CVE-2026-6686

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.

CVSS v3 4.6 MEDIUM

4.6^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 0.9 / Impact : 3.6

Attack Vector PHYSICAL

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact HIGH

Integrity Impact NONE

Availability Impact NONE

Scope UNCHANGED

References

Link	Resource
https://elm-chan.org/fsw/ff/	Product
https://github.com/runZeroInc/vulns-2026-fatfs-chance	Third Party Advisory Exploit
https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/	Third Party Advisory
https://www.runzero.com/blog/fatfs-bugs/	Third Party Advisory Exploit
https://github.com/runZeroInc/vulns-2026-fatfs-chance	Third Party Advisory Exploit

Configurations

Configuration 1 (hide)

cpe:2.3:a:elm-chan:fatfs:*:*:*:*:*:*:*:*

History

02 Jul 2026, 14:38

Type	Values Removed	Values Added
CPE		cpe:2.3:a:elm-chan:fatfs::::::::
First Time		Elm-chan Elm-chan fatfs
References	~~() https://elm-chan.org/fsw/ff/ -~~	() https://elm-chan.org/fsw/ff/ - Product
References	~~() https://github.com/runZeroInc/vulns-2026-fatfs-chance -~~	() https://github.com/runZeroInc/vulns-2026-fatfs-chance - Third Party Advisory, Exploit
References	~~() https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/ -~~	() https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/ - Third Party Advisory
References	~~() https://www.runzero.com/blog/fatfs-bugs/ -~~	() https://www.runzero.com/blog/fatfs-bugs/ - Third Party Advisory, Exploit

01 Jul 2026, 16:16

Type	Values Removed	Values Added
References	~~() https://github.com/runZeroInc/vulns-2026-fatfs-chance -~~	() https://github.com/runZeroInc/vulns-2026-fatfs-chance -

01 Jul 2026, 15:17

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-07-01 15:17

Updated : 2026-07-02 14:38

NVD link : CVE-2026-6686

Mitre link : CVE-2026-6686

CVE.ORG link : CVE-2026-6686

JSON object : View

Products Affected

elm-chan

fatfs

CWE

CWE-908

Use of Uninitialized Resource

{"id": "CVE-2026-6686", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-6686", "role": "CISA Coordinator", "options": [{"exploitation": "poc"}, {"automatable": "no"}, {"technicalImpact": "partial"}], "version": "2.0.3", "timestamp": "2026-07-01T15:05:23.138301Z"}}], "cvssMetricV31": [{"type": "Secondary", "source": "44488dab-36db-4358-99f9-bc116477f914", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 4.6, "attackVector": "PHYSICAL", "baseSeverity": "MEDIUM", "vectorString": "CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "NONE", "privilegesRequired": "NONE", "confidentialityImpact": "HIGH"}, "impactScore": 3.6, "exploitabilityScore": 0.9}]}, "affected": [{"source": "44488dab-36db-4358-99f9-bc116477f914", "affectedData": [{"vendor": "ChaN", "product": "FatFs", "versions": [{"status": "affected", "version": "0", "versionType": "custom", "lessThanOrEqual": "R0.16"}], "defaultStatus": "unaffected"}]}], "published": "2026-07-01T15:17:12.763", "references": [{"url": "https://elm-chan.org/fsw/ff/", "tags": ["Product"], "source": "44488dab-36db-4358-99f9-bc116477f914"}, {"url": "https://github.com/runZeroInc/vulns-2026-fatfs-chance", "tags": ["Third Party Advisory", "Exploit"], "source": "44488dab-36db-4358-99f9-bc116477f914"}, {"url": "https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/", "tags": ["Third Party Advisory"], "source": "44488dab-36db-4358-99f9-bc116477f914"}, {"url": "https://www.runzero.com/blog/fatfs-bugs/", "tags": ["Third Party Advisory", "Exploit"], "source": "44488dab-36db-4358-99f9-bc116477f914"}, {"url": "https://github.com/runZeroInc/vulns-2026-fatfs-chance", "tags": ["Third Party Advisory", "Exploit"], "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Secondary", "source": "44488dab-36db-4358-99f9-bc116477f914", "description": [{"lang": "en", "value": "CWE-908"}]}], "descriptions": [{"lang": "en", "value": "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."}], "lastModified": "2026-07-02T14:38:35.823", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:elm-chan:fatfs:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "B99160BB-C4F5-4DA0-9FC9-0723E0633E00", "versionEndIncluding": "r0.16"}], "operator": "OR"}]}], "sourceIdentifier": "44488dab-36db-4358-99f9-bc116477f914"}