# CVE-2026-6686: FatFs Use of Uninitialized Clusters After Seek Past EOF

> Attackers with physical access can read uninitialized memory from newly allocated clusters when extending files past EOF in FatFs.

- **Severity:** Medium 4.6 (`CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 1 Jul 2026
- **Updated:** 1 Jul 2026
- **Also known as:** GHSA-h94p-vjqx-v7rr

## What is CVE-2026-6686?

CVE-2026-6686 is a medium-severity vulnerability in ChaN FatFs, published on 1 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

FatFs R0.16 and earlier contains an uninitialized cluster exposure when f_lseek() extends files beyond EOF without zero-filling newly allocated clusters. This maps to CWE-908 (Use of Uninitialized Resource). Estimated CVSS v3.1 vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N (4.6, Medium). The estimated CISA SSVC vectors are Exploitation: PoC, Technical Impact: Partial.

Weakness: CWE-908 (Use of Uninitialized Resource)

## Which versions are affected by CVE-2026-6686?

It affects 1 product.


Products: Elm-Chan Fatfs.

## How do I fix CVE-2026-6686?

No fix has been published for CVE-2026-6686 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-6686 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6686 exists.

## Other vulnerabilities in ChaN FatFs

- [CVE-2026-6682](https://insights.sbomify.com/CVE-2026-6682/): FatFs Integer Overflow in FAT32 Volume Mount (high 7.6)
- [CVE-2026-6687](https://insights.sbomify.com/CVE-2026-6687/): FatFs Stack Buffer Overflow via Uncapped exFAT Label Length (high 7.6)
- [CVE-2026-6688](https://insights.sbomify.com/CVE-2026-6688/): FatFs Buffer Overflow via Unbounded LFN Filename Copy (high 7.6)
- [CVE-2026-6683](https://insights.sbomify.com/CVE-2026-6683/): FatFs Divide-by-Zero in exFAT Sync (medium 4.6)
- [CVE-2026-6684](https://insights.sbomify.com/CVE-2026-6684/): FatFs Infinite Loop in GPT Partition Scan (medium 4.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6686 and fix it if it is affected.

What it is: CVE-2026-6686 is a medium-severity vulnerability in ChaN FatFs, published on 1 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6686 exists.

Affected product: ChaN FatFs. 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-2026-6686/ (Markdown: https://insights.sbomify.com/CVE-2026-6686/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6686
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6686
- OSV: https://osv.dev/vulnerability/CVE-2026-6686

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6686/
Summaries are generated and may contain mistakes; the official description above is the record.