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Vulnerability record · CVE-2026-6686 · published 1 July 2026

CVE-2026-6686: Elm-chan fatfs use of uninitialized resource vulnerability

Elm Chan · Fatfs

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.

4.6 CVSS 3.1 Medium EPSS 0.26% · top 84.6% CWE-908 · Use of uninitialized resource
4.6CVSS 3.1 base score
0.26%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
5References, 3 tagged exploit
2 Jul 2026Last modified by NVD

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.

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

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

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

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

7.6CVE-2026-6688Elm-chan fatfs classic buffer overflow vulnerabilityFatFs R0.16 and earlier contains a downstream-caller vulnerability pattern associated with FatFs long filename handling. With LFN enabled, fno.fname …EPSS 0.32%7.6CVE-2026-6682Elm-chan fatfs integer overflow vulnerabilityIn FatFS R0.16 and earlier contains a FAT32 integer overflow bug in mount_volume() where fasize *= fs->n_fats can wrap, leading to attacker-controlle…EPSS 0.32%7.6CVE-2026-6687Elm-chan fatfs stack-based buffer overflow vulnerabilityFatFs R0.16 and earlier contains a stack overflow bug in f_getlabel() because exFAT label length (XDIR_NumLabel) is trusted without enforcing spec ma…EPSS 0.35%4.6CVE-2026-6683Elm-chan fatfs divide by zero vulnerabilityFatFs R0.16 and earlier contains a divide-by-zero in exFAT sync logic bug when crafted metadata causes n_fatent - 2 to be zero during write/sync oper…EPSS 0.31%4.6CVE-2026-6684Elm-chan fatfs vulnerabilityFatFs prior to R0.16 that use GPT scanning with 'FF_LBA64 = 1' contains an issue where an unbounded loop count derived from GPT header field GPTH_PtN…EPSS 0.31%7.8CVE-2026-85880Windows ALPC heap buffer overflow allows local privilege escalationA heap-based buffer overflow in the Windows ALPC subsystem, combined with use of an uninitialized resource, lets an attacker with existing local acce…KEVEPSS 3.6%analysed9.3CVE-2025-5777Citrix NetScaler ADC/Gateway memory overread via insufficient input validationCVE-2025-5777 is an insufficient input validation flaw in Citrix NetScaler ADC and NetScaler Gateway that causes a memory overread when the appliance…KEVEPSS 100%analysed5.5CVE-2024-50302Linux kernel HID core uninitialized report buffer leaks kernel memoryThe Linux kernel HID core allocates a report buffer without zero-initializing it, so residual kernel memory can be exposed through crafted HID report…KEVEPSS 0.81%analysed

Source: NIST National Vulnerability Database (record CVE-2026-6686), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.