TI-2026-05865 Aliases CVE-2026-6686 GHSA-h94p-vjqx-v7rr

FatFs Use of Uninitialized Clusters After Seek Past EOF

MEDIUM 54CWE-908Published 2026-07-01 · Updated 2026-07-01

Key facts

Attack vector: Physical
Privileges required: None
User interaction: None
Attack complexity: Low
Impact type: Information disclosure
Confidentiality · integrity · availability: High · None · None
Fix status: No fix listed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-908 and the CVSS impact.

Summary

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 it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV

EPSS 0.00255 17th percentileScored 2026-11-07

Public exploit Yes 2 exploit referencesTagged by NVD

Fix No fix listed No fixed version in the sources yet

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: Poc
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
ChaN	FatFs	up to R0.16	—	No fix listed	CNA

FatFs VendorChaN Affected up to R0.16 Fixed in — No fix listed CNA

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:elm-chan:fatfs::::::::	up to r0.16	Vulnerable	ChaN

cpe:2.3:a:elm-chan:fatfs:*:*:*:*:*:*:*:* Versions up to r0.16 Vulnerable ·ChaN

Remediation

No fixed version listed by the sources yet.

Timeline

2026-10-02 Threatist record created from CVE-2026-6686
2026-10-03 Alias added: GHSA-h94p-vjqx-v7rr

References

Exploit 2

Advisory 1

https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/

Other 1

https://elm-chan.org/fsw/ff/

TI-2026-05867 FatFs Buffer Overflow via Unbounded LFN Filename Copy CVSS 3.1HIGH 7.6
TI-2026-05866 FatFs Stack Buffer Overflow via Uncapped exFAT Label Length CVSS 3.1HIGH 7.6
TI-2026-05862 FatFs Integer Overflow in FAT32 Volume Mount CVSS 3.1HIGH 7.6
TI-2026-05864 FatFs Infinite Loop in GPT Partition Scan CVSS 3.1MEDIUM 4.6
TI-2026-05863 FatFs Divide-by-Zero in exFAT Sync CVSS 3.1MEDIUM 4.6

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences