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CVE-2026-6686
FatFs Use of Uninitialized Clusters After Seek Past EOF
Medium4.6EPSS 0.26%
- Published
- Updated
- Reserved
- Assigned by
- runZero
Description
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.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| runZeroCNA (assigner) | 3.1 | Medium4.6 | CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N |
| NVDNVD · Analyzed | 3.1 | Medium4.6 | CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N |
- EPSS (exploitation in 30 days)
- 0.26% · top 83% of all CVEs (6 Nov 2026)
- CISA SSVC
- Exploitation: poc · Automatable: no · Technical impact: partial
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The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 4.6.
Weakness
Affected products
References (4)
- https://www.runzero.com/blog/fatfs-bugs/ (opens in a new tab)third-party-advisory
- https://github.com/runZeroInc/vulns-2026-fatfs-chance (opens in a new tab)technical-descriptionexploit
- https://elm-chan.org/fsw/ff/ (opens in a new tab)product
- https://www.runzero.com/advisories/fatfs-uninit-cluster-exposure-cve-2026-6686/ (opens in a new tab)third-party-advisory
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://trackcve.app/cve/CVE-2026-6686)Record last updated .