CVE-2026-6686 in FatFs
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.
Quick answer
Elm-Chan FatFs should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- FatFs R0.16 and earlier when f_lseek() extends files beyond EOF without zero-filling new clusters
Fixed versions
- Review documented FatFs behavior, clear newly allocated clusters in downstream code, or apply vendor firmware guidance
How to fix it
CVE-2026-6686 affects FatFs. This is usually fixed in firmware or by applying the vendor patch or guidance. Check every device or product that ships FatFs code.
- List all firmware and devices that include FatFs.
- Check if they match FatFs R0.16 and earlier when f_lseek() extends files beyond EOF without zero-filling new clusters.
- Apply this fix or guidance: Review documented FatFs behavior, clear newly allocated clusters in downstream code, or apply vendor firmware guidance.
- Rebuild and sign firmware after the code change.
- Do not process untrusted disk images until the fix is applied.
- Test long file names, exFAT, FAT32, and GPT cases where relevant.
- Send the updated firmware to users or devices.
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Verify the fix
- Confirm the patched FatFs code is in the final firmware build.
- Run tests with malformed file systems and disk images.
- Check that the device does not crash, hang, or leak data.
- Run a full regression test for file read and write paths.
- Keep the patch source in the release notes.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-6686?
Elm-Chan FatFs versions listed as affected should be reviewed: FatFs R0.16 and earlier when f_lseek() extends files beyond EOF without zero-filling new clusters.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.