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

Severity: medium (CVSS 4.6 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-07-01 · updated 2026-10-25
Weakness: CWE-908

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.

Affected products

ChaN FatFs: through r0.16

References

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

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