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

Quick assessment

Affected
ChaN FatFs
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

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。

CVSS 4.6 · Medium EPSS 0.26% · P16

Possible ATT&CK Techniques 1 AI

T1005 · Data from Local System

Affected Version Matrix 1

VendorProduct Version RangeStatus
ChaN FatFs ≤ R0.16 affected
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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

Vendor Product Affected Versions CPE Subscribe
ChaN FatFs 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-2026-6688 7.6 HIGH FatFs Buffer Overflow via Unbounded LFN Filename Copy
CVE-2026-6687 7.6 HIGH FatFs Stack Buffer Overflow via Uncapped exFAT Label Length
CVE-2026-6682 7.6 HIGH FatFs Integer Overflow in FAT32 Volume Mount
CVE-2026-6684 4.6 MEDIUM FatFs Infinite Loop in GPT Partition Scan
CVE-2026-6683 4.6 MEDIUM FatFs 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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