CVE-2026-6686

MEDIUM 4.6

FatFs Use of Uninitialized Clusters After Seek Past EOF

CVSS Score

4.6

EPSS Score

0.0%

EPSS Percentile

0th

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.

CWE	CWE-908
Vendor	chan
Product	fatfs
Published	Jul 1, 2026
Last Updated	Jul 1, 2026

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CVSS v3 Breakdown

CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Attack Vector

Physical

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

None

Availability

None

Affected Versions

ChaN / FatFs

0 ≤ R0.16

References

NVD ↗ CVE.org ↗ EPSS Data ↗

runzero.com: https://www.runzero.com/blog/fatfs-bugs/ github.com: https://github.com/runZeroInc/vulns-2026-fatfs-chance elm-chan.org: https://elm-chan.org/fsw/ff/ runzero.com: https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/

Credits

HD Moore of runZero, Inc. Tod Beardsley of runZero, Inc.