Vulnerability 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.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Elm-Chan | Fatfs | <= r0.16 |
Related Weaknesses (CWE)
References
- https://elm-chan.org/fsw/ff/Product
- https://github.com/runZeroInc/vulns-2026-fatfs-chanceThird Party AdvisoryExploit
- https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/Third Party Advisory
- https://www.runzero.com/blog/fatfs-bugs/Third Party AdvisoryExploit
- https://github.com/runZeroInc/vulns-2026-fatfs-chanceThird Party AdvisoryExploit
FAQ
What is CVE-2026-6686?
CVE-2026-6686 is a vulnerability with a CVSS score of 4.6 (MEDIUM). 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 Res...
How severe is CVE-2026-6686?
CVE-2026-6686 has been rated MEDIUM with a CVSS base score of 4.6/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-6686?
Check the references section above for vendor advisories and patch information. Affected products include: Elm-Chan Fatfs.