FatFs Use of Uninitialized Clusters After Seek Past EOF

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Summary

CVECVE-2026-6686
StatePUBLISHED
AssignerrunZero
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-01 15:17:12 UTC
Updated2026-07-01 18:31:17 UTC
DescriptionFatFs 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.

Risk And Classification

Primary CVSS: v3.1 4.6 MEDIUM from 44488dab-36db-4358-99f9-bc116477f914

CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Problem Types: CWE-908 | CWE-908 CWE-908 Use of uninitialized resource

VersionSourceTypeScoreSeverityVector
3.144488dab-36db-4358-99f9-bc116477f914Secondary4.6MEDIUMCVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
3.1CNACVSS4.6MEDIUMCVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

CVSS v3.1 Breakdown

Attack Vector
Physical
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA ChaN FatFs affected R0.16 custom Not specified

References

ReferenceSourceLinkTags
www.runzero.com/blog/fatfs-bugs 44488dab-36db-4358-99f9-bc116477f914 www.runzero.com
www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686 44488dab-36db-4358-99f9-bc116477f914 www.runzero.com
github.com/runZeroInc/vulns-2026-fatfs-chance 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
elm-chan.org/fsw/ff 44488dab-36db-4358-99f9-bc116477f914 elm-chan.org
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: HD Moore of runZero, Inc. (en)

CNA: Tod Beardsley of runZero, Inc. (en)

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