Medium 4.6Published Jul 1, 2026

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

Updated under an hour ago (Oct 26, 2026, 4:10 PM ET)

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It affects ChaN FatFs and 1 other product. Add what you run to find out whether that includes you.

No exploitation reported and no EPSS estimate yet.

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Every CVE for ChaN FatFs

Exploited in the last 30 days

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: 4.6 MEDIUM (v3.1, CNA)CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness (CWE): CWE-908
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: runZero
NVD analysis: Analyzed
Record last updated: Jul 1, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

ChaN FatFs chan:fatfs (from the name)	through R0.16	CNA record
elm-chan fatfs elm-chan:fatfs	through r0.16	NVD

References

References

https://www.runzero.com/blog/fatfs-bugs/	third-party-advisory, exploit
https://github.com/runZeroInc/vulns-2026-fatfs-chance	technical-description, third-party-advisory, exploit
https://elm-chan.org/fsw/ff/	product
https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/	third-party-advisory

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.