Is CVE-2026-6686 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-6686 being exploited in the wild.

How severe is CVE-2026-6686?

Medium severity, with a CVSS score of 4.6, published 1 July 2026.

Is there a fix for CVE-2026-6686?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-6686 attacked?

It is attacked with physical access, in a software library.

Am I affected by CVE-2026-6686?

Check whether you run ChaN FatFs at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-6686: ChaN FatFs uninitialized cluster exposure in f_lseek

CVE-2026-6686 · Severity: medium · CVSS 4.6 · Published 2026-07-01

FatFs is a widely used file system module for small embedded systems, such as those found in industrial controllers, medical devices, and consumer electronics. A security flaw allows sensitive data from previously deleted files to be exposed when a new file is created or extended. An attacker with physical access to the device's storage could potentially recover old encryption keys, logs, or firmware fragments that were supposed to be erased.

Technologies: ChaN FatFs. Vendors: ChaN.

Quick answers

Is CVE-2026-6686 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-6686 being exploited in the wild.
How severe is CVE-2026-6686?: Medium severity, with a CVSS score of 4.6, published 1 July 2026.
Which products and versions are affected?: ChaN FatFs R0.16 and earlier.
Is there a fix for CVE-2026-6686?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-6686 attacked?: It is attacked with physical access, in a software library.
Am I affected by CVE-2026-6686?: Check whether you run ChaN FatFs at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A 'Use of Uninitialized Resource' (CWE-908) vulnerability exists in the f_lseek() function of FatFs R0.16 and earlier. When f_lseek() is used to extend a file beyond the current End-of-File (EOF), the library updates the logical file size and allocates new clusters but fails to initialize these clusters with zeroes. Consequently, if the newly allocated sectors previously contained data from deleted files, that stale content becomes readable through the extended file region. This enables information disclosure of sensitive remnants like firmware chunks or keys on shared media. The vulnerability requires physical access to the storage media or a local interface capable of triggering file seeks and subsequent reads.

Affected products

ChaN FatFs R0.16 and earlier

Timeline

2026-07-01: advisory: NVD publication date
2026-06-30: disclosed: runZero advisory update date

References

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