CVE-2026-6686: fatfs — Exploit

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.

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CVSS v3.1 Score

4.6

MEDIUM

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

EPSS — Exploit Prediction

0.0038

Probability of exploitation

0.30%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-908 CWE-908

Affected Products

Vendor	Product	Versions
elm-chan	fatfs	< r0.16

References

Exploits

https://github.com/runZeroInc/vulns-2026-fatfs-chance https://www.runzero.com/blog/fatfs-bugs/ https://github.com/runZeroInc/vulns-2026-fatfs-chance

Other References

https://elm-chan.org/fsw/ff/ https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/

Frequently Asked Questions

What is 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. It has a CVSS v3.1 base score of 4.6 (MEDIUM).

How severe is CVE-2026-6686? +

CVE-2026-6686 has a CVSS v3.1 score of 4.6 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0038, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-6686? +

CVE-2026-6686 affects products from elm-chan, specifically: fatfs. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-6686? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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