CVE-2026-6686

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.

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

0.0018

Probability of exploitation

0.07%

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 0.0018, 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.

Related Vulnerabilities

CVE-2025-2329

In high traffic environments, a Silicon Labs OpenThread RCP (see impacted versions) fails to clear the SPI transmit buffer and …

CVE-2025-48513

Use of uninitialized resource within the AMD Platform Management Framework (PMF) could allow an attacker to read a uninitialized kernel …

CVE-2025-1942 9.8

When String.toUpperCase() caused a string to get longer it was possible for uninitialized memory to be incorporated into the result …

CVE-2024-32611 9.8

HDF5 Library through 1.14.3 may use an uninitialized value in H5A__attr_release_table in H5Aint.c.

CVE-2025-50165 9.8

Untrusted pointer dereference in Microsoft Graphics Component allows an unauthorized attacker to execute code over a network.

CVE-2024-47540 9.8

GStreamer is a library for constructing graphs of media-handling components. An uninitialized stack variable vulnerability has been identified in the …