CVE-2026-6686

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.

Score

CVSS 3.1 4.6 Medium

Status

Analyzed - 44488dab-36db-4358-99f9-bc116477f914

Published

2026-07-01

Last modified

2026-10-25

Weaknesses

CWE-908

Affected products

• Fatfs (Chan)
• Fatfs (Elm-chan)

References

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