CVE-2026-6686: FatFs R0.16 and earlier contains an uninitialized cluster exposure when f_lseek() extends files beyond EOF wit

Q: How do I fix CVE-2026-6686?

fatfs: upgrade to newer than r0.16. Exact commands for each OS are on this page.

Q: Is CVE-2026-6686 being exploited?

CVE-2026-6686 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-6686 affect?

CVE-2026-6686 affects fatfs. Severity is medium (CVSS 4.6). 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

Q: How urgent is CVE-2026-6686?

Priority 25/100. Public exploit reference

Routine

MEDIUM 4.6UPGRADE

CVE-2026-6686 is a medium-severity flaw (CVSS 4.6) affecting fatfs. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	4.6 (Medium)
Attack vector	Physical
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.3% (top 83%)
CISA exploited list	No
Weakness	CWE-908
Published	2026-07-01
Last modified	2026-10-25

Publisher (CNA): runZero · Vendor: elm-chan

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-01 15:17 UTCPublished CVE record published
2026-10-25 17:10 UTCUpdated Record last modified

elm-chan fatfs UPGRADE

Affected: <= r0.16

Fixed in: newer than r0.16

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy fatfs || rpm -q fatfs || fatfs --version

2. UPGRADE to newer than r0.16 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade fatfs

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade fatfs

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-6686?

fatfs: upgrade to newer than r0.16. Exact commands for each OS are on this page.

Is CVE-2026-6686 being exploited?

CVE-2026-6686 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-6686 affect?

CVE-2026-6686 affects fatfs. Severity is medium (CVSS 4.6). 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

How urgent is CVE-2026-6686?

Priority 25/100. Public exploit reference

Related CVEs

CVE-2026-6687 HIGH 7.6 UPGRADE FatFs R0.16 and earlier contains a stack overflow bug in f_getlabel() because exFAT label length (XDIR_NumLabe
CVE-2026-6682 HIGH 7.6 UPGRADE In FatFS R0.16 and earlier contains a FAT32 integer overflow bug in mount_volume() where fasize *= fs->n_fats
CVE-2026-6688 HIGH 7.6 UPGRADE FatFs R0.16 and earlier contains a downstream-caller vulnerability pattern associated with FatFs long filename
CVE-2026-6683 MEDIUM 4.6 UPGRADE FatFs R0.16 and earlier contains a divide-by-zero in exFAT sync logic bug when crafted metadata causes n_faten
CVE-2026-6684 MEDIUM 4.6 UPGRADE FatFs prior to R0.16 that use GPT scanning with 'FF_LBA64 = 1' contains an issue where an unbounded loop count

References

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