What is CVE-2021-3304?

CVE-2021-3304 is a critical-severity vulnerability in Sagemcom F\@st 3686, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-01-26.

How severe is CVE-2021-3304?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2021-3304 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Buffer overflow in Sagemcom F\@st 3686

CVE-2021-3304

Sagemcom F@ST 3686 v2 3.495 devices have a buffer overflow via a long sessionKey to the goform/login URI.

Vulnerability class: Buffer Overflow

Published 2021-01-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.016 (75.1th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 75th percentile — 75.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.016 · newest: 0.016 · change: 0.000

Affected products

Sagemcom F\@st 3686 — versions v2
Sagemcom F\@st 3686 Firmware — versions 3.495

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

Public proof-of-concept exploits

ExpLangcn/FuYao-Go

References

twitter.com/j0nh4t/status/1349649975479840769 (Exploit, Third Party Advisory)

Frequently asked questions

What is CVE-2021-3304?: CVE-2021-3304 is a critical-severity vulnerability in Sagemcom F\@st 3686, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-01-26.
How severe is CVE-2021-3304?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2021-3304 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.