CVE-2026-6685

MEDIUM 6.1

FatFs Integer Underflow in Dirty-Sector Cache Flush

CVSS Score

6.1

EPSS Score

0.0%

EPSS Percentile

0th

FatFs R0.16 and earlier exhibits a stale dirty-cache skip via unsigned-subtraction wrap in f_read() / f_write() (fp->sect - sect < cc) during interleaved read/write on fragmented filesystems. This maps to CWE-191 (Integer Underflow). Estimated CVSS v3.1 vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H (6.1, Medium). The estimated CISA SSVC vectors are Exploitation: PoC, Technical Impact: Total.

CWE	CWE-191
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:N/I:H/A:H

Attack Vector

Physical

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

High

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-unsigned-sub-wrap-cve-2026-6685/

Credits

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