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CVE-2026-6686— FatFs Use of Uninitialized Clusters After Seek Past EOF

CVSS 4.6 · Medium EPSS 0.38% · P30

Possible ATT&CK Techniques 1AI

T1005 · Data from Local System

Affected Version Matrix 1

VendorProductVersion RangeStatus
ChaNFatFs≤ R0.16affected
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I. Basic Information for CVE-2026-6686

Vulnerability Information
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Vulnerability Title
FatFs Use of Uninitialized Clusters After Seek Past EOF
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
对未经初始化资源的使用
Source: CVE Program / CVE List V5
Vulnerability Title
ChaN FatFs 资源管理错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
ChaN FatFs是ChaN公司的一个为嵌入式系统设计的通用 FAT/exFAT 文件系统模块。 ChaN FatFs R0.16及之前版本存在资源管理错误漏洞，该漏洞源于未初始化的簇暴露问题，当f_lseek()函数在没有对新分配的簇进行零填充的情况下将文件扩展到EOF之外时，可能导致未初始化资源使用。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

VendorProductAffected VersionsCPESubscribe
ChaNFatFs 0 ~ R0.16 -

II. Public POCs for CVE-2026-6686

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III. Intelligence Information for CVE-2026-6686

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Vendor Advisories for CVE-2026-6686 (1)

Security Blog Posts for CVE-2026-6686 (1)

Vendor Pages for CVE-2026-6686 (1)

Other References for CVE-2026-6686 (1)

Same Patch Batch · ChaN · 2026-07-01 · 6 CVEs total

CVE-2026-66887.6 HIGHFatFs Buffer Overflow via Unbounded LFN Filename Copy
CVE-2026-66877.6 HIGHFatFs Stack Buffer Overflow via Uncapped exFAT Label Length
CVE-2026-66827.6 HIGHFatFs Integer Overflow in FAT32 Volume Mount
CVE-2026-66844.6 MEDIUMFatFs Infinite Loop in GPT Partition Scan
CVE-2026-66834.6 MEDIUMFatFs Divide-by-Zero in exFAT Sync

IV. Related Vulnerabilities

Same product: FatFs
Same vendor: ChaN
Same weakness: CWE-908

V. Comments for CVE-2026-6686

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