CVE-2026-93203 batman-adv: bla: avoid CRC corruption due to parallel claim add
A race condition in the Linux kernel's batman-adv module allows parallel claim additions to corrupt CRC values, leading to incorrect network state.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-93203?
CVE-2026-93203 is a high-severity vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: bla: avoid CRC corruption due to parallel claim add
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
Which versions are affected by CVE-2026-93203?
How do I fix CVE-2026-93203?
Apply the vendor's patch.
Is CVE-2026-93203 being exploited?
There is no known exploitation of CVE-2026-93203 in the wild and no public exploit code.
Sources
Also known as AZL-102131, BELL-CVE-2026-93203, DEBIAN-CVE-2026-93203, GHSA-jrfx-xv62-wfwq, UBUNTU-CVE-2026-93203.