Vulnerability · Published Jul 1, 2026 · Assigned by runZero

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

High 7.6

In plain English

A high-severity buffer overflow in FatFs from ChaN. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).

Severity (CVSS 3.1)7.6High

Exploit odds (30 days)0.3%top 74.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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.

Affected products (as reported)

Vendor	Product	Versions
ChaN	FatFs	0 – through R0.16

Weakness: CWE-120 · Vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

References

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