CVE-2026-6688: fatfs Buffer Overflow — Exploit

Description

FatFs R0.16 and earlier contains a downstream-caller vulnerability pattern associated with FatFs long filename handling. With LFN enabled, fno.fname can be up to 255 characters; many callers copy it into short fixed buffers without bounds checks, causing overflow. This maps to CWE-120 (Buffer Copy without Checking Size of Input). Estimated CVSS v3.1 vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H (7.6, High). The estimated CISA SSVC vectors are Exploitation: PoC, Technical Impact: Total.

Is your site exposed to CVE-2026-6688?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

7.6

HIGH

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

EPSS — Exploit Prediction

0.0021

Probability of exploitation

0.11%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-120 CWE-120

Affected Products

Vendor	Product	Versions
elm-chan	fatfs	< r0.16

References

Exploits

https://github.com/runZeroInc/vulns-2026-fatfs-chance https://www.runzero.com/blog/fatfs-bugs/ https://github.com/runZeroInc/vulns-2026-fatfs-chance

Other References

https://elm-chan.org/fsw/ff/ https://www.runzero.com/advisories/fatfs-long-fn-of-downstream-cve-2026-6688/

Frequently Asked Questions

What is CVE-2026-6688? +

FatFs R0.16 and earlier contains a downstream-caller vulnerability pattern associated with FatFs long filename handling. With LFN enabled, fno.fname can be up to 255 characters; many callers copy it into short fixed buffers without bounds checks, causing overflow. This maps to CWE-120 (Buffer Copy without Checking Size of Input). Estimated CVSS v3.1 vector: CVSS:3.1/AV:P/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H (7.6, High). The estimated CISA SSVC vectors are Exploitation: PoC, Technical Impact: Total. It has a CVSS v3.1 base score of 7.6 (HIGH).

How severe is CVE-2026-6688? +

CVE-2026-6688 has a CVSS v3.1 score of 7.6 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0021, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-6688? +

CVE-2026-6688 affects products from elm-chan, specifically: fatfs. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-6688? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-27820

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain …

CVE-2025-11698

A denial-of-service issue exists in 5380/5480/5580 controllers boot firmware lower than version 1.072. This vulnerability could potentially allow a malicious …

CVE-2025-12012

A denial-of-service issue exists in 5380/5480/5580 controllers. This vulnerability could potentially allow a malicious user to write invalid file data …

CVE-2025-12011

A denial-of-service issue exists in 5370/5570 controllers. This vulnerability could potentially allow a remote user to load an invalid project, …

CVE-2024-13503

Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') vulnerability in Newtec NTC2218, NTC2250, NTC2299 on Linux, PowerPC, ARM …

CVE-2024-6199

An unauthenticated attacker on the WAN interface, with the ability to intercept Dynamic DNS (DDNS) traffic between DDNS services and …