claim doesn't belong to B: CRC is still part of backbone B's CRC This wrong CRC is never recomputated from the stored claims. For local backbone claims, this can also not recovered using syncs. To avoid this, split the functionality in clear separate parts: * new claim which always adds claim CRC to the backbone CRC (but never changes the already set backbone_gw of the claim back) * update of existing claim which automatically changes the backbone_gw entry and only updates both backbone CRCs when there was an actual change
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/bridge_loop_avoidance.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 -wE 'CONFIG_BATMAN_ADV|CONFIG_BATMAN_ADV_BLA' /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 switches
- CONFIG_BATMAN_ADV, CONFIG_BATMAN_ADV_BLA
- 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
- 2026-09-17 (31 days ago)source
- CVSS
- 7.1 high
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:Hsource recompute it - Attack vector
- adjacent network (AV:A)source
- EPSS
- 0.33%, higher than 24.2% 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 |
|---|---|---|
| 3.5 | 5.10.270 | 3d4a9ae2c69c |
| 3.5 | 5.15.221 | ad982d925f00 |
| 3.5 | 6.1.188 | d5b0e7574da5 |
| 3.5 | 6.6.157 | a3e07db74f22 |
| 3.5 | 6.12.110 | 1d390db003cb |
| 3.5 | 6.18.52 | 8bcfa58c4277 |
| 3.5 | 7.2.6 | e54602d9e38d |
| 3.5 | 7.3 | 08645ab95768 |
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
- BATADV_CLAIM_TYPE_CLAIM ARP frames (ETH_P_ARP with ARP target hardware magic ff:43:05) reach batadv_bla_add_claim via batadv_interface_tx -> batadv_bla_tx -> batadv_bla_process_claim -> batadv_handle_claim. Those frames stay on the bridged bat0 backbone LAN and are not IP-routable, so the attacker must share that L2 segment.
- AC:LAttack complexity, low
- The attacker sends two concurrent CLAIM frames for the same client MAC with different ARP hardware-source backbone addresses, racing CPU0's batadv_hash_add of a new claim against CPU1's ownership-change path in batadv_bla_add_claim. Both sides of the race are attacker packets; CONFIG_BATMAN_ADV_BLA and bridge_loop_avoidance default on.
- PR:NPrivileges required, none
- batadv_check_claim_group() accepts a CLAIM once bla_dst->magic is ff:43:05 and bla_dst->group matches the on-wire 16-bit group id (crc16 of the primary hardif MAC, carried in every ANNOUNCE/CLAIM). No victim account, capability, or authentication is required.
- UI:NUser interaction, none
- After the mesh is BATADV_MESH_ACTIVE, the attacker triggers the race solely by injecting the two CLAIM ARP frames onto bat0; no victim mount, click, or other user action is required.
- S:UScope, unchanged
- The race only corrupts backbone_gw->crc and claim->backbone_gw inside the victim kernel's batman-adv BLA tables; it does not cross a VM, hypervisor, or IOMMU boundary.
- C:NConfidentiality impact, none
- The bug only XORs crc16(client MAC) into the wrong backbone_gw->crc and leaves claim->backbone_gw pointing at gateway A; there is no OOB read, UAF, or other kernel-memory disclosure.
- I:LIntegrity impact, low
- After the race, A's CRC lacks the claim while B's CRC still contains it, so a later ANNOUNCE mismatch makes batadv_bla_send_request() flush B's claims and can mis-assign which backbone forwards a client. That is limited BLA forwarding-state corruption, not an arbitrary kernel write.
- A:HAvailability impact, high
- The corrupted backbone CRC is never rebuilt from stored claims, so batadv_handle_announce() never returns that gateway to BATADV_BLA_BACKBONE_GW_SYNCED and bla.num_requests stays non-zero. batadv_bla_rx/tx then drop all multicast/broadcast, persistently denying ARP/DHCP/ND on the mesh/LAN.