What is CVE-2020-27633?

CVE-2020-27633 is a critical-severity vulnerability in Butok Fnet, classified under Use of Insufficiently Random Values. CVSS score: 9.1/10. Published 2023-10-10.

How severe is CVE-2020-27633?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Vulnerability in Butok Fnet

CVE-2020-27633

In FNET 4.6.3, TCP ISNs are improperly random.

Published 2023-10-10 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.

EPSS exploit prediction

EPSS: 0.009 (57.2th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 57th percentile — 57.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: 0.000

Affected products

Butok Fnet — versions 4.6.3

Weakness classification (CWE)

CWE-330 — Use of Insufficiently Random Values

References

www.forescout.com (Third Party Advisory)
www.cisa.gov/news-events/ics-advisories/icsa-21-042-01 (US Government Resource, Third Party Advisory)
www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-st… (Third Party Advisory)

Frequently asked questions

What is CVE-2020-27633?: CVE-2020-27633 is a critical-severity vulnerability in Butok Fnet, classified under Use of Insufficiently Random Values. CVSS score: 9.1/10. Published 2023-10-10.
How severe is CVE-2020-27633?: Critical severity. CVSS v3 base score is 9.1 out of 10.