# 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.

- **Severity:** High 7.1 (`CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102131, BELL-CVE-2026-93203, DEBIAN-CVE-2026-93203, GHSA-jrfx-xv62-wfwq, UBUNTU-CVE-2026-93203

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-93203 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-93203 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-93203/ (Markdown: https://insights.sbomify.com/CVE-2026-93203/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-93203
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-93203
- OSV: https://osv.dev/vulnerability/CVE-2026-93203

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-93203/
Summaries are generated and may contain mistakes; the official description above is the record.