mediumCVE-2026-6685

CVE-2026-6685 in FatFs

Q: What is affected by CVE-2026-6685?

Elm-Chan FatFs versions listed as affected should be reviewed: FatFs R0.16 and earlier dirty-cache arithmetic in f_read() and f_write() on fragmented filesystems.

Q: 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.

Q: 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.

Q: 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.

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.

ProductFatFs

CVSS6.1

EPSS0.00205

UpdatedJuly 15, 2026

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 dirty-cache arithmetic in f_read() and f_write() on fragmented filesystems

Fixed versions

Elm-Chan notes this wraparound is intentional; verify downstream code, test fragmented filesystems, and apply vendor guidance if your product is affected

How to fix it

CVE-2026-6685 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 dirty-cache arithmetic in f_read() and f_write() on fragmented filesystems.
Apply this fix or guidance: Elm-Chan notes this wraparound is intentional; verify downstream code, test fragmented filesystems, and apply vendor guidance if your product is affected.
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.

Scan now. Google sign-in is only needed to unlock fix guidance.

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

Supply Chain Input Validation

Trusted references

Source advisoryNVD Source advisoryNVD Source advisoryNVD Source advisoryNVD Source advisoryNVD Source advisoryNVD

FAQ

What is affected by CVE-2026-6685?

Elm-Chan FatFs versions listed as affected should be reviewed: FatFs R0.16 and earlier dirty-cache arithmetic in f_read() and f_write() on fragmented filesystems.

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.